FOUS11 KWBC 280750
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EDT Mon Oct 28 2024

Valid 12Z Mon Oct 28 2024 - 12Z Thu Oct 31 2024


...Western U.S....
Days 1-3...

An amplified upper trough moving across the western U.S. is
forecast to produce widespread mountain snow from the Pacific
Northwest and the Sierra Nevada to the northern and central
Rockies. Areas of heavy accumulations are expected, especially for
portions of the central Rockies on Tuesday.

A well-defined shortwave moving through the base of the upper
trough is expected to lift northeast from the Southwest through the
central Rockies on Tuesday. In addition to strong mid-to-upper
level forcing, upslope flow on the northwest side of an associated
low level wave moving across Wyoming, will likely support periods
of heavy snowfall along the favored terrain. Ranges that will be
impacted include the Wind River, southern Absaroka, and the Big
Horns. WPC probabilities are 50 percent or greater for
accumulations of 8 inches or more across parts of these areas.
Locally heavy snows are also likely in the Black Hills, with
amounts of 6 inches or more likely, according to WPC probabilities.

As the system lifts out of the Southwest, other areas impacted
will include the southwestern and west-central Colorado ranges,
including the San Juan and Elk mountains, where heavy heavy snows
are forecast to begin tonight, with storm totals, according to WPC probabilities, likely reaching over a foot before snow diminishes
on Wednesday. Further west, widespread mountain snows are expected
along the Utah mountains as well, from the southern mountains to
the Uintas, with WPC probabilities indicating that locally heavy
amounts of 8 inches or more likely.

In the Northwest, onshore flow along the backside of the trough,
will continue to support unsettled weather across the region
through today. Snow levels will remain low enough to support light accumulations along the higher Cascades passes. Snow is expected to
diminish on Tuesday as a shortwave ridge builds ahead of the next
approaching system. Precipitation will then return by late
Wednesday with the arrival of a frontal band associated with a deep
upper low dropping south from the Gulf of Alaska. This will bring
the return of mountain snow to the Olympics and the Cascades and
the potential for impacts across the higher Cascades passes.

The probability of significant ice across the CONUS is less than
10 percent.

Pereira


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FOUS11 KWBC 290730
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EDT Tue Oct 29 2024

Valid 12Z Tue Oct 29 2024 - 12Z Fri Nov 01 2024


...Central Rockies onto High Plains....
Day 1...

Widespread mountain snow, with locally heavy amounts, is expected
as a well-defined shortwave moving through the base of a broader-scale
trough lifts northeast from the Southwest, crossing the central
Rockies into the High Plains over the next 12 hours. Areas likely
impacted will extend from the southern Utah and southwestern
Colorado mountains to the Wyoming ranges and the Black Hills.
Strong upward ascent afforded in part by a developing upper jet
couplet and low-to-mid level frontogenesis, will generate moderate
to heavy snow that will translate northeastward across the region
today. HREF guidance indicates that locally intense bands,
producing rates of 1-2 in/hr, can be expected to develop initially
over the Utah and western Colorado mountains this morning before
lifting into Wyoming, where more widespread heavy snow is likely
to develop by the afternoon and continue into the evening. WPC
guidance shows locally high probabilities for accumulations of 8
inches or more along the central Utah and west-central Colorado
ranges. More widespread coverage is shown across the central
Wyoming ranges, including the Wind River and eastern Absaroka
ranges, the Owl Creek Mountains, and Big Horns. WPC guidance
suggests widespread amounts of 8 inches or more across these areas,
with totals topping 18 inches across some of the higher terrain.
Rain changing to snow will produce some lighter accumulations,
generally around 1-2 inches across the northeastern Wyoming Plains.
However, heavier accumulations of 8 inches or more can be expected
across the higher elevations of the Black Hills.

...Northwest to the Northern Rockies...
Days 2/3...

Much of the Northwest will be dry today as a shortwave ridge
builds and moves east across the region ahead of the next
approaching system. This next system will be a deep low that is
expected to rapidly develop over the Gulf of Alaska today. This
system is forecast to drop south, with its associated frontal band
expected to move into the Pacific Northwest on Wednesday. As the
low reaches the Northwest on Thursday, it is forecast to turn to
the east and move toward the northern Rockies as it gradually
weakens. The heaviest precipitation is expected to focus across
western Oregon and northwestern California, generated by strong
forcing and moisture advection. Significant snow accumulations are
likely along the Oregon passes as snow levels dip below 4500 ft. By
Wednesday night, snow will begin to develop and spread east from
the northeastern Oregon into the central Idaho ranges, where some
locally heavy amounts of 6 inches or more can be expected by Friday
morning.

The probability of significant ice across the CONUS is less than
10 percent.


Pereira


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FOUS11 KWBC 300830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Wed Oct 30 2024

Valid 12Z Wed Oct 30 2024 - 12Z Sat Nov 02 2024

...Central Rockies...
Day 1...

Snow showers will continue across portions of the central Rockies,
as an upper trough moves across the region today. While widespread
heavy amounts are not expected, WPC probabilities indicate that
additional accumulations of 4 inches or more are possible across
the higher elevations of the west-central to north-central Colorado
mountains. Snows are expected to diminish as the upper trough moves
east into the central Plains this evening.

...Northwest to the Northern Rockies...
Days 1-3...

Precipitation will return to the Northwest and northern California
as a well-defined frontal band associated with a deep low dropping
out of the Gulf of Alaska moves onshore later today. Precipitation
rates are expected to increase later this morning and continue into
the afternoon as the front moves across the region. Rates will
diminish by the evening and overnight, but onshore flow will
continue to support unsettled conditions. Snow levels are expected
to quickly drop behind the front, supporting accumulating snow
across many of the Cascade passes.

The upper low is expected to support another period of organized
heavier precipitation as it continues to drop south on Thursday.
The heaviest precipitation is expected to center from southwestern
Washington through western Oregon into northwestern California.
Accumulations of a foot or more are likely for portions of the
Oregon Cascades, where snow levels are forecast to increase but
remain near or below 5000 ft through much of the period.

Further to the west, this series of systems will bring snow into
the eastern Oregon and northern Nevada mountains beginning this
evening, and the central Idaho ranges overnight, with periods of
snow continuing through Friday. Storm total accumulations of a foot
or more are likely across portions of the Blue Mountains in
northeastern Oregon and the central Idaho ranges, including parts
of the Salmon River Mountains and the Sawtooth Range.

As the upper low remains centered over Washington through much of
Friday, a well-defined shortwave digging to its west will move into
northern California late Friday into early Saturday, bringing
heavier precipitation across northwestern California into the
Shasta-Cascade region and the northern Sierra, where high
elevation heavy snow accumulations can be expected.

...Northeastern Minnesota to Upper Michigan...
Day 2...

The previously noted upper trough crossing the central Rockies
today is forecast to lift northeast through the central Plains and
into the upper Midwest by early Thursday. Guidance shows the upper
trough briefly assuming a negative tilt, with a closed 500 mb
center developing as moves into the upper Midwest Thursday morning.
As the surface, low pressure is expected to quickly deepen and
track northeast across the upper Mississippi Valley into the Great
Lakes. Beginning Thursday morning, models are showing rain
changing to snow within the strong low-to-mid level frontogenesis
band on the northwest side of the low, with some hi-res guidance
indicating snowfall rates increasing upward of 2 in/hr from east-
central Minnesota to the North Shore and northwestern Wisconsin.
While warm ground temperatures and solar insolation may help to
keep amounts in check, these rates, should they develop, will
likely to be sufficient to produce at least a few inches in some
locations. The overall trend in the overnight models was toward a
colder and wetter solution. So while probabilities for anything
above light accumulations have been low, it is expected these will
increase should these trends persist.

The probability of significant ice across the CONUS is less than
10 percent.

Pereira


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FOUS11 KWBC 301932
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 PM EDT Wed Oct 30 2024

Valid 00Z Thu Oct 31 2024 - 00Z Sun Nov 03 2024

...Northeastern Minnesota to Upper Michigan...
Days 1-2...

A vigorous upper level trough entering the Central Plains tonight
will lead to a strengthening area of low pressure that tracks into
the Upper Mississippi Valley by Thursday morning. This setup is
driven heavily by strong synoptic-scale ascent over eastern MN,
northern WI, and upper MI Thursday morning thanks to robust
250-500mb upper level divergence. As the storm strengthens Thursday
morning, dynamic cooling within the column coupled with intense
vertical velocities just below of, and including some lower
sections, of a saturated DGZ will support periods of heavy snow
from east-central MN to the MN Arrowhead, northern WI, and the
western most communities of MI's Upper Peninsula. Latest WPC
Snowband Probability Tracker does suggest the potential for 1-2"/hr
snowfall rates from just north of the Twin Cities metro area to
more prominently around Duluth, its surrounding communities, and on
north and east to include the coastal portions of the Arrowhead and
northwest WI. This event is a fairly fast moving setup with snow
starting mid-morning in parts of eastern MN, then concluding close
to sunset. The start and end times across northwest WI and the MI
Upper Peninsula would be closer to early afternoon for a start time
and ending around midnight local time.

Snowfall totals will likely be heavily driven by which areas reside
beneath the deformation axis, which is still not fully agreed upon
by guidance across the board. That said, most guidance does show an
opportunity for heavy snow in areas that would be experiencing
there first measurable snowfall of the season. Given that is would
be the first snow, the potential impacts are more amplified for
residents in these areas. The latest WSSI does show a large Minor
Impact area that stretches from the northern Twin Cities suburbs to
as far north as the MN Arrowhead and as far east as the western
most section of MI's Upper Peninsula. It is northwest WI to the
east of Duluth that show localized areas of Moderate to even Major
Impacts. These impacts are driven largely due to a combination of
the algorithm's Snow Amount and Snow Load components, signifying
that this is likely to be a heavy, wet snow in areas that could
result in detrimental impacts to trees branches and possibly power
lines. The good news is the storm will be racing east Thursday
night and any lingering snowfall is effectively finished around
sunrise Friday morning.

...Northwest to the Northern Rockies...
Days 1-3...

A potent storm system is arriving in the Pacific Northwest today
that is ushering in the next round of unsettled weather as far east
as the Northern Rockies and northern Great Basin. Initially, a cold front
will track through the Pacific Northwest today that begins the process
of lowering snow levels below 4,000ft by this evening throughout much
of the Pacific Northwest. NAEFS shows an upper level trough, sporting
500mb and 700mb heights below the 10th climatological percentile,
barreling into the Pacific Northwest and southern British Columbia
early Thursday morning. This upper level feature is also directing
a 300-400 kg/m/s IVT (above the 90th climatological percentile
over northern CA) into portions of the Northwest. There is not much
in the way of a sub-freezing air-mass either in front of, or in
wake of the upper low. This should keep most heavy snow to
elevations above 4,000ft in the Cascade Range, Siskiyou, and
Shasta. This same stream of moisture will reach the northern
Rockies late tonight and into Thursday where heavy snowfall

5,000ft is expected in the Blue, Sawtooth, Boise, and Teton

Mountains.

The upper low lingers along the Pacific Northwest coast through
Friday morning, then finally advances inland through the Northwest
Friday night. A cold front will advance south through northern
California and help to cause lowering snow levels throughout the
northern and central Sierra Nevada on Saturday. Periods of snow are
possible in the higher ridge lines of the northern Great Basin on
Saturday and as far east as the Tetons and Wasatch Saturday
afternoon.

WPC 72-hr PWPF shows high chances (>70%) of snowfall totals >18"
in the Oregon Cascades above 5,000ft, with some of the tallest
peaks above 6,000ft having similar high probabilities for snowfall
totals >30". Farther east, the Blue and Sawtooth Mountains at
elevations above 6,000ft have moderate-to-high chances (50-70%) for
snowfall amounts >18". Even portions of the Washington Cascades
are forecast to see some heavy snow with moderate-to-high chances

70%) for >12" of snowfall at elevations above 4,000ft. Farther

south, the Sierra Nevada and Siskiyou sport moderate-to-high odds
(50-70%) for snowfall >8" on Saturday with additional snowfall
likely to occur Saturday night. In terms of impacts, the
Probabilistic WSSI shows Moderate chances (40-60%) for Moderate
Impacts along the Oregon Cascades and the southern peaks of the
Washington Cascades. These areas can expect the more treacherous
travel conditions, particularly where heavy snow falls around pass
level. Elsewhere, the other mountain ranges listed sport generally low-to-moderate chances (30-50%) for Minor Impacts.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax





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FOUS11 KWBC 010623
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
223 AM EDT Fri Nov 1 2024

Valid 12Z Fri Nov 01 2024 - 12Z Mon Nov 04 2024

...Mountainous West...
Days 1-3...

Active period in the Western CONUS over the next few days
downstream of a very strong upper ridge (~99th percentile) between
Alaska and Hawaii. This will help guide several northern stream
systems into the West Coast and into the Intermountain West this
weekend into early next week. With the source region in the mid-
latitudes, moisture anomalies with each system will be modest and
thus the brunt of the snowfall will be driven by orographic upslope
into the Cascades initially then into the Rockies thereafter. Snow
levels will be on the higher side but still low enough at times to
bring some snow to the higher mountain passes.

D1 system will have the largest QPF amounts, primarily into the
WA/OR Cascades, as a cold front comes ashore. D1 probabilities of
at least 8 inches of snow are >50% above ~5000-6000ft. Into the
weekend, the lead system will move through the Great Basin with
light to moderate snow across central Idaho/western Montana and
into the Tetons southward through the Uintas and Wasatch. WPC
probabilities for at least 8 inches of snow D2 are low (<40%) in
these areas but a bit higher into the OR Cascades with the next
system quickly approaching the coast. By D3, that system will slip
through the Great Basin and into the central Rockies, with more
moderate totals possible into northern CO (Medicine Bow). Light
snow will work its way into the Front Range by the end of the
period as low pressure deepens over southeastern CO. WPC
probabilities of at least 8 inches of snow are >50% above 10,000ft
or so and for at least 4 inches are >50% above 8000ft.

The probability of significant ice across the CONUS is less than
10 percent.

Fracasso





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FOUS11 KWBC 020637
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 AM EDT Sat Nov 2 2024

Valid 12Z Sat Nov 02 2024 - 12Z Tue Nov 05 2024

...Mountainous West...
Days 1-3...

An active pattern remains for much of the West this period,
downstream of a strong upper ridge between Alaska and Hawaii. This
will take a series of systems on a NW to SE path from the PacNW
through the Great Basin and into the Rockies this weekend into
early next week. For day 1, ongoing system in the NW will continue
to bring snow to the Cascades eastward to the northern Rockies and
southward through the Bitterroots, Tetons, Wind River, into the
Wasatch. WPC probabilities for at least 8 inches of snow are >50%
above 5000ft or so in the NW to about 9000ft over the Uintas.

On day 2, the trough will dig into the Four Corners region and
support light to modest snow for the CO Rockies, especially above
10,000ft where WPC probabilities for at least 8 inches of snow are

50%. Light snow is probable into the I-25 corridor but amounts are

quite uncertain. Accumulating snow is favored farther south into
the Palmer Divide. By day 3, that upper trough will continue to
move slowly through NM with a focus for snowfall over the Sangre de
Cristos into the Raton Mesa. There, WPC probabilities for at least
8 inches of snow are moderate (40-70%). Back to the PacNW, the
next system will move ashore with another surge in moisture to the
area and significant snow for the WA Cascades. Snow levels will
rise to 4500ft or so with a decent moisture plume (300-400 kg-m/s
IVT) then fall behind the front. Higher mountains passes will
likely be affected with significant accumulations, but lowering
snow levels will likely affect some lower passes as well. WPC
probabilities for at least 8 inches of snow are >50% above
3000-3500ft or so.


The probability of significant ice across the CONUS is less than
10 percent.

Fracasso


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FOUS11 KWBC 030705
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
205 AM EST Sun Nov 3 2024

Valid 12Z Sun Nov 03 2024 - 12Z Wed Nov 06 2024

...Mountainous West...
Days 1-3...

Upper troughing over the Great Basin this morning will continue to
dig into the Four Corners region into tomorrow, promoting broad
lift over the Wasatch and especially the CO Rockies. WPC D1
probabilities of at least 8 inches of snow are >50% above 10,000ft
or so.

Into D2, upper low will be moving out of NM with lingering snow
for the San Juans, Raton Mesa, and Jemez Mountains where WPC
probabilities for an additional 8 inches of snow are moderate
(40-70%). Concurrently, a shortwave out of the northeastern Pacific
will push into Washington with a brief surge in moisture aimed at
the WA Cascades before weakening as the cold front moves inland.
Snow levels will rise from ~4000 to 6000ft as the core of the WAA
surges in ahead of the front, then will fall back below 4000ft
post-FROPA. Moisture should reach into the northern ID mountains
and into NW MT as well, where WPC probabilities for at least 8
inches of snow are >50% above 4000-5000ft or so.

By D3, the system in the NW will continue toward the Rockies with
upstream vorticity at the nose of a 140kt jet streaming in across
the PacNW, helping to amplify the pattern over the eastern Pacific
into western North America. This, in turn, will carve out a broad
upper trough that will dig into the Four Corners region through the
end of the period (and then beyond). Broad lift combined with more
localized upslope will maximize snow over the higher terrain as
snow levels fall to the valley floors in most areas of the Northern
Tier. WPC probabilities for at least 8 inches of snow are highest

70%) over the Little Belt Mountains in MT as well as the Big Snow

Mountains due to the favorable northerly flow. Lighter snow is
forecast for the central Idaho ranges, NW MT around Glacier NP,
southward through western WY and into the CO Rockies.

The probability of significant ice across the CONUS is less than
10 percent.

Fracasso


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FOUS11 KWBC 040704
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
204 AM EST Mon Nov 4 2024

Valid 12Z Mon Nov 04 2024 - 12Z Thu Nov 07 2024

...CO/NM border...
Day 1...

Upper low over NM this morning will lift northeastward across the
TX Panhandle this evening. Snow on the NW side of the low within a
region of modest PVA and enhanced upslope into the Raton Mesa will
continue for the first part of D1 before tapering off later this
evening and overnight. Lighter snow will extend eastward into the
far western OK Panhandle as rain changes to snow. WPC probabilities
for at least 8 inches of snow are >50% above 9000-10,000ft over
portions of central/northern NM and across the CO line.


...Northwest and Northern/Central Rockies...
Days 1-2...

Next system coming into the PacNW will be the last for at least a
little bit in what has been quite a busy stretch. Warm front will
move ashore today bringing in a surge of moisture and rising snow
levels up to around 4000ft to the north and 6000ft to the south
over the WA Cascades, but only rising to around 5000ft farther east
toward the Divide. Strong and favorable upslope flow will maximize
over the WA Cascades where a few feet of snow will be possible at
the higher peaks. By late in the evening and overnight into
Tuesday, snow levels will fall to around 3000ft bringing
accumulating snow to many of the lower passes across the Cascades.
WPC probabilities for at least 8 inches of snow are >50% above
3500ft or so. Upper jet will continue inland and promote moderate
snow across the northern ID ranges into northwestern Montana. Into
D2, the mid-level vort will deepen and split, with the northern
portion closing off along the MT/Canadian border before pivoting
back southward Tuesday evening over central MT. This will maintain
modest snow over the central MT terrain (i.e., Little Belt and Big
Snowy Mountains) on upslope northerly flow. There, WPC
probabilities for at least 8 inches of snow D2 are >50% above
4000ft or so. The southern portion of the mid-level trough will dig
into the Great Basin through early Wednesday, spreading snow
across UT/CO where several inches are likely, especially into the
CO Rockies. By the end of D2, an upper low will start to close off
over the Four Corners region, setting up the next phase of the
system.


...Four Corners...
Day 3...

Aforementioned upper low is forecast to be situated over eastern
UT early Wednesday morning and will likely sink southward through
the day to a position over eastern AZ by the end of D3. This will
keep a relatively consistent SW to S flow over northern NM into
southern CO, which is quite favorable for upslope enhancement over
the San Juans and into the Sangre de Cristos among other ranges.
Multiple waves of PVA will rotate around the upper low and across
the region, favoring moderate to heavy snow at times with a high
likelihood (>60% chance) of moderate impacts per the
probabilistic WSSI. WPC probabilities for at least 8 inches of
snow are >50% above 6000-8000ft and probabilities for at least 4
inches >50% down to around 5000-7000ft. Colder air filtering into
northeastern CO will also promote light to perhaps modest snow over
the I-25 corridor with heavier totals likely into the Palmer
Divide. Heavy snow is forecast to continue past the end of D3 as
the system slowly moves through the region. Please see the Winter
Weather Outlook and the Probabilistic Winter Storm Severity Index
(WSSI-P) for more information.

The probability of significant ice across the CONUS is less than
10 percent.

Fracasso


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FOUS11 KWBC 050721
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
221 AM EST Tue Nov 5 2024

Valid 12Z Tue Nov 05 2024 - 12Z Fri Nov 08 2024


...Northern/Central Rockies...
Days 1-2...

Vigorous upper trough moving through the NW will start its
transition to separate systems... the northern entity becoming an
upper low that will wobble through eastern Montana later today and
overnight. Broad divergence will favor light snow over much of
Montana but the eastward then southward movement of the forming
upper low will turn the flow northerly over central portions of the
state, which will enhance upslope flow into the Little Belt, Big
Snowy, and Big Horn Mountains. Snow will fall at a modest to heavy
rates per the 00Z HREF which shows >1"/hr probs of 50-90%. Lighter
snow is expected farther south through WY into CO as the southern
portion of the trough begins to split off and sink toward the Four
Corners, with some higher totals in the high terrain. Snowfall
should gradually taper off in most areas by Wednesday morning
(north) or overnight (south). WPC PWPF shows high chances (>70%)
for snowfall totals >8" over many of the MT ranges, especially

6,000ft, through Wednesday afternoon. Some of the tallest peaks of

these ranges could surpass 24" locally.

...Four Corners...
Day 2-3...

The southern part of the upper level feature responsible for the
heavy snow in the northern Rockies will sink southward through
eastern UT on Wednesday into AZ by early Thursday. After that, it
should slowly turn eastward into NM, but the guidance remains
inconsistent in the evolution/track. A cold front will bring in
colder air to the southern Rockies onto the High Plains as strong
upper level divergence aloft will support broad ascent over the
region. Moisture levels will be modest, but IVT on southwest to
southerly flow is forecast to be near the 90th percentile which
should act in concert with upslope enhancement to promote moderate
to heavy snow over the southern Rockies. Trend has been a bit less
and farther south with the QPF, resulting in a bit less snow for
northern areas in CO and continued uncertainty in snow amounts over
NM. Nevertheless, significant snow is still expected in at least
portions of southern CO into northern NM with this dynamic system.

The upslope component should favor both the SW/S-facing slopes of
the San Juans and Sangre de Cristos as well as farther north into
the Front Range via the cold front. The key on Thursday will be
where the upper low sets up and the extent of the moisture/upslope
flow into the Southern Rockies. By Friday morning, the upper low is
forecast to drift into NM with the brunt of the snowfall over NM.

WPC PWPF for the event through 12Z Fri shows high chances (>70%)
for snowfall >8" in the San Juans, the Sangre De Cristo, and into
parts of the far western High Plains. Highest totals are likely
over the Sangre De Cristo where 2ft+ is possible (>50% chance).
Around the Denver metro area, WPC PWPF for >4" of snow are around
30% but increase to the south (i.e., Palmer Divide) and then
increase again near/south of Colorado Springs and Pueblo to reach

70%.

The probability of significant ice across the CONUS is less than
10 percent.


Fracasso


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FOUS11 KWBC 060807
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 AM EST Wed Nov 6 2024

Valid 12Z Wed Nov 06 2024 - 12Z Sat Nov 09 2024


...Central/Southern Rockies and Adjacent High Plains...
Days 1-3...

Upper low at the base of a larger western CONUS trough lies over
southeastern UT this morning, and will continue sinking southward
today into AZ. It will slowly detach from the northern stream and
cut off as it wobbles southward then eastward into southern NM by
early Friday. Then, feeling the push of the subtropical jet, it
will lift northeastward across the northern TX Panhandle and far
western OK Panhandle to the CO/KS border by early Saturday. The
slow movement of the upper low combined with a fairly steady
stretch of moisture as well as upslope enhancement will lead to
significant snow accumulations in parts of the NM mountains and
across the CO line along the Sangre de Cristo range. Impacts may be
major to extreme in some locations.

For D1, frontal boundary over southeastern CO on the edge of the
height falls will see modest to locally heavy snow continue this
morning in an area of surface convergence and beneath bouts of PVA.
Through the day, as the upper low tucks farther southwest of CO,
heights should rise and the snow should lessen. Nevertheless,
snowfall rates of >1"/hr should yield an additional several inches
near/south of DEN and eastward over the High Plains. There, WPC
probabilities for at least 8 inches of snow after 12Z are moderate
(40-70%). Farther south over NM, closer to the stream of moisture
and vort maxes rotating around the upper low center, snow will
increase in intensity over much of the NM terrain where snowfall
just on D1 will likely exceed 8 inches (probabilities >70%) in the
terrain (e.g., Sangre de Cristos/Raton Mesa/Jemez Mnts).

By D2, slow movement of the upper low will allow for continued
snowfall to its east via persistent IVT values near/above the 90th
percentile. This could result in 1-2ft of snow over the NM terrain
(WPC probabilities for at least 12 inches are >70% over much of
northeastern NM) along with areas of blowing/drifting snow eastward
into the western OK Panhandle. Snow will continue over northern NM
into southern CO along the Sangre de Cristos and onto the High
Plains as the upper low sloshes toward the TX/OK Panhandles.
Moisture source will add in then largely switch to the western
Gulf, wrapping around the upper low and capitalizing on upslope
from the northeast through the column. Snow will finally start to
taper off by the end of the period (early Saturday).

All told, 3-day WPC probabilities for at least 24 inches of snow
are >50% above 6000-7000ft over the Sangre de Cristos and Raton
Mesa, with a large footprint of at least 6 inches over much of
eastern CO southward into central NM.


The probability of significant ice across the CONUS is less than
10 percent.


Fracasso



$$
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FOUS11 KWBC 090736
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
236 AM EST Sat Nov 9 2024

Valid 12Z Sat Nov 09 2024 - 12Z Tue Nov 12 2024


...Colorado Rockies...
Day 1...

The historic winter storm that has brought multiple feet of snow to
many parts of the Southern Rockies and central High Plains will
wane quickly Saturday morning as the low becomes vertically stacked
and occludes, shutting off the primary moisture feed. Despite this
occurring, the primary upper low will remain intense, and the
strong deformation axis responsible for the heavy snow banding
overnight will pivot across north-central CO into southeast WY
before slowly eroding through the day. The thermal profile to the
northeast is less conducive for snow, but some dynamic cooling due
to strong ascent could still result in some modest snowfall through
early Saturday. However, the greatest potential for any additional
significant accumulations will be across the higher terrain of
north-central CO, including the Park Range where WPC probabilities
for an additional 4+ inches of snow are 50-70%.


...Northwest...
Days 2-3...

A potent upper trough digging from the Gulf of Alaska will dig
towards Washington State, but likely come ashore over British
Columbia Monday night. This will have a two-pronged effect of
causing broad height falls across the Pacific and Interior
Northwest (height anomalies -1 sigma according to NAEFS) while also
causing some pinched broadly cyclonic flow into the Northwest
CONUS. Together, this will drive a corridor of moderate IVT for
which CW3E probabilities from the ECENS and GEFS suggest will
exceed 500 kg/m/s as an AR surges onshore, accompanied by a surface
cold front advecting eastward. This will result in a stripe of
heavy precipitation ahead of the cold front, with a secondary wave
of precipitation following in its wake embedded within the
confluent mid-level flow.

Snow levels ahead of the front will be generally 6000-7000 ft, so
the initial precipitation should fall as primarily rain except in
the highest terrain of the Cascades. However, as snow levels crash
behind the front and precipitation persists, accumulating snow will
occur as low as 3500-4000 ft by the end of the period. This will
result in significant snowfall in the WA/OR Cascades and the
Olympics, with increasing pass-level impacts possible.
Additionally, by Tuesday morning /end of D3/ precipitation falling
as snow is expected to spread as far east as the Northern Rockies
and areas around Yellowstone NP.

WPC probabilities for more than 4 inches of snow on D2 are confined
to the far northern WA Cascades and the higher terrain around Mt.
Rainier where they are above 70%. By D3, snowfall expands along the
Cascades and as far south as the Sierra, while additionally
pushing inland to the Northern Rockies. WPC probabilities for more
than 6 inches D3 are above 70% along the Cascades and near Mt.
Shasta, where 1-2 feet of snow is likely in the highest terrain.
Elsewhere, WPC probabilities are as high as 30-50%, highest near
the Salmon River and Sawtooth Ranges of Idaho. With snow levels
falling, impactful pass-level snow is also likely, especially at
some of the higher passes like Washington Pass and Stevens Pass in
the WA Cascades.

The probability of significant ice across the CONUS is less than
10 percent.

Weiss

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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FOUS11 KWBC 100738
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
238 AM EST Sun Nov 10 2024

Valid 12Z Sun Nov 10 2024 - 12Z Wed Nov 13 2024

...Northwest...
Days 2-3...

Dual amplified shortwaves will bring an active period of weather to
the Pacific and Interior Northwest through mid-week. The first of
these will dig out of the Gulf of Alaska and approach the British
Columbia coast Sunday night, before sharpening into a short-
wavelength but amplified open wave advecting onshore WA/OR Monday
evening. This feature will then move quickly eastward into the
Northern Rockies while weakening through Tuesday. Although this
trough will be of modest amplitude and progressive, height falls,
divergence and dual jet streaks (one poleward arcing downstream of
the trough axis, another more zonally oriented upstream) will lead
to widespread deep layer ascent from the Pacific Northwest eastward
through the Northern Rockies. At the same time, this trough will
drive a cold front eastward, surging onshore the WA/OR coasts
Monday morning before racing into the Northern High Plains by the
end of the forecast period.

This synoptic lift will act upon an increasingly moist column noted
by rapidly rising 1000-500mb RH as warm/moist advection occurs
within the aforementioned jet streaks, driving IVT to 250-500
kg/m/s. This will support widespread moderate to heavy
precipitation, and as snow levels collapse from around 6000 ft
early to as low as 3500 ft D2, snow will become widespread in the
terrain of the Olympics, Cascades, Shasta/Trinity region of CA, the
Sierra, and then into the Northern Rockies. WPC probabilities for
at least 6 inches of snow are 70+% along the spine of the Cascades
of WA and OR, with amounts reaching multiple feet likely in the
highest volcanoes. There is also a 30-50% chance of more than 6
inches in the Shasta/Trinity region, the northern Sierra, the Blue
Mountains of OR, and the far Northern Rockies.

After a brief period of shortwave ridging across the Pacific
Northwest, a more pronounced impulse digs into the region during D3,
nearly reaching the coast by the end of the period. This will be
accompanied by more impressive moist advection with ECENS IVT
probabilities reaching above 90% for 500 kg/m/s, with several
members exceeding 750 kg/m/s. The accompanying WAA will surge snow
levels back to around 5000-6000 ft along the immediate coast, but
rise only to around 4000 ft across the Cascades. This will support
more widespread heavy snow, with generally below climo SLRs leading
to high snow load and greater impacts, especially across the WA
Cascades. WPC probabilities are above 9)% in the WA Cascades,
primarily above 5000 ft, with 1-2 feet likely in the higher
elevations. Some pass level impacts are also expected, especially
at Washington Pass, with lesser but still potential impacts at
Stevens Pass.


The probability of significant ice across the CONUS is less than
10 percent.

Weiss


$$
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FOUS11 KWBC 110728
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
228 AM EST Mon Nov 11 2024

Valid 12Z Mon Nov 11 2024 - 12Z Thu Nov 14 2024

...Western U.S...
Days 1-3...

Two distinct and anomalous shortwaves will amplify and move
progressive across the Western CONUS through Thursday, bringing an
extended period of unsettled weather to the region, with several
days of moderate to heavy snow expected in the higher terrain.

The first of these will approach the Pacific Coast Monday morning,
with downstream SW flow driving robust IVT approaching 500 kg/m/s
onshore into CA/OR/WA. This IVT will help surge PWs to above climo
directly ahead of a surface cold front which will be positioned
beneath the trough axis. As this trough/front advect onshore Monday
aftn, the overlapping ascent into the moistening column will result
in heavy precipitation spreading from central CA through the WA
Cascades, and then pushing east, while weakening due to lessening
IVT, reaching the Northern Rockies and eastern Great Basin by
Tuesday morning. Snow levels ahead of this front will be generally
around 6000 ft during the period of heaviest precipitation, but
will fall quickly to as low as 3500 ft behind the front. At the
same time, while the primary forcing mechanisms will shift east,
continued onshore moist advection in pinched mid-level flow will
drive persistent moisture onshore, resulting in snow accumulations
lowering across the Cascades/Olympics. WPC probabilities D1 for
more than 6 inches of snow are above 70% along the spine of the
Cascades in OR and WA, with up to 2 feet possible in the highest
terrain. Lower probabilities exist into the Shasta/Trinity region,
the northern Sierra, and as far east as the Blue Mountains of OR.

During D2 /Tuesday and Tuesday night/ the lead trough and
accompanying cold front continue to push east as far as the Central
Plains while weakening, while shortwave ridging briefly builds in
across the Northwest. This suggests that much of D2 will feature
waning snowfall with lesser accumulations. However, before D2 ends,
the subsequent trough will approach the Pacific Northwest once
again. This trough is progged to be deeper, noted by 700-500mb
height anomalies reaching -2 sigma according to NAEFS, with the
subsequent downstream warm/moist advection in the mid and upper
levels driving IVT to as high as 750 kg/m/s, and ECENS
probabilities suggest even a 30% chance of IVT exceeding that
value. This WAA will occur within a narrow channel ahead of yet
another cold front, driving a corridor of heavy precipitation in
conjunction with snow levels rising once again to 6000-7000 ft.
This will limit snowfall before D2 ends except in the highest
terrain of the Olympics and Cascades.

The trough axis responsible for pushing the front eastward and
driving the pronounced deep layer ascent across the Northwest will
struggle to move east as repeated lobes of vorticity rotate down
from the Gulf of Alaska, resulting in little movement of the trough
axis and persistent, although weaker, moist advection onshore. This
suggests that snow levels will only slowly fall, progged to drop
to 4500-5000 ft by the end of the forecast period, with primarily
light to moderate precipitation rates from northern CA through the
Northern Rockies. This will still result in heavy snow
accumulations at the highest elevations, reflected by WPC
probabilities for more than 6 inches exceeding 90% in the northern
WA Cascades, and then expanding east into Selkirk Range, the Blue
Mountains, and the Sawtooth/Salmon River area. The higher passes
above 5000 ft will also likely experience significant impacts from
snow accumulations, but in general most of the passes should be
below the heavier snow accumulation levels.


The probability of significant ice across the CONUS is less than
10 percent.


Weiss


$$
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FOUS11 KWBC 120820
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Tue Nov 12 2024

Valid 12Z Tue Nov 12 2024 - 12Z Fri Nov 15 2024

...Western U.S...
Days 1-3...

An active period of winter weather continues across the Western
CONUS this period as dual shortwave trough cross the region.,

The first of these will be moving from the eastern Great Basin this
morning into the Central Plains by Wednesday evening, with minimal
amplitude gain. The combination of the progressive nature of this
feature with modest PW surge (at least until the Plains when
thermals support only rain) will somewhat limit snowfall potential
D1. However, sharp height falls downstream of the short-wavelength
trough combined with low-level convergence along the accompanying
cold front and post-frontal upslope flow into terrain features will
still support rounds of moderate to heavy snow. The greatest
potential for significant accumulations will be across the Wasatch
and Colorado Rockies, including the Park Range, where WPC
probabilities for more than 6 inches are above 70%, and locally up
to 10 inches is possible, primarily above 5500 ft.

Brief shortwave ridging blossoms across the Northwest behind this
first trough, but is quickly replaced by a more impressive impulse
digging along the Pacific Coast as it drops from the Gulf of
Alaska. Downstream divergence and accompanying WAA/moist advection
will begin to spread precipitation back into the Pacific Northwest
late tonight. Unlike its predecessor, this secondary trough will be
very slow to move east as it gets repeatedly reinforced just off
the coast by renewed lobes of vorticity swinging cyclonically
around it. This will have the two-pronged effect of driving waves
of ascent and persistent moisture onshore, while also keeping the
primary trough axis positioned just west of the region until Friday
when a more pronounced vort max swings through the base and pushes
the trough onshore CA late in the forecast period.

Within the most impressive WAA, a narrow channel of IVT exceeding
750 kg/m/s is likely, focused into the WA/OR and northern CA coast
late D2 into D2 before weakening with inland extent. This channel
/AR/ will also provide the highest increase in snow levels,
reaching as high as 7000 ft. However, dual cold front progged to
push onshore, one Wednesday morning and another Thursday morning,
driven east by periodic shortwaves, will cause gradual lowering of
snow levels to as low as 3000 ft by Friday morning across the
Cascades, and 4500 ft into the Interior Northwest and northern CA.
The lower snow levels will be accompanied by weaker forcing and
more transient moisture however, so heavy pass-level snow is still
not anticipated at this time.

Despite that, heavy snow accumulations are likely, especially on D2
when WPC probabilities for more than 6 inches spike above 90% in
the Cascades, Shasta/Trinity region, the Okanogan Highlands,
Selkirk Mountains, and Blue Mountains. Some of these areas could
receive as much as 2 feet of snow D2, and 6-12 inches is possible
at some of the elevated passes including Washington Pass. By D3,
the snow intensity wanes across most of the NW, but an increase in
WPC probabilities for 6+ inches reaching 30-50% in the Sierra as
the trough finally pushes onshore into CA.

The probability of significant ice across the CONUS is less than
10 percent.


Weiss

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FOUS11 KWBC 130751
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
251 AM EST Wed Nov 13 2024

Valid 12Z Wed Nov 13 2024 - 12Z Sat Nov 16 2024

...Western U.S...
Days 1-3...

The weather pattern over the next few days will be driven largely
by a longwave upper level trough with a pair of upper level
disturbances tracking into the West Coast. The first storm system
is already impacting the Pacific Northwest with an IVT up to 750
kg/m/s (topping the 97.5 climatological percentile) escorting
anomalous moisture as far south as northern California and as far
east as the Lewis Range in Montana. Elevations above 5,000ft,
whether it be in the Cascades, Olympics, northern Sierra Nevada, or
inland through the Blue, Sawtooth, Bitterroots, and Lewis Range,
will be favored for witnessing accumulating snowfall that could
result in treacherous travel conditions. The WSSI does depict some
localized areas of Minor to Moderate Impacts in the >5,000ft
elevations of the Olympics, Cascades, Blue, and Shasta Mountains
today.

Onshore flow into the Pacific Northwest will keep high elevation
snow in the forecast through Thursday in the Northwest. By
Thursday night, the next Pacific storm system will track farther
south into California with 500mb heights that below the 2.5
climatological percentile on NAEFS. Heights will be low enough
Thursday night and into Friday morning that snow levels in the
Sierra Nevada will be as low as 4,000ft (5,000ft in the San Gabriel
and San Bernadino of southern California). The heaviest snowfall
will be observed above 6,000ft in the central Sierra Nevada. WPC
PWPF shows moderate-to-high chances (50-70%) for snowfall totals

4" in the central Sierra Nevada between Thursday evening and

Friday morning. By Friday afternoon, the upper trough will track
into the heart of the Intermountain West with strong upper level
ascent over the Northern Rockies. WPC PWPF shows moderate chances
(40-60%) for snowfall totals >4" in the >7,000ft ranges of the
Absaroka.

...Northern New England...
Day 3...

An inverted trough revolving around the northern flank of a
retrograding storm system in the northwest Atlantic will direct a
plume of low level moisture back towards northern New England on
Friday. The air-mass over northern New England (particularly
Maine) will be sufficiently cold and dry enough for wet-bulb
temperatures to remain below freezing within the boundary layer.
Strong easterly mid-level flow with air-mass origins over the
Atlantic will lead to warm air-advection that causes a >0C nose
between 850-750mb Friday morning. This is a setup that will likely
result in periods of sleet/freezing rain over northern Maine and
possibly as far west as northern New Hampshire. WPC PWPF does show
low chances (10-30%) for ice accumulations >0.1" in parts of the
North Woods and Central Highlands of northern Maine. One notable
location with those potential ice accumulations includes Baxter
State Park. These icy conditions could result in Minor Impacts
according to the Probabilistic WSSI with low-moderate chances
(30-50%) through Friday afternoon. The primary impacts would be icy
roads in some areas that could lead to slick travel conditions.

Mullinax


$$
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FOUS11 KWBC 140802
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Thu Nov 14 2024

Valid 12Z Thu Nov 14 2024 - 12Z Sun Nov 17 2024

...Western U.S....
Days 1-3...

A pair of Pacific storm systems will produce heavy mountain snow
from the West Coast to the Northern Rockies over the next few days.
Today, broad scale upper level ascent out ahead of a longwave
trough off the West Coast will also direct Pacific moisture at the
Northwest and northern California. Periods of snow will occur for
many of the Northwest U.S. mountain ranges which include (but are
not limited to) the Cascades, Siskiyou, northern Sierra Nevada,
Blue, Sawtooth, Bitterroots, Idaho Panhandle, and northern Montana
Mountains). As a potent upper level trough approaches California
Thursday night (500mb heights below the 2.5 climatological
percentile via NAEFS), snow levels will fall to as low as 4,000ft
in the Sierra Nevada and as low as 6,000ft in the San Gabriel and
San Bernadino of southern California. The heaviest snowfall will
be mainly confined to elevations at/above 6,000ft in the Sierra
Nevada. By Friday morning, the upper level trough will advance
through the Great Basin and into the Rockies by Friday afternoon.
Periods of snow will move through northern Nevada and into the
northern Rockies with the Tetons, Wind River, and Absaroka ranges
seeing the heaviest snowfall Friday evening. Snow will also pick up
in intensity along the Wasatch, Uinta, and Big Horn ranges Friday
night and into Saturday morning. WPC PWPF shows moderate-high
chances (50-70%) for >4" of snowfall along the Sierra Nevada above
6,000ft, the northern Great Basin above 7,000ft, and the Absaroka,
Tetons, and Wind River ranges above 8,000ft.

Meanwhile, the Pacific Northwest will see periods of snow gradually
diminish throughout the day today with a fairly tranquil day
weather-wise on tap for Friday. By Saturday, the next Pacific
trough approaches with a stronger IVT (>500 kg/m/s, or above the
90th climatological percentile via NAEFS) will direct another plume
of Pacific moisture at the region. Strong synoptic-scale support
will also be present as 200-500mb winds over western Washington top
the 90th climatological percentile. As the warm front arrives
Saturday night, snow levels will rise to as high as 5,500ft in some
cases, but once the cold front pushes through on Sunday, snow
levels will crash and result in precipitation changing over to
heavy snow Sunday afternoon in the Olympics and Cascades. Through
12Z Sunday, the northern Cascades and the Idaho Panhandle above
5,000ft sport high chances (>70%) for snowfall >6" with a low
chance (10-30%) for >12" in the tallest peaks of the northern
Washington Cascades.

...Northern Maine...
Days 1-2...

An inverted trough revolving around the northern flank of a
retrograding storm system in the northwest Atlantic will direct a
stream of low-level moisture back west towards northern New England
late Thursday night into Friday. The air-mass over northern New
England (particularly Maine) will be sufficiently cold and dry
enough for wet-bulb temperatures within the boundary layer to
remain below freezing for at least a few hours. Strong easterly
mid-level flow with air-mass origins over the Atlantic will lead to
warm-air advection (WAA) that causes a >0C nose between 850-750mb
Friday morning. This setup will result in periods of sleet/freezing
rain over northern Maine and possibly into far northern New
Hampshire. WPC PWPF does depict some spotty areas with low chances
(10-30%) for ice accumulations >0.1" in portions of the North Woods
and Central Highlands of northern Maine. One notable location with
those potential ice accumulations includes Baxter State Park. These
icy conditions could result in Minor Impacts according to the
Probabilistic WSSI with low chances (10-30%) through Friday
afternoon. The primary impacts would be icy roads in some areas
that could lead to slick travel conditions.

Mullinax



$$
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FOUS11 KWBC 251905
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 PM EDT Fri Oct 25 2024

Valid 00Z Sat Oct 26 2024 - 00Z Tue Oct 29 2024


...Pacific Northwest...
Days 2-3...

An impressive closed low spinning off the British Columbia coast
will shed a shortwave and accompanying vorticity maxima into the
Pacific Northwest Sunday. This feature will spread height falls and
PVA to drive ascent eastward, with a collocated upper jet streak
also pivoting onshore to enhance lift. The best moisture advection
will occur downstream of this lead trough, but snow levels D2 will
be quite high, 7000-8000 ft, so despite impressive IVT exceeding
500 kg/m/s leading to widespread precipitation, snow will be
confined to only the highest peaks of the Cascades and Olympics.
However, during D3, the parent upper low shifts southward and opens
into a still sharp upper trough, lifting onto the WA/OR coast
Monday. While residual moisture will gradually decay through D3,
sufficient ascent downstream of this trough combined with upslope
ascent will drive continued moderate precipitation, with snow
levels falling to 4500-5000 ft. This will allow snow to accumulate
into lower elevations by the end of the forecast period, but
most snow is still expected only above pass level.

WPC probabilities for more than 6 inches of snow are only above
30% in the highest peaks of the WA Cascades D2, where locally 2
feet of snow or more is probably near the top of Mt Rainier. By D3
snowfall becomes more widespread, with WPC probabilities for more
than inches reaching 30-50% across much of the spine of the
Cascades from WA to OR, and several feet of event-total snow is
likely on the higher mountain tops.

The probability of significant ice across the CONUS is less than
10 percent.

Weiss



$$
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FOUS11 KWBC 260845
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
445 AM EDT Sat Oct 26 2024

Valid 12Z Sat Oct 26 2024 - 12Z Tue Oct 29 2024


...Pacific Northwest...
Days 2-3...

Upper level ridging will give way to a broad upper trough, carved out
by a series of shortwaves moving south of a low moving into British
Columbia. This will support the return of unsettled weather and
falling snow levels in the wake of a strong cold front moving from
the Northwest into the northern Rockies on Sunday. Strong mid-to-
upper level forcing and a shot of deeper moisture are expected to
generate some of the heaviest precipitation across western
Washington and Oregon Saturday night into Sunday morning. However,
with snow levels above 6000 ft in most locations, winter weather
impacts will be limited, initially. Snow levels are expected to
steadily drop through Sunday, dipping to around 4500-5000 ft along
the Washington and Oregon Cascades by Monday morning. Conditions
will remain unsettled as onshore flow persists. WPC probabilities
indicate that most significant snow accumulations will remain above
the major Washington and Oregon passes through Monday Morning.

Unsettled weather will continue across the Northwest through
Monday, with snow levels dipping further as additional shortwaves
move through the base of the broader scale trough as its axis
shift inland. WPC probabilities indicate that potentially
impactful accumulations will become more likely along the Oregon
Cascades, with probabilities for accumulations of 4 inches or more
climbing above 50 percent in some of the passes.

Snow will also be spreading south into the Sierra Nevada and east
into portions portions of the northern and central Rockies Monday
into Tuesday. The highest probabilities for significant
accumulations through Tuesday morning center over north-central
Idaho and southwestern Montana and northwestern Wyoming -- in and
around Yellowstone NP. WPC guidance shows at least low
probabilities for accumulations of 6 inches or more in the higher
elevations of these areas.

The probability of significant ice across the CONUS is less than
10 percent.

Pereira


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FOUS11 KWBC 271922
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 PM EDT Sun Oct 27 2024

Valid 00Z Mon Oct 28 2024 - 00Z Thu Oct 31 2024


...Pacific Northwest to the central Rockies...
Days 1-3...

A much more active pattern is set to develop this week as a large
trough begins to expand across the western CONUS.

A Pacific trough will encroach on the coast D1, with the primary
trough axis pivoting onshore by Tuesday morning. The most
impressive vorticity impulse embedded within this trough is progged
to surge into central CA, which will then help to amplify this
feature as it advects east on Tuesday into the Great Basin and then
Four Corners. Initially, this trough will remain progressive and
positively tilted, but should deepen, and some of the guidance
suggests a negative tilt or even closing off of the upper height
fields late in the forecast period. Although there is, as expected,
some uncertainty into the placement and speed of this evolution,
500mb heights across the West feature minimal spread in the IQR
according to DESI, so confidence is pretty high in the progression.

As this trough surges eastward, it will drive increasingly intense
ascent through height falls and downstream divergence, especially
during the period of best amplification D2, and combine with a
strengthening 250mb jet streak pivoting from CA into the central
Rockies to force impressive deep layer ascent, especially in the
Rockies. At the same time, moisture will become increasingly robust
due to onshore flow of Pacific moisture streaming ahead of the
trough axis, and pushing PWs to +1 to +2 sigma according to NAEFS
ensemble tables. This will result in widespread and expanding
precipitation, first in the WA/OR Cascades, but then becoming more
robust across the central Rockies beginning Tuesday aftn.

The challenge with this event will be how much snow will spread out
of the higher terrain and into the High Plains Tuesday and
Wednesday. While forcing will remain impressive, a cold front
digging southward will begin to cool the column, while also
providing a region of enhanced baroclinicity leading to stronger
frontogenesis. This should result in a surface wave developing near
UT/CO and then lifting E/NE into the Plains by Wednesday. The
combination of fgen, this modestly strengthening surface low, and
the southward push of the cold front should help cause snow levels
to crash from around 8000ft early to 4500-5000 ft late. This,
combined with periods of dynamic cooling, will result in
significant snow accumulations in many areas.

The other challenge with this system will occur D3 as the low
ejects into the Plains and leaves what could be a potent comma
head/deformation axis with precipitation to its NW. There is
considerable model spread in the placement/timing of this feature,
so confidence is lower, but some of the more aggressive guidance
indicates the band will be strong enough to cause dynamical
cooling to support of swath of snow into the Dakotas and maybe even
MN before the end of the forecast period. Despite favorable omega
into a subjectively deep DGZ, the weaker/less snowfall solutions
are preferred based on consensus and ensemble means, but it will be
worth monitoring the next few model runs to see if this stripe of
heavier snow could materialize.

At this time, while uncertainty still exists in both the elevations
at which significant snow can accumulate, and the eastern footprint
of the heavy snow axis, heavy snow is likely in a lot of the higher
elevations. On D1, WPC probabilities for more than 6 inches are as
high as 70-90% in the WA Cascades as a weak low pressure approaches
the coast beneath the primary mid-level trough. By D2, more
expansive precipitation begins in the central Rockies, and WPC
probabilities for more than 6 inches are above 50% in the San
Juans, Uintas, Big Horns, Wind Rivers, and around Yellowstone NP.
By D3 precip begins to wane, but periods of moderate to heavy snow
will persist across the higher terrain of CO and WY, and as far
east as the Black Hills of SD where WPC probabilities are 30-70%
for an additional 6+ inches. Storm total snow of 12-18" is likely
in some of the highest elevations by Wednesday night.


The probability of significant ice across the CONUS is less than
10 percent.

Weiss



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FOUS11 KWBC 230703
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 AM EDT Wed Oct 23 2024

Valid 12Z Wed Oct 23 2024 - 12Z Sat Oct 26 2024

...Northern Rockies...
Day 1...

Mid-level low off the WA/OR coast opens into compact shortwave
and ejects east over the OR Coast this afternoon before weakening
as it reaches western WY late tonight. Lee-side low pressure
develops over northern WY overnight which aids upslope flow back on
the western WY terrain where snow reaches moderate rates overnight.
Snow levels drop from around 8000ft to 7000ft overnight during this
precip, particularly around greater Yellowstone. Day 1 PWPF are
10-40% for >4" over the northern Absarokas in southern MT and in
the Tetons.


The probability of significant ice across the CONUS is less than
10 percent.


Jackson


$$
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FOUS11 KWBC 250730
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EDT Fri Oct 25 2024

Valid 12Z Fri Oct 25 2024 - 12Z Mon Oct 28 2024


...Pacific Northwest...
Day 3...

Upper level ridging will give way to a broad upper trough, carved
out by a series of shortwaves moving south of a low moving into
British Columbia. This will support the return of unsettled weather
and falling snow levels in the wake of a strong cold front moving
from the Northwest into the northern Rockies on Sunday. Strong mid-
to-upper level forcing and a shot of deeper moisture are expected
to generate some of the heaviest precipitation across western
Washington and Oregon Saturday night into Sunday morning. However,
with snow levels above 6000 ft in most locations, winter weather
impacts will be limited. Snow levels are expected to steadily drop
through Sunday, reaching to around 4500 ft in the northern Cascades
by Monday morning. Conditions will remain unsettled as onshore
flow persists. However, WPC probabilities indicate that most
significant snow accumulations will remain above pass level.

The probability of significant ice across the CONUS is less than
10 percent.

Pereira

$$
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FOUS11 KWBC 210820
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Mon Oct 21 2024

Valid 12Z Mon Oct 21 2024 - 12Z Thu Oct 24 2024

...Colorado Rockies...

Day 1...
An upper low centered over central CO early this morning will shift
east into KS by early afternoon. Comma head precip, moderate at
times, on the back side of the low will persist over the north-
central CO Rockies through this morning with snow levels around
9000ft. Probs for an additional >6" snow between 12Z and 18Z are
around 50 percent in portions of the Front Range.


...Glacier National Park...
Day 1...

An upper trough axis reaches the PacNW coast early this morning and
shift ENE through western MT this evening. An inland surge of
Pacific moisture ahead of this trough maintains high snow levels
over the Northwest today, but height falls are sufficient by this
evening to allow snow levels to drop on the Lewis Range through
Glacier NP to drop from around 7000 to 5000ft tonight. PWPF for >6"
is around 30 percent on the higher peaks of the park.


The probability of significant ice across the CONUS is less than
10 percent.

Jackson


$$
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FOUS11 KWBC 220712
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 AM EDT Tue Oct 22 2024

Valid 12Z Tue Oct 22 2024 - 12Z Fri Oct 25 2024

...Greater Yellowstone...
Day 2...

A rather positively tilted trough extending WSW from southern BC
closed off into a mid-level low today west of WA. This low then
opens into a compact trough and ejects east Wednesday with the H5
trough axis crossing greater Yellowstone late Wednesday night. Snow
levels in the moisture ahead of this wave are 8000-9000ft. The
only snow probs for >4" in the WWD forecast period are on Day 2 and
in the 20-40% range over the northern Absarokas in MT and for the
Tetons. There is about a 10% risk for 6" in the Red Lodge portion
toward Beartooth Pass.


The probability of significant ice across the CONUS is less than
10 percent.

Jackson


$$
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FOUS11 KWBC 180830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Fri Oct 18 2024

Valid 12Z Fri Oct 18 2024 - 12Z Mon Oct 21 2024

...Central and Southern Rockies...
Days 1-2...

Potent low closes off over northern AZ this morning where it stalls/occludes/fills through Saturday. This will result in a
prolonged period of intense synoptic ascent focused across the Four
Corners and southern/central Rockies, where height falls,
downstream divergence, and upper level diffluence in response to
modestly coupled jet streaks overlap. At the same time, a surface
cold front will shift east over the Four Corners today and over the
southern Rockies tonight. This will have a two-pronged effect of
enhancing ascent, as a wave of low pressure develops along it,
while impressive baroclinicity results in sharpening fgen to drive
locally more intense lift. Where this most intense ascent occurs,
especially in areas of upslope flow, precipitation rates will be
heavy.

Downstream of this upper low, strong moisture advection leads to
PWs +2 to +3 sigma, additionally supporting widespread heavy
precipitation. Snow levels ahead of the low are quite high,
varying between 9000 to 10000ft, but will fall to around 6500ft
underneath the upper low over AZ tonight. Day 1 PWPF for >8" is
over 50% in the higher southern UT ranges and the Kaibab Plateau
and around Humphreys Peak in northern AZ along with over 80% for

12" additional in the San Juans of CO.

The filling low causes precip rate to drop to light to moderate
with snow levels lingering around 9000ft over the San Juans. Day 2
PWPF for additional >8" is 30-50%.


The probability of significant ice across the CONUS is less than
10 percent.

Jackson


$$
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FOUS11 KWBC 150737
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Tue Oct 15 2024

Valid 12Z Tue Oct 15 2024 - 12Z Fri Oct 18 2024

...Northeast...
Day 1...

Following a brief tapering off of snowfall overnight, periods of
snow will pick back up over the northern Appalachians Tuesday and
linger into Tuesday night. This is due to an elongated upper level
trough pivoting around the western flank of a closed 500mb low
situated over the St. Lawrence Seaway that will traverse the
Northeast from west to east. In addition to the usual upslope flow
into the Adirondacks, Green, and White Mountains, the steep lapse
rates aloft will trigger potent snow showers that could cause brief
bursts of heavy snow at elevations >2,000ft. A few snow showers may
linger around northern New England Wednesday morning, but
accumulations would be minor and limited to the elevations >3,000ft
on Wednesday. WPC probabilities show low chances (10-30%) for
additional snowfall totals >4" along the higher elevations of the
Adirondacks, Green, and White Mountains through Wednesday morning.

...Pacific Northwest...
Days 2-3...

An upper level shortwave trough bringing a surge in 850-700mb
moisture and causing 500-700mb heights falls will cause snow levels
to gradually drop from 6,000ft Tuesday night to around 4,000ft
Wednesday evening over Washington's Olympic and Cascade Ranges. As
this initial 500mb vort max exits to the east Wednesday night, a
second trough will dive south through the region on Thursday with
snow levels bottoming out just below 4,000ft in some cases. Periods
of heavy snow are expected along the Oregon Cascades during the day
on Thursday. Snow should taped off some time Thursday night. WPC
PWPF showed high chances (50-70%) for snowfall totals >4" above
5,000ft in the Cascade Range through Thursday.

...Northern & Central Rockies...
Day 3...

Pacific moisture out ahead of the first upper level shortwave to
traverse the Pacific Northwest is forecast to produce periods of
snow in the higher elevations (>6,000ft in northern ID, >7,000ft in
the Bitterroots, Sawtooth, Absaroka, Tetons, Wind River, and Big
Horn Ranges) early in the day on Thursday, but it is with the
approach of the second trough where snowfall rates look to
increase. Stronger height falls Thursday evening will accompany an
air-mass that NAEFS shows at the 500mb and 700mb layers to be below
the 10th climatological percentile. Plus, NAEFS shows an anomalous
IVT that tops the 90th climatological percentile from the Great
Basin to Utah. The region will also be placed favorably beneath a
coupled jet structure that will help to maximize upper level
divergence from the Absaroka on south to the Wasatch Thursday night
and into Friday morning. Waves of low pressure will develop along
a strengthening frontal boundary that will track through the
Wasatch and into southern WY by Friday morning, prompting
precipitation that starts out as rain to make a quick changeover to
heavy snow.

WPC PWPF sports moderate-to-high chances (50-70%) for >6" of
snowfall in the Absaroka of southwest Montana with some moderate
chances (40-60%) for >12" in some of the Absaroka's tallest peaks.
Farther south, the Bighorns and Wind River Ranges sport low-
moderate chances (30-50%) of seeing >8" of snowfall through 12Z
Friday, while the Uinta how moderate-high chances (50-70%) for the
same snowfall totals through 12Z Friday. This is likely to be the
first winter storm of the season for these mountain ranges this
season and will result in impacts residents have not dealt with in
months (poor visibility, slick/snow covered roads). The WSSI-P does
sport moderate-high chances (50-70%) for Minor Impacts in the
Absaroka, Wind River, Big Horn, and Uinta ranges, as well as some
of the tallest peaks of the Wasatch through Friday morning.


The probability of significant ice across the CONUS is less than
10 percent.


Mullinax




$$
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FOUS11 KWBC 160752
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Wed Oct 16 2024

Valid 12Z Wed Oct 16 2024 - 12Z Sat Oct 19 2024

...Pacific Northwest...
Days 1-2...

An upper level shortwave trough ushering in a surge of 850-700mb
moisture and resulting in upper level height falls will cause snow
levels to gradually drop from 6,000ft early this morning to around
4,000ft by Wednesday evening over Washington's Olympic and Cascade
Ranges. As this initial 500mb vort max exits to the east Wednesday
night, a second trough will dive south through the region on
Thursday with snow levels bottoming out just below 4,000ft in some
cases. The heaviest snowfall is likely to occur along the
Washington Cascades Wednesday evening with moderate-to-heavy
snowfall possible along the tallest peaks of the Oregon Cascades
Thursday morning. WPC probabilities depict high chances (>70%) for

4" of snowfall in the Washington Cascades and peaks above 5,000ft.

The taller volcanic peaks of the Washington Cascades sport
moderate-high chances (50-70%) for snowfall >8" through Thursday.


...Northern & Central Rockies...
Days 2-3...

**First Significant Winter Storm of the Season to Impact the
Central Rockies**

The evolution of the first significant winter storm of the season
across portions of the Intermountain West begins as an upper level
trough enters the Pacific Northwest on Thursday. As Thursday
progresses, the upper trough will deepen as it tracks south through
the Great Basin Thursday night, then becoming a closed low along
the UT/AZ border by midday Friday. The low continues to slowly
track across northern Arizona Friday night but remain stalled over
the Four Corners region early Saturday morning.

NAEFS shows upper level evolution that is quite anomalous from a
variety of parameters. At 250mb, a jet streak over the northern
High Plains will be coupled with a jet streak at the base of the
amplifying upper trough, thus maximizing upper level divergence
over the Intermountain West on Thursday. As the upper trough cuts
off into an upper low by Friday, strong divergence beneath the left
exit region of the 250mb jet streak will be placed over Utah and
the Central Rockies. NAEFS shows 500-700mb heights and temperatures
that are routinely below the 10th climatological percentile,
particularly over the Great Basin and Utah late Thursday into
Friday. Perhaps most impressive are the IVTs which will eclipse
300 kg/m/s (above the 99th climatological percentile) in southern
UT Thursday night. Mean flow out of the SW will result in strong
upslope ascent into the Uinta and San Juans in particular.

In the Absaroka, Big Horns, and Wind River Ranges, WPC PWPF shows
high chances (>70%) for storm event snowfall totals >8" with at
least moderate chances (>50%) for snowfall >12" in the tallest
peaks. Farther south, the Uinta and San Juan sport high chances

70%) for snowfall totals surpassing 12". In fact, the Uinta sport

moderate chances (40-60%) for >18" of snowfall through Saturday
morning, with high chances (>70%) in the San Juan for >18" of
snow. The WSSI-P Moderate probabilities are keying in on the San
Juans as seeing the most impactful snow from this event, primarily
due to a combination of the algorithm's focus on snow totals,
snowfall rates, and snow load. Probabilities are showing moderate
chances (40-60%) for snowfall that would result in hazardous
driving conditions and potential closures and disruptions to
infrastructure in the San Juans, Wasatch, and Uinta. While Moderate
Impacts are deemed lower chance (10-30%) in the Absaroka, Wind
River, and Big Horn Ranges, potential impacts are elevated compared
to what the algorithm suggests given it is the first significant
winter storm of the season in these Ranges above 7,000ft. There are moderate-high chances (50-70%) for Minor Impacts in these Ranges,
as well as far south as parts of the Sangre De Cristo and far
northern mountains of Arizona.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax



$$
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FOUS11 KWBC 140709
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 AM EDT Mon Oct 14 2024

Valid 12Z Mon Oct 14 2024 - 12Z Thu Oct 17 2024

...Northeast...
Days 1-2...

An anomalous upper level trough over the Northeast will lead to a
strengthening cyclone over northern New England this morning. The
850mb low positioned over Upstate New York will track north and
east through the Green and White mountains by this afternoon. To
the north and west of the 850mb low, a surge in both moisture and
NW winds will result in a stronger upslope component into the
Adirondacks, Green, and White mountains throughout the day. In
addition to the sufficient upper level ascent, an ari-mass that
is quite cold by mid-October's climatology will support sub-
freezing boundary layer temps for in the northern Appalachians. Locally
heavy snowfall rates are expected with 0.5-1"/hr rates possible in
the tallest peaks of the Adirondacks this morning and afternoon
according to the 00Z HREF on WPC's Snowband Probability Tracker. As
the storm tracks into Quebec this evening, snow will taper off and
be limited to just snow showers through Tuesday morning. Another
round of snow is likely to occur Tuesday late morning and through
the afternoon as a trough axis pivoting on the back side of the
storm system in eastern Quebec brings about an additional surge of
low- mid level moisture. In addition, steep lapse rates could
trigger potent snow showers, especially in the higher terrain of
northern New England above 2,000ft. Snow showers look to linger in
the northern Appalachians until Wednesday morning when the trough
axis moves east and off the New England coast.

Any locally heavy snowfall totals (>4") would likely be confined
to elevations above 2,000ft in the Adirondacks, Green, and White
mountains. WPC probabilistic guidance does depict some low-
moderate chances (20-40%) for >6" of snowfall in the tallest

3,000ft peaks of the Adirondacks. Below 2,000ft, snowfall amounts

will struggle to top 2" in the 1,000-2,000ft elevation layer, with
mainly a coating to 1" in the elevations below 1,000ft. Given this
snowfall is the first of the season for parts of the region,
motorists will be subjected to poor visibility and some snow
covered roads for the first time in months. The WSSI-P does show
low-moderate chances (20-40%) for Minor Impacts in parts of the
Adirondacks through Monday evening.

...Northwest...
Days 2-3...

A 500mb shortwave trough bringing with it a surge in 850-700mb
moisture and associated height falls will cause snow levels to
gradually drop to as low as 4,000ft by Wednesday. The air-mass is
not particularly cold for the time of year, so this setup favors
snowfall accumulation in the Olympics and Cascades to elevations

5,000ft. WPC probabilities do suggest low-moderate chances

(20-40%) for >4" of snowfall on Wednesday along the windward side
of the Cascade Range at elevations >4,000ft. As 500mb heights and
snow levels continue to fall, some of the higher level Washington
passes may start to see accumulating snow by Wednesday night. The
potential for accumulating snowfall reaches the Northern Rockies
(the Bitterroots, Absaroka, and Tetons most notably) by early
Thursday morning. WPC probabilistic guidance suggests the Lewis
Range of western MT could see locally higher totals surpassing 4"
at elevations >7,000ft, while the other mountain ranges mentioned
are most likely to see their best accumulating snowfall above
8,000ft on Thursday.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax


$$
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FOUS11 KWBC 150811
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
311 AM EST Fri Nov 15 2024

Valid 12Z Fri Nov 15 2024 - 12Z Mon Nov 18 2024

...Western U.S....
Days 1-3...

This morning, an upper level trough will advance through the Great
Basin and into the Rockies by this evening. Periods of snow
along the Sierra Nevada and northern Great Basin this morning will
move into the northern Rockies tonight with the Tetons, Wind
River, and Absaroka ranges seeing the heaviest snowfall. Snow will
also pick up in intensity along the Wasatch, Uinta, Big Horn,
Little Belt, and Big Snowy ranges tonight and into Saturday
morning. WPC PWPF shows moderate chances (40-60%) for >6" of
snowfall along the Sierra Nevada above 9,000ft. High probabilities

70%) for snowfall >8" are forecast along the ridgelines of the

northern Great Basin above 6,000ft and the Absaroka, Tetons, and
Little Belt, and Big Snowy ranges above 7,000ft.

Elsewhere, the Pacific Northwest will witness a fairly tranquil
day weather-wise today. This changes by Saturday as the next
Pacific trough approaches with a stronger IVT (>500 kg/m/s, or
above the 90th climatological percentile via NAEFS) will direct
another plume of Pacific moisture at the region. Strong synoptic-
scale support will also be present as 200-500mb winds over western
Washington top the 90th climatological percentile. As the warm
front arrives Saturday night, snow levels will rise to as high as
5,500ft, but once the cold front pushes through on Sunday, snow
levels will crash and result in precipitation changing over to
heavy snow Sunday afternoon in the Olympics and Cascades. Heavy
snow will continue to push inland through the interior Northwest
Sunday night while persistent upslope flow leads to a prolonged
stretch of accumulating snowfall above 3,000ft in the Cascades.

Through this weekend, the northern Cascades and the Idaho
Panhandle above 5,000ft sport high chances (>70%) for snowfall >12"
with low-moderate chances (30-50%) for >18" in the tallest peaks.
In the Oregon Cascades and Blue Mountains, WPC PWPF shows
moderate-high chances for >12" of snowfall. WSSI-P depicts
moderate-high chances (50-70%) for Major Impacts in the WA/OR
Cascades above 4,000ft, which includes some notable passes such as
Snoqualmie Pass.

...Northern Maine...
Day 1...

Some light ice accumulations are expected across portions of
northern Maine this morning. Warm air at mid levels overrunning
sub-freezing low level temperatures will support periods of
freezing rain/drizzle, especially from the St. John Valley on south
through the Southern Aroostook. The primary impacts would be icy
roads in some areas that could lead to slick travel conditions. WPC
PWPF shows low-moderate chances for ice accumulations >0.01"
through this afternoon.


Mullinax


$$
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FOUS11 KWBC 160820
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Sat Nov 16 2024

Valid 12Z Sat Nov 16 2024 - 12Z Tue Nov 19 2024

...Western U.S....
Days 1-3...

As one storm system originally over the Intermountain West last
night departs (leaving some light-moderate mountain snow over parts
of the central Rockies in its wake this morning), attention shifts
to the next Pacific storm system that is set to bring an onslaught
of heavy snow to the Pacific Northwest and Northern Rockies this
weekend and into early next week. Precipitation will arrive
Saturday morning as a warm front approaches from the West. Snow
levels start out around 3,000ft Saturday morning, but strong warm
air advection associated with the warm front will cause snow levels
to rise above 7,000ft in the OR Cascades, while they are more
focused around 5,000ft in the northern WA Cascades Saturday night.
This same moisture plume, which is associated with a >500 kg/m/s
IVT that also happens to be above the 90th climatological
percentile according to NAEFS, will advect rich low-mid level
moisture well inland into the northern Rockies Saturday night. By
Sunday, while moisture advection into the Cascades will not be as
strong, a cold front passing through on Sunday will cause snow
levels to plummet to as low as 2,000ft in the WA Cascades and
2,500ft in the Oregon Cascades by Sunday evening.

These falling snow levels will also occur inland across the
Northern Rockies Sunday night and into Monday. NAEFS by 00Z Monday
shows 500mb heights that are below the 10th climatological
percentile throughout the northwestern U.S.. By Monday afternoon,
700mb temperatures are unusually cold across the Pacific NW with
temps as cold as the 2.5 climatological percentile. This unusually
cold air-mass is in place at the same time as steady onshore flow
perpendicular to the Cascades and northern Rockies continues to
support moderate-heavy snow from the Olympics, Cascades, and
northern Rockies to as far south as the Salmon Mountains of
northern California. Heavy snow not only looks to continue through
Monday night in the Cascades and Olympics, but a much stronger
Pacific storm system arrives on Tuesday with more heavy snow and
much stronger winds.

Through 12Z Tuesday, WPC PWPF shows which chances (>70%) for
snowfall >18" in the Oregon Cascades, the northern Bitterroots of
the Idaho Panhandle, and the tallest peaks of the Lewis Range. The
northern Washington Cascades show high chances (>70%) for over 2
feet of snowfall with some of the tallest peaks above 7,000ft
topping 3 feet. Impacts will be the worst in these aforementioned
areas with Moderate to Major Impacts shown on the WSSI, which
includes some of the notable passes in the OR/WA Cascades.
Elsewhere, WPC PWPF shows high chances (>70%) for >12" of snowfall
in the Blue, Boise, and Sawtooth Mountains. Similar high chance
probabilities exist in the Tetons for >8" of snowfall. The WSSI
shows Minor to localized Moderate Impacts in these mountain ranges.

...Eastern MT & Western ND...
Day 1...

Over the past 24 hours, guidance shows the upper level trough
tracking through southern Montana tonight to be more potent and
compact than originally shown. The region remains beneath the
divergent left-exit region of a 250mb jet streak located over the
Four Corners region. As the 700mb low tracks through eastern
Montana this morning, a ribbon of anomalous 700mb moisture aloft
will wrap around the northern and western flank of the low while
strong vertical velocities aloft and dynamic cooling allow for
heavy snow to ensue over northeast Montana. Latest CAMs show
excellent upward vertical velocities within a highly saturated DGZ
around 600-550mb. The 00Z HREF Snowband Probability Tracker (SPT)
does show a swath of snowfall rates that could hover around 2"/hr,
if not higher in some cases. Residents in these areas can expect
rapidly accumulating snowfall this morning with US Routes 2 and 85
to the north and west of Williston likely to feature treacherous
travel conditions. Slick roads and significantly reduced
visibilities are expected where the heaviest snowfall occurs. By
this afternoon, the storm system will be over eastern North Dakota
and tracking to the northeast. Periods of snow will continue across
western North Dakota before finally concluding Saturday evening.


The probability of significant ice across the CONUS is less than
10 percent.

Mullinax

$$
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FOUS11 KWBC 170812
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 AM EST Sun Nov 17 2024

Valid 12Z Sun Nov 17 2024 - 12Z Wed Nov 20 2024

...Western U.S....
Days 1-3...

A developing active pattern is in its early stages as the first in
a series of Pacific storm systems revolves beneath the base of a
longwave trough that will become quasi-stationary in the Gulf of
Alaska to start the period. Then, as a strong ridge builds over the
Aleutians mid-week, the longwave trough will dive south into the
northeast Pacific and direct a steady barrage of rich Pacific
moisture at the region into the second half of the week. Tonight,
the warm front associated with the initial surge in Pacific
moisture is working across the interior Northwest. Meanwhile, a
cold front will push across western WA/OR Sunday morning that will
cause snow levels to plummet to as low as 2,000ft in the WA
Cascades and 2,500ft in the OR Cascades by this afternoon. By
Sunday evening, some locations on the windward side of the Cascades
could see snow levels as low as 1,000ft. It is also worth noting
that there are likely to be some instances of freezing rain today
with low-moderate probabilities for >0.01" of freezing rain in
parts of eastern OR and the northern ID Panhandle.

The falling snow levels will occur well inland across the Northern
Rockies Sunday night and into Monday. NAEFS shows 500-700mb temps
and heights that are below the 10th percentile throughout the
Northwest on Monday. This unusually cold air-mass is in place at
the same time as persistent onshore flow, courtesy of the longwave
trough to the north, strikes mountain ranges such as the Cascades
at an orthogonal angle to maximize the snowfall potential through
Monday. Following a brief lull Monday night, a rapidly
strengthening storm system (both the GFS and ECMWF have the low sub
950mb by Tuesday afternoon!) will direct a stronger atmospheric
river at the Pacific Northwest with not only exceptional moisture
advection (>750 kg/m/s IVT on NAEFS would be above the 97.5
climatological percentile Tuesday afternoon), but intense winds
along the mountainous terrain. Robust warm air-advection late
Tuesday and into Tuesday night will lead to snow levels rising to
above 3,000ft in the WA Cascades and Olympics, around 4,000ft in
the OR Cascades, and 6,000ft in the northern California ranges by
Wednesday morning. The concern would be for the potential for a
heavier/wet snow type to rapidly accumulate, while powerful wind
gusts potential lead to tree damage and power outages Tuesday
night and into Wednesday.

All told, there is no shortage of snow in the forecast for the
Pacific Northwest and Northern Rockies. Snow is likely to be
measured in feet along not just the Cascades above 3,000ft, but the
Olympics, the Salmon/Shasta/Siskiyou of northern California, and
well inland through the Blue, Sawtooth, Boise, Bitterroot, Tetons,
and Lewis Mountain Ranges. The latest WSSI shows Moderate to Major
Impacts in the Cascades and Bitterroots with impacts including not
just dangerous travel conditions (including notable Cascade
passes), but the weight of the snow and strong winds by Tuesday
night could result in tree damage and power outages in some areas.

...Northern Plains...
Day 3...

Confidence is increasing in a powerful late November cyclone to
track from the Southern Plains on Monday and race north into the
Midwest Monday night. Both NAEFS and ECMWF SATs support a highly
anomalous area of low pressure Monday night, but the ECMWF is
farther west and closer to the Missouri River than the NAEFS, which
is located farther east over central Iowa. By Tuesday morning, the
NAEFS (a GFS/CMC ensemble based tool) shows the storm about to
cross the Mississippi River in southeast MN while the ECMWF is over
southwest MN. The primary reason for this is the ECMWF shows a
slightly slower and stronger 500mb solution than the GFS, albeit
they are not overly different in the storm evolution.

Where they differ is that the ECMWF (RGEM as well) phases sooner
with an upper level disturbance over the northern High Plains and
is pulled farther west than the GFS. By 21Z Tuesday, both the ECMWF
and RGEM show a closed 500mb low near Bismarck, while the GFS is
more elongated and does not close off fully until a few hours later
near the ND/Canada border. These details are critical in
determining where the deformation zone (or heaviest snow bands) set
up; over the Dakotas or more over south-central Canada. For the
moment, WPC PWPF shows moderate-high probabilities (50-70%) for

4" of snowfall over north-central ND. It is worth noting that the

WPC PWPF does show low chances (10-30%) for snowfall >8", and near
the North Dakota/Canada border, a small 10% chance area for >12" of
snowfall. Residents in the Dakotas should monitor the forecast
closely over the next 24-48 hours.

While exact amounts are unclear, the exceptionally strong storm
system over Minnesota coupled with high pressure building in over
the Northern Rockies will make for a very windy setup over the
Northern Plains. Snow is still expected to fall across much of
central and western North Dakota and wind gusts of 30-40 mph would
result in whiteout conditions for areas that, in some cases, would
experience their first round of accumulating snowfall this season.
The Probabilistic WSSI (WSSI-P) shows moderate-high chances
(50-70%) for Minor Impacts across much of central North Dakota with
the highest probabilities focused in north-central North Dakota.
The WSSI-P shows Snow Amount and Blowing Snow to be the primary
impacts in these areas. In summary, while exact amounts remain low
confidence at this range, the combination of some accumulating snow
and hazardous visibility reductions due to blowing snow are still
likely to cause some treacherous travel conditions in these areas
late Tuesday and into Wednesday morning.


The probability of significant ice across the CONUS is less than 10
percent.

Mullinax
$$
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FOUS11 KWBC 220831
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Fri Nov 22 2024

Valid 12Z Fri Nov 22 2024 - 12Z Mon Nov 25 2024

...Pacific Coast through the Interior Northwest and Great Basin... Days 1-3...

Persistent closed low centered off the Pacific Northwest coast will
remain energized by vorticity maxima swinging through the
associated trough this period. This will result in amplification of
the gyre, noted by an expansive region of 500-700mb height
anomalies reaching below -1 sigma, and approaching -3 sigma in the
core of the trough. Beneath this trough, deepening surface low
pressure will approach the WA coast today, but likely get pulled
back to the west on Saturday before landfall in response to
secondary shortwave energy digging to its south. This will result
in persistent onshore and divergent mid-level flow, accompanying
upper level jet energy to spread moisture onshore as a continuation
of the ongoing atmospheric river (AR) across northern CA through
early Saturday. While IVT values exceeding 500 kg/m/s wane and
pivot southward by the end of D1, this will still result in
plentiful moisture and widespread moderate to at times heavy
precipitation pushing onshore the Pacific Coast into the interior Northwest.

Snow levels within this AR will be quite high, generally 6000-8000
ft, except east of the WA Cascades on D1 where they will remain
entrenched around 4000 ft before rising. This indicates that most
of the heavy snow will be above pass levels, except in the higher
Cascade Passes D1, and the Sierra Passes D1-D2. WPC probabilities
D1 are high for more than 6 inches of snow in the northern WA
Cascades, the Salmon River Range, and portions of the Northern
Rockies near Glacier NP. On D2, snowfall is more robust and
expansive, with a greater than 90% chance of 6 inches again in the
Northern Rockies and Salmon River/Sawtooth Ranges. More
impressively, the Sierra will be favorably located to be impinged
upon by moisture by the end of D1, leading to pronounced upslope
snow. WPC probabilities are above 80% for 12+ inches through
Saturday, with 2 feet possible above 8000 ft.

By D2 onward, snow levels crash again as a cold front pushes
onshore, but this is accompanied by reduced moisture advection as
the column begins to dry out. Some moderate snow will persist
through onshore flow, especially in the Sierra where an additional
6-12 inches is possible, but otherwise WPC probabilities for more
than 6 inches are between 30-50% and focused across the Wasatch,
Uintas, and ranges of NW WY.

...Appalachians and Northeast... Days 1-2...

Impressive upper low over the Northeast will feature a complex
evolution through the weekend resulting in a couple surface lows and
varying areas of heavy snow.

The Central Appalachians, especially from the Laurel Highlands
southward across WV and as far south as the Great Smokey Mtns of
TN/NC, will benefit from prolonged upslope flow on the western
periphery of the upper low resulting in waves of heavy snowfall
through Saturday. WPC's snowband probability tracker depicts an
extended period of 1-2"/hr snowfall rates during the D1 period
across the Allegheny Mtns. WPC probabilities in this area are high

70%) on D1 for more than 8 inches of snow. Event total snowfall

could peak around 2 feet in central WV.

The ongoing potent and compact surface low churning near Long
Island is forecast to loop westward and eventually toward the south
today over eastern PA as it remains embedded within the large
500mb gyre overhead. As this occurs, an impressive plume of low-
level moisture will continue to wrap northwest into the system,
driving rich theta-e advection into a TROWAL to support some
elevated instability and enhance available moisture for heavy
precipitation early on D1. Increasing deformation is noted on
current radar trends on the W/SW side of the system as it becomes
exceptionally wrapped. This setup will support intense mesoscale
ascent and bands of heavy precipitation rotating W/SW as the low
retrogrades. Although the column will be marginally supportive of
accumulating snow outside of higher elevations (above about 1500
ft), the intense lift should dynamically cool the column and allow
efficient snowfall accumulation at times even in lower terrain.
SLRs for this event will likely be below climo due to near freezing
temps and warm soils/compaction, but still, rates of 1-2"/hr will
promote heavy accumulations, especially in the Poconos and
Catskills and surrounding areas early this morning. Most of the
ongoing snowfall is expected to occur before or right around the
start of the D1 period (12z Fri), so WPC probabilities don't
reflect the ongoing event. Nonetheless, the greatest snowfall
amounts are expected in northeast PA and neighboring areas of New
York and northern NJ, with 2-day snowfall potentially reaching
above 12" in a few highly elevated spots. This will likely result
in dangerous travel and impacts to infrastructure including
scattered power outages.

In the lower elevations of the Mid-Atlantic, some snow bands could
pivot as far SE as the I-95 corridor, so while accumulations in
the major urban areas from Washington, D.C. to New York City are
expected to be minimal, if any, these cities could see their first
"falling" snow of the season Friday morning/aftn.

Along the Adirondacks, Green, and White Mountains, the
upper low will draw moist easterly flow northward and into these
ranges Friday night through Saturday while another surface low
develops east of Maine and pivots northwest. Snow levels should be
high enough that only the highest terrain will receive significant
snow, but WPC probabilities have increased and now feature a 30-50%
chance on D2 of more than 4 inches, primarily in the White
Mountains of NH and the peaks of northern ME.

...Northern Rockies through the Northern Plains... Days 1-2...

A shortwave ejecting through the base of an anomalous trough off
the Pacific Coast will swing eastward into the Northwest Saturday
and then begin to amplify into a potent but fast moving impulse
across the Northern Rockies and into the Northern Plains Sunday.
This deepening system will drive downstream ascent through height
falls, PVA, and divergence. At the same time, a zonally oriented
jet streak will pivot south of the region, placing its favorable
LFQ atop the far northern High Plains, interacting with increasing baroclinicity as a front wavers near the international border. The
overlap of synoptic lift with the enhanced mesoscale ascent
through fgen will result in weak cyclogenesis, leading to an
expanding area of precipitation falling as snow. The column will be
plenty cold for wintry precipitation thanks to high pressure from
Canada extending southward, and as the WAA along the warm front
lifts north, it will result in an exceptionally deep DGZ to support
fluffy aggregate dendrites which should accumulate efficiently. The
biggest question remains whether or not this favorable environment
and most impactful snowfall will span south over the U.S.-Canada border.

The heaviest snowfall is likely in the terrain near the Northern
Rockies where upslope flow will enhance ascent, and WPC
probabilities support a high risk (>70%) for at least 8 inches near
Glacier NP on D2. However, heavy snowfall is also possible farther
east across northern MT along the international border D1-D2 where
the most robust synoptic ascent will materialize. A leading surge
of modest upper diffluence and WAA may lead to a burst of snow
today across northwest and northern MT. WPC probabilities for at
least 2" on D1 are high throughout northern MT. For the main
activity related to the approaching shortwave on D2, the guidance
has continued to trend a bit farther north from central MT eastward
to ND, with current WPC probabilities suggest around a 30% chance
for more than 4 inches right along the international border as far
east as north- central MT on D2, with higher probabilities for
heavy snowfall located in southern Alberta and Saskatchewan.

The probability of significant ice across the CONUS is less than
10 percent.

Snell

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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FOUS11 KWBC 230755
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
255 AM EST Sat Nov 23 2024

Valid 12Z Sat Nov 23 2024 - 12Z Tue Nov 26 2024

...Pacific Coast through the Intermountain West and Rockies... Days 1-3...

Strong upper low off the Pacific Northwest finally begins to
unravel while ejecting shortwaves inland across the West, with
persistent onshore flow and lowering snow levels leading to
widespread heavy mountain snowfall.

The first shortwave ejecting through the base of an anomalous
trough off the Pacific Coast will swing eastward into the Northwest
Saturday and then begin to amplify into a potent but fast-moving
impulse across the Northern Rockies and into the Northern Plains
Sunday. This deepening system will drive downstream ascent through
height falls, PVA, and divergence. At the same time, a zonally-
oriented jet streak will pivot south of the region, placing its
favorable LFQ atop the far northern High Plains and southern Canada, interacting with increasing baroclinicity as a front wavers near
the international border. The overlap of synoptic lift with the
enhanced mesoscale ascent through fgen will result in weak
cyclogenesis, leading to an expanding area of precipitation falling
as snow. The column will be plenty cold for wintry precipitation
thanks to high pressure from Canada extending southward. As the WAA
along the warm front lifts north, an exceptionally deep DGZ will
support fluffy aggregate dendrites which should accumulate
efficiently, but the most favorable environment and most impactful
snowfall will likely be just north of the U.S.-Canada border. The
greatest impacts over the U.S. from this system appear to occur
over the northern Rockies on D1, where upslope flow will enhance
ascent. WPC probabilities for at least 8 inches of snow are high
(<70%) across northwest MT and Glacier NP, as well as the Salmon
River and Clearwater Mts of Idaho.

The Pacific mountain ranges spanning from the Cascades to the
Sierra Nevada will see a few separate shortwaves move inland
before the parent upper low finally pushes eastward on D3. The
ongoing AR over central CA will come to an end by the start of D1
and allow for snow levels to fall back to around 4000-5000 ft
across the Sierra and lower (2500-3000 feet) across the Cascades.
However, IVT values will also become very weak along the West Coast
until a resurgence across the Sierra on D3 in response to the
parent upper lows eastward movement. A few locally enhanced axes
of moisture are also possible across the Pac NW D2-D3 due to the
lingering low off the coast. This results in high probabilities

70%) for at least a foot of total snowfall over the next three

days across the Sierra Nevada, Shasta Siskiyous of northern
California, Oregon Cascades, and Olympic Mts. Areas above 5000 feet
can expect the potential for up to two feet of snow.

Leftover moisture spilling across the Intermountain West and
central Rockies could combine with favorable west-east upslope
enhancement over the Wasatch Mts of Utah and Colorado ranges on D2.
WPC probabilities for at least 8 inches of snow are low (10-30%).

...Northern New England... Days 1-2...

Stacked/wrapped upper and sfc low over the Northeast will complete its counterclockwise loop this morning before a separate sfc low
develops and lifts north through Atlantic Canada along with the
larger upper low this weekend. Upslope snow into the central
Appalachians will persist through this evening via NW flow as
moisture has circulated from the Atlantic up and around
southeastern Canada back across the lower Great Lakes. A few
additional inches of snow are possible in elevations above 3000 ft
in West Virginia, with marginal surface temperatures limiting the
potential for heavier amounts.

Into New England, trailing vorticity lobe will aid this new coastal
sfc low east of Cape Cod that will track northward into Nova
Scotia by tonight, favoring increased northerly/cooler flow over
interior portions of Maine southwestward into VT. The column is
fairly mild to start over New England via SE flow ahead of the
initial front, but that will change on D1 with rain changing to
snow as the low approaches and deepens. QPF will be focused closer
to the coast, but light amounts of snow are likely for the interior
and especially the higher terrain above 2500ft or so (White
Mountains and northern ME) where WPC probabilities of at least 4
inches are >40%.

Additionally, lake effect snow showers are possible downwind of
Lake Ontario D1-D2 due to this persistent west-northwest flow
around the large low pressure system to the east. Warm boundary
layer temperatures will limit snowfall accumulations as upper
ridging quickly builds from the west. In, fact most showers may
simply fall as rain outside the higher elevations of Upstate New
York or within some of the locally heavier bands where dynamic
cooling can lower surface temperatures to near freezing.

...Upper Great Lakes... Day 3...

The fast-moving shortwave exiting the northern Plains on Sunday is
forecast to reach the Upper Great Lakes on Monday while also
taking on a negative tilt in response to a piece of upper energy
quickly rounding the southern end of the trough over the Midwest.
Surface low is expected to then deepen over the Great Lakes by the
end of D3 with upper diffluence and lake enhancement off Lake
Superior supporting periods of light to moderate snow over the
U.P. of Michigan. WPC probabilities for at least 4 inches of snow
are low (10-30%) for most of this region on D3, with medium
chances (40-60%) over the Porcupine Mts.

Snell
$$
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FOUS11 KWBC 240835
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Sun Nov 24 2024

Valid 12Z Sun Nov 24 2024 - 12Z Wed Nov 27 2024

...Northern New England...
Days 1-2...

Upper low moving over Atlantic Canada is expected to stall today
and absorb approaching upper shortwave diving southeastward over
Ontario. These features combined with strong northwest flow will
support upslope snow over the higher terrain of far northern New
England (mainly the White Mts through northern Maine). WPC
probabilities of an additional 4 inches of additional snow are
medium (40-70%) over these areas. Light snow is also favored over
eastern NY (Adirondacks into the Catskills) but amounts should be
under 4 inches.

...Upper Great Lakes...
Days 1-3

Tonight, energy from two separate disturbances, one an upper level
low over the Canadian prairies, and the other an ejecting
shortwave out of central Great Basin, will merge over a strong
surface cold front diving southward over the central Plains. An
area of low pressure will then form and progress northeastward from
the Midwest to the Great Lakes on D2. The combination of upper
level energy and upper diffluence beneath the favorable left- exit
region of a quick moving 250 mb 105 kt jet streak will aid in a
blossoming precipitation shield from northern MN to northern WI and
the U.P. of Michigan into Monday morning. As the low tracks
northeast across the L.P., much colder air wrapping around the back
side of the low will allow for additional widespread lake-effect
snow and snow showers persisting into D3 for much of the U.P. WPC
probabilities for at least 4" of snow are medium (30-60%) across
the Arrowhead of MN, far northern WI, and the U.P. of Michigan.

As this low and precipitation moves eastward over the Northeast
and New England on D3, light freezing rain is possible over
interior New England as low- level cold air remains locked in
place as mid- level temperatures rapidly warm above freezing along
with the precipitation shield. Surface temperatures should also
eventually rise above freezing given the strong low forecast to
track north of the region into southern Quebec. Only the northern
White Mts and far northern Maine could see temperatures remain
around the freezing mark and the entire column potentially
remaining cold enough for light snow.

...Sierra, Wasatch, CO Rockies...
Days 1-3...

Ejecting upper shortwave and surface cold front crossing the
Intermountain West and central Rockies today will include residual
moisture from the atmospheric river impacting northern California
over the last few days and will have the potential to produce a
burst of snow over the region. Snow levels starting around
5000-6000ft will fall below 4500-5000ft by the end of D1 and
behind the cold front, but also as the column gradually dries out.
WPC probabilities for at least 6 inches of snow are mostly low
(10-30%) for the Wasatch Mts and CO Rockies, with higher chances
above 9000ft.

Troughing off the West Coast will continue during the short range
period, with a rather steady stream of moisture into the region.
Generally lighter snow is forecast for D1 above 4000ft or so
over northern CA and the Cascades WPC probabilities for at least 6
inches of snow are medium (40-70%) for these ranges and high

70%) above 6000ft. By D2, the next heavy snow event enters the

West as another surge of moisture will aim toward the Sierra with
PW and IVT values climbing above the 95th percentile per the NAEFS
ESAT. With a favorable WSW flow into the central/southern Sierra,
moderate to very heavy snow is likely for areas south of Lake Tahoe
into D3 as PW/IVT anomalies peak above the 99th percentile. Snow
levels will rise well above 7000ft to 9000ft (north to south) but
those high elevations may see several feet of snow over the two-day
period. WPC probabilities for at least 24 inches of snow are
highest above 8000ft with the highest peaks possibly receiving
closer to four feet.

Farther east, 120-140kt upper jet will stream across the Great
Basin with the axis of moisture focused across southern NV/UT. This
will set up favorable westerly flow into the Wasatch and then the
CO Rockies with moderate-to-heavy snow accumulations on D3 as snow
levels increase (7500-9000ft). The extended duration of elevated
moisture and favorable westerly flow could lead to higher
elevations receiving multiple feet of snowfall by the end of the
event on Thursday. Currently, WPC probabilities for at least 12
inches are >50% above 9000ft in the CO Rockies and Wasatch MTs
through Wednesday morning.


Snell
$$
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FOUS11 KWBC 250743
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
243 AM EST Mon Nov 25 2024

Valid 12Z Mon Nov 25 2024 - 12Z Thu Nov 28 2024

...Upper Great Lakes to New England...
Days 1-2...

Negatively tilted trough over the Upper Midwest along with a closed
low churning over south-central Canada will slide east today and
work to provide ample lift over the Upper Great Lakes during the D1
period. Additionally, broad upper diffluence within the left exit
region of a zonally oriented upper jet into the central Plains will
help create the widespread precipitation shield from northern MN
and WI into the U.P. of MI. Meanwhile, a sure low is expected to
deepen as it crosses the Great Lakes and enters southern Ontario on
Tuesday morning with the added forcing to the north of the low. Periods
of heavy snow with rates up to around 1"/hr are possible associated
from this low pressure system between northern WI to the U.P. from
late this afternoon through the evening hours per the 00z HREF
SPT. Lake -enhanced snowbands are also likely as cold west-
northwest flow follows in the wake of the low before precipitation
turns strictly lake-effect on D2. WPC probabilities are high (>70%)
across the U.P. of Michigan and medium (40-60%) across the MN
Arrowhead.

Into New England, which will be dealing with WAA and the passage of
an occluded front as the low moves into Canada Tuesday, will see some
of the colder valleys remain below freezing for a period of time
and allow for light icing. This is most likely to occur near the
White MTs of northern New England, including parts of VT, NH, and
ME. PWPF for ice >0.10 inches are about 20-40%.


...Pacific Northwest...
Day 1...

Upper low off the coast will finally move inland over SW OR and
weaken on Tuesday, with onshore flow ahead of it for D1 over the
Cascades where several inches of snow are likely. Snow levels are
around 3000-4000ft from north to south (WA to OR) and will fall by
D2 but as QPF wanes and finally moves away from the area. WPC
probabilities are most notable across the Cascades and Blue Mts in
OR, as well as the northern CA ranges.


...Sierra Nevada...
Days 1-2...

Persistent WSW flow will continue for the next few days as waves
of anomalous moisture flow across the central and especially
southern Sierra, bringing significant heavy, wet snow to higher
elevations. Precipitation is beginning to move onshore this morning
and quickly become heavier today before peaking early Tuesday as
PW/IVT approach record levels for this time of year per the NAEFS
ESAT and CFSR period. 700mb 0C line will lie W-E from central CA
eastward to near UT/AZ border, suggesting high snow levels and
generally a heavy/wet snow for many areas (and likely lower SLR
values). NBM shows snow levels around 7500ft (north) to perhaps
10,000ft (south) coinciding with the heaviest QPF. With >2-3"
liquid, even at <10:1 ratios, multiple feet of snow are likely in
the highest elevations along with snowfall rates up to 2-3"/hr.
Snow levels will drop as QPF starts to decrease towards the end of
D2 once the shortwave to the north moves out of the Great Basin.
For the two- day period, WPC probabilities for at least 18 inches
of snow are high (>70%) above ~8000ft.


...Wasatch, CO Rockies...
Days 1-3...

Several rounds of forcing in the upper levels and a potent 140 kt
upper jet initially over UT and CO will impact the region through
Wednesday and produce widespread heavy mountain snowfall. This
strong upper flow will also take advantage of ample amounts of
atmospheric moisture advecting into CA from continued onshore flow
from the Pacific. IVT values are expected to reach all-time highs
from southern UT/northern AZ to the Four Corners per NAEFS
climatological percentiles. Once the zonal jet shifts eastward, a
strong shortwave trough will approach the area on Wednesday,
resulting in much heavier snow across the Wasatch and CO Rockies.
Snow may also bleed over into the CO Front Range as the core of the
upper jet sinks south, as well as better upslope components on the
east-facing slopes of the Sangre de Cristo and Sawatch range. Snow
levels are expected to start out at the beginning of D2 in the
6000ft-8000ft range (higher south) and rise slightly on D2 before
falling rapidly below 5000 ft on Day, but while precipitation also
weakens. In total, multiple days of very favorable forcing and
continued moisture advection from the Pacific will result in a
broad 2 to 3 feet of snow over many of the higher elevations of UT
and CO. WPC probabilities are high (>70%) for at least 24 inches of
snow during the three day period for elevations in UT and CO above
10,000ft, with low probabilities (10-30%) of 4 inches of snow
across the CO Front Range on D3.

The probability of significant ice >0.25 inches across the CONUS
is less than 10 percent.

Snell


$$
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FOUS11 KWBC 030859
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Tue Dec 3 2024

Valid 12Z Tue Dec 03 2024 - 12Z Fri Dec 06 2024

...Michigan...
Days 1-3...

LES from NNWly flow continues through mid-morning over the eastern
U.P. and down the western shore of the L.P. Localized additional
amounts of 4" are possible between 12Z and 18Z in far SW MI given
the orientation of the flow into that area as well as the current
radar coverage for the strong banding shifting down Lake Michigan.

Surface ridging ahead of the next wave spreads over MI today
shifting flow to the SW and bringing a reprieve to snow this
afternoon to the state. However, the SWly flow quickly saturates
and warm air advection snow begins over the U.P. this evening as
the moisture plume from the next low shifts in. Lake Michigan
enhancement to this snow will really kick in overnight with
heavy/repeating snow north from Grand Traverse Bay and over the
southern shore of the eastern U.P. which continues into Wednesday
morning. Day 1.5 probabilities for >12" snow are around 60% north
of Petoskey and across the Mackinac Straits.

A strong cold front from the low tracking north of the Great Lakes
sweeps across MI Wednesday afternoon with a quick return to LES
banding. Potent cold air advection over the Lakes brings heavy
bands to NW flow prone snow belts into Thursday. Particularly
strong winds can be expected given the pressure gradient between
the 990mb low and the 1037mb high over the Dakotas, so impacts will
be enhanced by the wind. Day 2 PWPF for >6" is 50-80% over the U.P.
north shore and the NW sector of the L.P. with potential prolonged
banding southeast from Grand Traverse Bay with a Superior and Lake
Michigan fetch contributing given the NW flow turning NNWly. LES
slowly decays and the wind gradiently quickly drops off on
Thursday.


Eastern Great Lakes and Northeast...
Day 1...

NWly flow will continue to produce heavy LES over the Chautauqua
Ridge from fetch over Lake Huron and then over Lake Erie through
much of today before it diminishes/pivots east as flow shifts SWly
behind a ridge axis that moves through. An additional 3-6" can be
expected today over the Chautauqua Ridge area. This afternoon the
SWly flow will bring snow, possibly banded, to around Buffalo and
the easten shore of Lake Ontario that persists into the overnight;
Day 1 PWPF for >4" are 20-40%.


Days 2/3...

SWly flow ahead of a developing clipper-type low tracking north of
the Great Lakes saturates over the interior Northeast, increasing
coverage and intensity of snow on Wednesday that continues to
expand and shift north over New England through the overnight. The
Mohawk Valley/southern Adirondacks are favored for heavier snow
Wednesday afternoon where the combination of topographic lift and
antecedant cold air overlap. Higher terrain of the Greens and
Whites get snow in this time, but the greater coverage is expected
over central Maine, generally inland from Bangor Wednesday night.
Day 2 PWPF for >6" is 50-80% over the Mohawk Valley as well as in
central Maine. The WAA based precip spreads across the rest of
Maine Thursday morning with a rain/snow line developing interior
from the Maine coast with a quick transition to heavy snow
on the cold side.

The strong cold front crossing Michigan on Wednesday crosses the
eastern Great Lakes late Wednesday night with a quick onset back to
heavy snow again on NWly flow off eastern Erie starting before
sunrise Thursday with aided fetch from Lake Huron. A decent banding
situation arrises from Lake Ontario over the Tug Hill down the
Mohawk Valley, possibly overlapping well with the snow from
Wednesday.Day 2.5 PWPF for >8" is unfortunately 60-90% from Erie Co
PA to the South Towns of Buffalo. These bands may extend decently
far inland given the rather strong winds from the 990mb low moving
into southern Quebec and the 1035mb high over South Dakota Thursday
morning.

The low quickly exits to the Canadian Maritimes Thursday with
guidance wanting to end most of the heavier banding Thursday night.
Guidance is usually too quick to cut off LES banding, but at least
in this case the surface high tracking over the Ohio Valley on
Friday should cause disruption to the bands. Day 3 PWPF for >8" is
90% over Erie Co PA and around 30% over northern Maine where the
surface low tracks and for upslope flow over the northern Greens
and Adirondacks.

The probability of significant ice across the CONUS is less than
10 percent.

Jackson
$$
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FOUS11 KWBC 060718
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
218 AM EST Fri Dec 6 2024

Valid 12Z Fri Dec 06 2024 - 12Z Mon Dec 09 2024

...Great Lakes and Northeast...
Days 1-2...

Departing area of low pressure over Atlantic Canada and modest
high pressure over the Mid-MS Valley will maintain cyclonic flow
across the Great Lakes, especially off Lakes Erie/Ontario, for the
next day or two. A shortwave within the broad trough over the
region will help back the flow enough to instigate another strong
single band off Ontario into the Tug Hill between Watertown and
Syracuse this afternoon/evening and overnight. Robust snowfall
rates of 1-3"/hr (per the HREF) will help deposit another 12-18"+
of snow over favored areas before waning on Saturday. Downwind of
Lake Erie, multi-band streamers will result in a larger area of
light to moderate snow with WPC probabilities of at least another
6" of snow >50% from Erie, PA northeastward to the Chautauqua Ridge
in NY.

Another shortwave rounding the base of the longwave trough will
slip through the Great Lakes late Sat into early Sun with generally
light snow for the U.P., northern Lower MI, into NYS and northern
New England. WPC probabilities for at least 4 inches of snow are
generally low (10-40%) except for areas with lake enhancement and
parts of the Adirondacks, Green, and White Mountains where
probabilities are moderate (40-70%).


...Pacific Northwest/Northern Rockies...
Days 2-3...

An upper low currently south of the Gulf of Alaska will move
eastward over the next day or so, aimed toward British Columbia.
Ahead of it, a plume of moisture will be directed into the Pacific
Northwest starting early Saturday. Snow levels will start quite
high (around 8000-9000ft at precip onset very early Saturday) but
fall to around pass level (~4000ft) late Saturday into the
overnight hours. East of the crest, cold air in place at the
surface with overrunning WAA-driven precip supports a wintry mix of snow/sleet/freezing rain for eastern WA. Some icing is possible,
from a few hundredths to around a tenth of an inch.

A second surge in moisture will occur on Sunday associated with
the parent upper low as it opens up across the region. With snow
levels already around pass level, snow should accumulate on Sunday
with WPC probabilities for at least 6 inches of snow >50% above
about 3500ft or so.

Farther east, moisture will spread across northeastern OR and into
the northern Rockies from late Saturday through the end of the
period early Monday. Snow levels will lower through time, and
nearly all areas except the lowest valleys will change from rain to
snow. WPC probabilities for at least 8 inches of snow days 2-3 are

50% above about 5000ft or so, mostly in the Lewis Range and

across northern ID, the Blue Mountains, and into northwestern WY.


...Northern Minnesota and northeastern North Dakota...
Day 3...

Eastern extension of the northern Rockies system will manifest
itself as another upper low forming by Sunday over southern
Saskatchewan. That will move east-southeastward into early Monday
with WAA-driven precipitation to its east from northeastern ND
eastward across northern MN. Ptype will be an issue with milder air
overtopping the colder boundary layer, especially where precip
arrives near/after sunset Sunday. NBM ptype uncertainty maximizes
between all 4 types in a zone along the U.S.-Canadian border though
the majority of probabilities are either rain or snow. Still,
cannot rule out some light icing but will have time to narrow the
forecast with time. WPC probabilities for at least 4 inches of snow
are moderate (40-70%) over northern MN.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Fracasso


$$
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FOUS11 KWBC 070727
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 AM EST Sat Dec 7 2024

Valid 12Z Sat Dec 07 2024 - 12Z Tue Dec 10 2024

...Great Lakes and Northeast...
Day 1...

Shortwave on the southwest side of the expansive eastern North
American trough will zip quickly across the Great Lakes and
into/through the Northeast later today. With a surface low track
through southern Ontario, WAA-driven snow will spread across the
Northeast and end early Sunday. With a general westerly wind
component, upslope into N-S terrain (e.g., the Green and White
Mountains) will promote moderate amounts of snow with lighter
amounts in lower elevations. Some additional lake enhancement is
also likely off Lake Ontario into the Tug Hill. Quick movement of
the system should limit amounts, and WPC probabilities for at least
4 inches of snow are highest (>50%) north of I-90 in NY (Tug
Hill/Adirondacks) and into the Green and White Mountains. By later
Saturday night into early Sunday, as the system departs, could see
some late enhancement over eastern Maine where WPC probs for 4
inches are also near 50%.


...Pacific Northwest/Northern Rockies...
Days 1-3...

An upper low in the northeast Pacific heading into British
Columbia will direct two shortwave troughs into the Pacific
Northwest this weekend. Snow levels will initially start out
between 8,000-9,000ft this morning, but plummet to as low as
3,000ft by early Sunday morning. Rain at most locations will change
to snow at Stevens/Snoqualmie Passes as the first wave of
precipitation wanes. East of the Cascades, lingering sub-freezing
temperatures within the boundary layer will be trapped in the
Columbia Basin as WAA and overrunning moisture aloft provide a
favorable setup for an icy wintry mix in central and northeastern
Washington today and even into parts of southeastern
OR/southwestern ID tonight. Some minor icing accumulations are
possible up to about a tenth of an inch.

The second shortwave trough makes its way through the Pacific
Northwest tonight and into Sunday. Lower snow levels support
moderate snow at pass level (esp Stevens) with this second wave of
moisture. WPC probabilities for at least 8 inches of snow are
highest above about 3500ft.

These atmospheric disturbances will push their respective plumes
of moisture east into the Northern Rockies as well, with the
heaviest snow tonight and into Sunday as snow levels decrease, SLRs
increase, and northerly low level winds aid in upslope enhancement
for snowfall rates. Highest probabilities for >6" snow are over
the terrain including the Bitterroots, Lewis Range, Blue Mountains,
and into the Little Belt Mountains in central Montana, the
Absarokas in southern Montana, and the Tetons of western Wyoming.


...Northern Minnesota and northeastern North Dakota...
Days 2-3...

The first upper level shortwave trough that brought snow to parts
of the Pacific Northwest mountain ranges will spawn low pressure
over the Canadian Prairies Sunday morning. As its warm front moves
east, WAA across the Northern Plains will result in a shield of
precipitation that passes over a sub-freezing boundary layer. While precipitation may start out as snow along the Canadian border,
mid/low-level WAA may promote an area of freezing rain for portions
of eastern ND into MN late tonight into early Sunday. Depending on
the evolution of the main low along the Canadian border and other
frontal waviness over the Corn Belt moving up toward the MN
Arrowhead, snow could be maximized over northeastern MN from north
of Duluth to the border. WPC probabilities for at least 4 inches of
snow are >50% from around the Northwest Angle southeastward to
Embarrass and points northeast.


...CO Rockies/Sangre de Cristos...
Day 3...

Height falls associated with the second wave through the Pacific
Northwest will push through the central Rockies, spreading light
snow across the region on Monday into early Tuesday. Quick movement
and general lack of moisture will limit snowfall, with WPC
probabilities of at least 4 inches >30% limited to the Sangre de
Cristos in CO and just into NM.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Fracasso/Mullinax
$$
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FOUS11 KWBC 080812
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 AM EST Sun Dec 8 2024

Valid 12Z Sun Dec 08 2024 - 12Z Wed Dec 11 2024

...Northeast...
Day 1...

Quick-moving system crossing from Ontario into Quebec before 12Z
will continue eastward across northern Maine this morning and early
afternoon before moving into Atlantic Canada. Light snow is
forecast after 12Z this morning for the Adirondacks and northern
New England, with a bit higher amounts over eastern Maine where
snow will eventually taper off later this afternoon. WPC
probabilities for at least 4 inches of snow today after 12Z are
20-40% over far eastern Maine.

Day 2...

The next winter storm to affect the region comes from a
combination of a shortwave over the Ohio Valley and a northern
stream upper low over southern Canada, both acting in concert to
coax a moisture plume northward from the Gulf of Mexico on WAA. A
mix of some freezing rain on the southern side (northeast PA
northward through NYS and also over parts of western New England)
and snow on the northern side (north of I-90) may cause some minor
impacts on Monday. WPC probabilities for at least 4 inches of snow
are not higher than 10 percent, but are moderate (40-70%) for at
least 2 inches over northern NH into western ME.

Day 3... Yet another system will start to affect the Northeast as
the pattern becomes more amplified in the East. Through 12Z Thu,
strong WAA should scour out most of the cold air over the region,
but interior Maine may see some light icing very early Wednesday
with much more precipitation into D4.


...Pacific Northwest/Northern Rockies...
Days 1-2...

Second part of the two-part system will affect the NW today,
pushing moisture into a much colder environment. Moderate to heavy
snow at the higher passes may continue to cause moderate impacts
this morning before the snow starts to taper off from west to east
this afternoon. WPC probabilities depict high chances (>70%) for at
least 8 inches of snow above about 3,500ft while the more remote
areas above 5,000ft likely receive an additional 12"+ of snowfall.

Moisture will continue east of the Cascades across to the Rockies
as the cold front has moved well past the Divide. WPC probabilities
for >8" of snow are over the higher terrain including the
Bitterroots, Lewis Range, Blue Mountains, and into the Little Belt
Mountains in central Montana, the Absarokas in southern Montana,
and the Tetons of western Wyoming. Snow will continue into D2
farther east over the Black Hills where probabilities of at least 4
inches reach 60-90%.


...Northern Minnesota and northeastern North Dakota...
Day 1...

Low pressure over southern Saskatchewan this morning will move ESE
toward northern ND/MN this evening and overnight, lifting its warm
front through the region. While precipitation may start out as
snow along the Canadian border, mid/low-level WAA may promote an
area of freezing rain for portions of eastern North Dakota into
Minnesota this morning, with several hundredths of an inch of ice
possible after 12Z. Closer to the Canadian border, a colder column
supports light to modest snowfall, with the heaviest snow occurring
with the initial WAA over northern Minnesota (esp the Arrowhead,
which will be enhanced by easterly flow off Lake Superior). WPC
probabilities for at least 4 inches of snow are >50% across much of
the northern part of MN, with higher probabilities over the
Arrowhead (>80%). There, probabilities for at least 8 inches of
snow are moderate (40-70%).


...CO Rockies/Sangre de Cristos...
Day 2...

Height falls at the southwest side of a large positively-tilted
longwave trough over the middle of North America will swing through
the central Rockies on Monday, brining with it a plume of residual
Pacific moisture. High pressure building in from the north in the
wake of a cold frontal passage Monday morning will result in some
weak upslope flow into Colorado's Front Range during the day. This
upslope component will extend as far south as the Sangre de Cristos
in northern New Mexico by Monday night. Periods of light-to-
moderate snow across the region are expected as the system moves
quickly through the region and heights build in by Tuesday. WPC
probabilities show >50% probabilities for at least 4 inches mainly
limited to the Sangre de Cristos in southern Colorado and northern
New Mexico, which does include the Raton Mesa. The highest and more
remote elevations could see localized amounts approach 8" by the
time the snow comes to an end Tuesday morning.

The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Fracasso/Mullinax


$$
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FOUS11 KWBC 090741
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
241 AM EST Mon Dec 9 2024

Valid 12Z Mon Dec 09 2024 - 12Z Thu Dec 12 2024

...CO Rockies/Sangre de Cristos...
Day 1...

Height falls moving out of the Great Basin this morning will
continue southeastward behind a cold front that has pushed into the
central Rockies. High pressure over the northern Rockies and
nosing down the High Plains will promote some modest upslope flow
into Colorado's Front Range during the day today. This upslope
component will extend as far south as the Sangre de Cristos in
northern New Mexico by tonight. Periods of snow across the region
are expected as the system moves quickly through the region and
heights build in by Tuesday. WPC probabilities are high (>70%) for
at least 4 inches of snow around the Palmer Divide southward to
Raton Pass. The highest and more remote elevations could see
localized amounts around 8-10" by the time the snow comes to an end
Tuesday morning. Lighter amounts are expected through the I-25
corridor but could be heavier around Raton Pass.

...Northeast...
Days 1-2...

Shortwave moving through the Ohio Valley this morning will help
bring in another surge of moisture coincident with an advancing
warm front. Sub-freezing air mass largely resides north of I-90,
along with some higher elevations through the Catskills, this
morning. As the warm front approaches I-80 this afternoon, an area
of low pressure will form over southeastern New England, helping to
maintain some northerly flow over northern New England. However,
with broad SW to S flow between 850-700mb, warm nose will help
support areas of freezing rain over eastern NYS into New England
today, then into Maine D2. Around a tenth of an inch of icing is
expected D1-2, but WPC probabilities for at least 0.25" are 10-20%
over western Maine (into the Kennebec Valley). Snowfall may be
limited with time due to the marginal thermal profile and lack of
cold enough temperatures in the production zone for dendrites as
the warm front attempts to push through. A few inches will be
possible toward precip onset, focusing over the Green/White
Mountains into Maine where WPC probabilities for at least 4 inches
are 40-80%.


Day 3...

By D3 (Wed-early Thu), upper pattern becomes much more amplified
upstream as a vigorous system lifts northeastward out of the
Southeast. Digging upper low into MN will raise heights over the
East Coast with milder temperatures above freezing ahead of the
front. On the backside of the front, colder air will filter in atop
a lagging precip shield, supporting a rain-to-snow transition
initially over the eastern TN Valley and central/southern
Appalachians first (Wed afternoon), then eventually into NYS Wed
evening and overnight into early Thu. WPC probabilities for at
least 4 inches of snow through 12Z Thu are low to moderate
(10-40/40-70%) over the central Appalachians (eastern WV) and into
parts of central NYS into the Adirondacks which will be quicker to
change over from rain to snow. Additionally, as the low deepens
quite smartly into Canada, cyclonic flow will wrap across Lakes
Erie/Ontario as 850mb temps crash to -10 to -15C on westerly flow.
This will support single banded lake snows by early Thursday,
continuing beyond this forecast period.


...Upper Great Lakes...
Days 2-3...

Upstream anticyclonic wave breaking over northwestern Canada will
support a digging and deep upper low moving into the Great Lakes on
Wednesday. An arctic front will switch winds to northwesterly as
much colder air is dragged into the region (850mb temps -15 to -25C)
which will kick-start the lake-effect snow (LES) machine into
high gear toward the end of D3 and continuing beyond this forecast
period. WPC probabilities of at least 4 inches of snow are high

70%) over much of the northern shore of the Michigan U.P. and

along the western coast of Michigan's Mitten. For at least 8
inches of snow, the highest probabilities lie over northwestern
Lower Michigan. Snowfall rates of 1-2"/hr are likely in more intense bands.


Fracasso


$$
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FOUS11 KWBC 100759
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Tue Dec 10 2024

Valid 12Z Tue Dec 10 2024 - 12Z Fri Dec 13 2024

...Northeast... Day 1...

Broad SW flow in the mid-levels to the south/southeast of a
shortwave nearing James Bay will override a nose of high pressure
from Atlantic Canada, maintaining sub-freezing low-levels across
much of central/northern New England this morning. Wavy frontal
boundary will try to lift northward into the colder air, but will
be slow to succeed until tomorrow (Wed). As precipitation moves
into the region overnight tonight into Wednesday morning, freezing
rain will be likely over much of central/northern New England,
especially in areas with a little elevation. On Wednesday, southerly/southeasterly flow will eventually win out at the low
levels, turning all areas over to plain rain. Ice accumulations of
a tenth or two of an inch are likely over parts of the Green and
White Mountains and into western Maine where the cold air may hold
on the longest. WPC probabilities for at least a tenth of an inch
of ice are >50%. Though the probabilities of at least 0.25" are
low, this may be underdone as the models are sometimes too quick to
have the cold air retreat.


...Central Appalachians... Day 2...

The southern part of the system affecting the Northeast tonight
into Wednesday will be the evolution of an increasingly negatively-
tilted and deep upper trough over the eastern 1/3 of the CONUS.
Frontal boundary will clear past the Appalachians tonight, allowing
colder air to push in behind it and change rain to snow over the
Cumberland Plateau and points eastward. Upslope enhancement into
the central Appalachians will promote more modest snow totals, and
WPC probabilities for at least 4 inches of snow are >30% over
eastern WV northward through the western MD Panhandle (Garrett Co)
into the Laurel Highlands in PA.


...Northern Plains... Day 1...

The leading edge of the encroaching Arctic air-mass will track
into the Northern Plains today. Surface-based warming out in front
of the Arctic front and strong CAA aloft will allow for lapse rates
in the 500-700mb layer to be >7.5C/km in some cases, with just
enough low-level moisture for some snow squalls to develop. These
squalls could form as far north and west as eastern Montana and
western North Dakota this morning then race southward and eastward
into South Dakota this afternoon and the Missouri Valley by this
evening. Squalls could even continue to track into parts of Iowa,
northern Kansas, and northern Missouri tonight as the snow squall
parameter exceeds 1 in much of the CAM and even global guidance.
Though snow amounts will be light, squalls can lead to rapid
reductions in visibility with accumulating snow on roadways where
road temperatures drop below freezing and create icy hazards.


...Great Lakes/Northeast... Days 1-3...

Upper pattern will become increasingly amplified over the East as
ridging into the West weakens but builds quite smartly over the
northern Atlantic. This will help strengthen the jet to ~170kts out
of the southwest across the TN/OH Valley overnight tonight as
arctic air moves in from the northwest. Lake effect snow will pick
up in earnest starting this evening and overnight across the U.P.
of Michigan and then into the western side of Lower Michigan as
height falls rush through (500mb heights below the 1st percentile
per the CFSR) as the trough axis becomes quite negatively-titled
into the Southeast/Mid-Atlantic. 850mb temperatures will plunge to
-15C to -25C tomorrow with plenty of Lake-to-850 DeltaT. Snow will
continue through the period as winds slowly back from NW to WNW or
W as the upper center passes by. Into the Northeast, once the
front clears the area and temperatures fall below freezing area-
wide, snow will increase off Lakes Erie/Ontario, the latter of
which will support a strong single band into the Tug Hill Plateau
with lighter snow pretty far inland. Snow off Lake Erie may also
be a single banded aimed just into the BUF southtowns. Snow will
continue across all the Great Lakes through the end of this period
(12Z Fri) but will continue beyond then.

Through 12Z Fri, WPC probabilities of at least 12 inches of snow are
high (>70%) across the favored lake belts on NW flow over the U.P.
and northwest Michigan; between Erie, PA and Buffalo, NY (esp the
southtowns but perhaps into the southern Buffalo area); and in the
Tug Hill. Snowfall rates of 1-2"/hr (surpassing 3"/hr at times) are
likely in the more intense bands.


...WA/OR Cascades... Day 3...

Incoming shortwave or upper low will move into NorCal/Southwestern
OR Thursday, spreading some precipitation into the Cascades
southward into the Sierra. Snow levels will be around 3000ft to the
north and 5000ft to the south with QPF totals generally up to
0.50" or so. WPC probabilities for at least 6 inches of snow are
highest in the WA/OR Cascades.

Fracasso

$$
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FOUS11 KWBC 110710
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
210 AM EST Wed Dec 11 2024

Valid 12Z Wed Dec 11 2024 - 12Z Sat Dec 14 2024


...Central Appalachians...
Day 1...

The southern part of the system affecting the Northeast tonight
will evolve into a deep, negatively-tilted trough over the eastern
CONUS on Wednesday. As the associated frontal boundary moves east
of the Appalachians, colder air will fill in behind it, changing
rain to snow from the Cumberland Plateau to areas north and east.
Upslope enhancement in the central Appalachians will support modest
snow totals, with WPC probabilities of greater than 30 percent for accumulations of 4 inches or more largely confined to eastern West
Virginia but also northward into the Laurel Highlands.


...Great Lakes/Northeast...
Days 1-3...

Driven by falling 500mb heights and a negatively-tilted trough,
lake effect snow will intensify today across the Upper Peninsula of
Michigan and into western portions of Lower Michigan. 850mb
temperatures will drop to -15C and -25C (below the 5th percentile),
supporting significant lake-to-850 DeltaTs. Snow will continue
through Thursday while winds gradually turn from the northwest to
west- northwest, as the upper low dropping south through central
Canada pivots and moves east of the Great Lakes.

In the Northeast, once the front clears the region and
temperatures drop below freezing, snow will increase downwind of
lakes Erie and Ontario. Models continue to advertise intense,
single-bands targeting both the Tug Hill and the Buffalo Southtowns
beginning later today and continuing through Thursday. Snow will
gradually wind down from west to east across the Great Lakes
starting late Thu/early Fri before finally ending early Saturday
over NY as a surface ridge builds over the region.

For the event (next 72 hours), WPC probabilities for at least 12
inches of snow are greater than 70 percent across the favored lake-
effect snow belts of Upper Michigan, northwestern Lower Michigan,
the I-90 corridor from Erie to the Buffalo Southtowns, and the Tug
Hill Plateau. For some of these areas, especially those in New
York, WPC probabilities indicate that localized heavier amounts of
2 feet or more can be expected. Hazardous conditions can be
expected in the more intense bands where snowfall rates may exceed
2 inches per hour on Thursday per the 00Z HREF.

...Cascades/Sierra Nevada...
Days 1-3...

After a recent quiet period, the pattern will become more active
in the West over the next few days. A shortwave/compact upper low
will move ashore on Thursday, with some precipitation out ahead of
it today/tonight over the OR Cascades and the northern CA ranges.
Snow levels will be around 3000ft to the north and 5000ft to the
south, decreasing with the passage of the shortwave. Some
additional decreases are expected Thursday night before rebounding
late in the period as a ridge begins to build and precipitation
returns ahead of a low/trough amplifying over the eastern Pacific.
That system will have a little more moisture and amplitude,
focusing the precipitation northward into the WA Cascades by
Friday. Greatest impacts will be at the mountain passes throughout
the region from the WA Cascades southward into the norther Sierra.
WPC probabilities for 3-day snow totals of 12 inches or more are
highest over the Oregon Cascades into the Shasta-Siskiyous/Trinity
region and the northern Sierra Nevada.

...Corn Belt...
Day 3...

Upper low exiting the Rockies on Friday will start to tap limited
moisture out of the Gulf, eventually promoting an area of WAA-
driven precipitation across the Corn Belt. Cold air mass in place
beneath a warming ~850mb layer will favor an area of mixed
precipitation (sleet/freezing rain) just south of some light snow
on the northern side where the column remains below freezing.
Amounts may be light, but any freezing rain could be hazardous. WPC probabilities for at least 0.1" of ice accumulation are 10-30%
over IA through 12Z Saturday.


The probability of significant icing greater than 0.25 inches is
less than 10 percent across the CONUS this period.

Fracasso/Pereira

$$
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FOUS11 KWBC 120806
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
306 AM EST Thu Dec 12 2024

Valid 12Z Thu Dec 12 2024 - 12Z Sun Dec 15 2024

...Great Lakes.. Days 1-2...

The core of an anomalous upper low (-2 sigma at 500mb according to
NAEFS with a 510dm center) will begin to fill and drift eastward
across southern Canada on D1, with the primary mid-level trough
axis pivoting into New England by 00Z Saturday. This will be
replaced by brief shortwave ridging downstream of a potent trough
digging across the Central Plains. Until that happens, however, the
environment will remain favorable for another round of widespread
and heavy lake-effect snow (LES) in the W/NW snow belts.

At the start of the forecast period, 850mb temps will be generally
-15C to -20C, and continue to plunge to -18C to -23C by 00Z Friday
before slow warming begins. This CAA supplying the cold air will
move across Great Lakes water temperatures that are still warm,
generally +5 to +9C, supporting deltaT that is well above
threshold for heavy LES, and will drive lake-induced equilibrium
levels 10,000 - 15,000 ft, which will be well above the -10C
isotherm suggesting potential lightning in the heavier bands. This
will support snowfall rates of 1-2"/hr in many of the bands, with
locally 3-4"/hr possible downstream of Lakes Erie and Ontario.
Combined with the cold temperatures and gusty winds, significant
travel impacts are expected within LES.

The heaviest LES is likely D1 before a slow ease of CAA (and onset
of subtle WAA) wanes LES from west to east during D2, but not
before multiple feet of snow occur in the more persistent and
intense banding. WPC probabilities D1 are high (>70%) for more than
8 inches in the eastern U.P., near Traverse Bay, along the
Chautauqua Ridge, and into the Tug Hill Plateau, with high
probabilities continuing on D2 only in the Tug Hill.

...Pacific Coast and Interior Northwest... Days 1-3...

Two waves of precipitation will spread onshore the West Coast and
then inland through the forecast period bringing widespread snow
to the higher elevations.

The first wave of moisture will be ongoing to start the period as a
slowly filling closed low drops into northern CA with 500mb height
anomalies falling below -1 sigma according to NAEFS. Height falls
combined with the LFQ of an accompanying upper level jet streak
will drive ascent onshore, leading to locally heavy snow, primarily
confined to the Sierra Nevada. The strongest forcing is transient
and pushes south of the area halfway through D1, but at least a
short duration of 1"/hr snow rates is likely across the Sierra,
generally above 5000 ft. During D1, WPC probabilities for more than
6 inches of snow are above 70% in the Sierra, but probabilities for
more than 12 inches are only 10-30%.

A more significant system will then approach the coast from the
Pacific Friday into Saturday, leading to a more substantial
precipitation event after only a brief break Thursday night into
Friday. This system will again be driven by a pronounced closed low
offshore, opening into a potent trough as it shifts across CA/OR
late Saturday, reaching the Great Basin by the end of the forecast
period. Once again, downstream divergence, warm/moist advection,
and robust jet energy will combine to drive ascent. However, this
second system will be more broad, reflected by 500mb height
anomalies below -1 sigma across much of the Pacific Coast, combined
with modestly coupled jet streaks to push a strong surface low into
British Columbia and a second wave across CA. This overall more
impressive system is reflected as well by GEFS and ECENS
probabilities for 500 kg/m/s IVT lifting onshore exceeding 60%.

Snow levels across the West with this second system will generally
remain around 4000-5000 ft, with a narrow corridor exceeding 6000
ft in the strongest IVT/WAA. However strong ascent into an
environment with a deep layer of lapse rates stronger than moist-
adiabatic could lower snow levels beyond forecasts, and the NBM
25th percentile may be more representative, which reaches as low as
3000-4000 ft during the heaviest precipitation, and falls even
lower as precip begins to shift eastward. This suggests the
potential for more widespread pass-level impacts, reflected by
WSSI-P above 70% for moderate impacts due primarily to snow load
and snow rate in the Sierra and Shasta/Trinity region. Lesser, but
still notable impacts, are possible as far north as the WA Cascades.

WPC probabilities for more than 8 inches of snow on D2 are high

70%) but confined to the Shasta/Trinity region of northern CA and

the highest peak of the WA/OR Cascades. By D3, high WPC
probabilities for more than 8 inches spread across the length of
the Sierra and into the Sawtooth/Salmon River Ranges, while also
continuing near Mt. Shasta.

...Central Plains into the Upper Midwest... Days 2-3...

A closed 500mb low emerging from the Central Rockies will track
eastward while deepening and pulling south in response to a potent
vorticity lobe rotating around its base. The trend in the global
guidance has been for this feature to be a little weaker and
farther south, and while this will likely result in more
interaction with a stationary front/modest low-level baroclinic
zone, the resultant downstream moisture advection may be somewhat
muted. Still, as this advects east into Missouri by Saturday
evening, it will combine with strengthening fgen in response to
surging WAA to drive moisture northward reflected by a narrow
channel of +1 sigma PWs on the NAEFS tables, while also helping to
create a surface low moving across the Central Plains and the Upper
Midwest by the end of the forecast period.

Downstream of this low, a sprawling high pressure centered over the Mid-Atlantic states will gradually retreat to the east. This will
allow confluent flow to merge into the Corn Belt and then Upper
Midwest, with overrunning WAA leading to a period of mixed
precipitation including sleet and freezing rain, with snow farther
north. Even where precipitation is snow, it may begin as a period
of ZR/IP due to antecedent low RH within the DGZ (no ice growth)
noted in regional soundings.

There remains considerable spread in the latitudinal gain of warm
air, but with the high retreating steadily and WAA increasing,
solutions with a farther north depiction of accumulating snow and
ice seem more reasonable, and this could result in hazardous
accretions of freezing rain, and modest snowfall as well. Current
WPC probabilities for more than 4 inches of snow are less than 10%
with this system, but the probability for at least 0.1 inches of
ice are 10-30% D2 in central IA, increasing to 50-70% D3 when
locally as much as 0.2 inches is possible.

Weiss

$$
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FOUS11 KWBC 130813
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Fri Dec 13 2024

Valid 12Z Fri Dec 13 2024 - 12Z Mon Dec 16 2024


...Great Lakes.. Day 1...

Residual heavy lake-effect snow (LES) will persist through the
first half of Friday, especially east of Lake Ontario into the Tug
Hill Plateau, before increasing SW flow downstream of a shortwave
moving through the middle of the country causes WAA and shuts off
LES by the end of D1. Before that occurs, the environment remains
favorable for 2+"/hr snow rates, primarily downstream of Lake
Ontario, before intensity wanes and LES ends later in the day.
Despite a generally short duration of continuing heavy LES, WPC
probabilities for more than 6 inches are high (>70%) downstream of
Lake Ontario, with locally up to 12 inches possible.

...Western U.S...

A closed low over the eastern Pacific will amplify and shed
periodic vorticity lobes towards the Pacific Coast of the United
States and Canada. Each of these will combine with increased
moisture to cause widespread winter weather across the region.

California... Days 1-2...

One of these shortwaves will intensify into a pronounced trough,
taking on a negative tilt as it shifts into central CA Saturday
aftn /early D2/. This will act in tandem with modestly coupled jet
streaks to drive ascent, while impressive moist advection surges
IVT to 500-750 kg/m/s according to both GEFS and ECENS
probabilities. Snow levels will rise to around 5000 ft within the
core of this IVT, highest in the Trinity/Shasta/Siskiyou region,
but still support heavy snow in the higher terrain, with
accumulations aided by nearly ideal upslope flow. Although the
heaviest snowfall, which will likely feature rates above 1"/hr will
occur within the warmer snow levels/higher IVT, even as snow levels
fall behind the primary trough axis moderate to heavy snow will
persist through D2 before waning. Additionally, heavy snow through
synoptic and impressive upslope flow will extend along the length
of the Sierra, bringing substantial impacts to many of the Crest
Passes. WPC probabilities for more than 12 inches of snow is
extremely high (>90%) in the Shasta/Trinity region and the northern
Sierra D1, continuing above 70% D2 while extending down the length
of the Sierra. Local snowfall maxima of more than 4 feet are likely
in the highest terrain.

Pacific Northwest through Northern Rockies... Days 1-2...

Farther north, a stronger closed mid-level low will push into
British Columbia as it remains embedded in the downstream southerly
flow ahead of the larger scale trough axis. This will drive
persistent moist advection into the Pacific Northwest D1 into D2,
with this moisture then merging with the aforementioned shortwave
ejecting from CA to drive widespread heavy precipitation into the
Northern and Central Rockies. Snow levels will fluctuate through
the period, but generally remain around 4000-5000 ft. However,
strong ascent and ample moisture could result in locally lower snow
levels through cold-air dragging on intense snowfall rates,
especially in regions of intense upslope ascent or modest elevated
instability.

WPC probabilities for more than 6 inches of snow are high (>80%)
along the WA/OR Cascades, the Olympics, and into the Salmon
River/Sawtooth region on D1, with the highest probabilities
focusing across parts of ID and into the Tetons D2, while
remaining, albeit more modestly, in the Cascades. By D3 the highest
WPC probabilities for more than 6 inches of snowfall drop
considerably to 30-50%, focused almost exclusively in the Tetons.
Storm total snowfall of 20-30 inches likely in the higher terrain
of many of these ranges. With snow levels ranging between around
4000-5000 ft, some of the higher passes could experience
considerable impacts as well.

...Central Plains into the Upper Midwest... Days 1-2...

A trough ejecting from the Central Rockies Friday night will
amplify into a closed low over the Central Plains as it digs E/SE
through Saturday, and then lifts northeast towards the Great Lakes
on Sunday. Height falls downstream of this deepening trough will
combine with weak but coupled jet streaks aloft and a residual
low-level baroclinic gradient to drive surface cyclogenesis in the
lee of the Rockies Friday night. As this low moves progressively to
the east, it will draw increased moisture northward from the Gulf
of Mexico, channeling an axis of PW anomalies as high as +2 sigma
into MO/IA Saturday. This moisture has steadily shown an increased
trend with recent model runs, likely in response to a subtly deeper
upper low.

As this feature moves east and deepens, it will interact with a
retreating but sprawling surface high (max pressure around
1050mb!). This will leave an environment that is cold enough for
wintry precipitation, but as the WAA intensifies, the high will
lose its favorable position for cold advection leading to a column
that will become overwhelmed by the WAA and warm with time.
Additionally, it will take some time for the DGZ to saturate as the
antecedent air is quite dry, so this setup continues to look more
favorable for IP (and mostly ZR), with just some snow on the
northern side. However, the stronger low could also result in some
more intense deformation to the north of the surface low, leading
to at least some modest snowfall accumulations in MN/WI.

Despite that, the primary hazard appears to be freezing rain, and
WPC probabilities have increased for significant accretion, with
locally damaging icing now possible as reflected by max
probabilities reaching around 30% for > 0.25" of ice, highest
across central IA.

...Central Appalachians... Day 3...

The same high pressure retreating eastward from the Upper Midwest
(above) will push into New England and then favorable wedge down
the coast in a Cold Air Damming (CAD) setup east of the
Appalachians. As moisture from a low pressure moving into the Great
Lakes shifts eastward, it will encounter cold air, especially in
the higher elevations, supportive of wintry precipitation. However,
the robust WAA driving the expanding precipitation shield is
likely to overrun the cold air, leading to a corridor of
significant freezing rain, with only light snow accompanying. WPC
probabilities for more than 0.1" of freezing rain have increased to
as high as 50-70% across the higher terrain from eastern WV,
through the western Panhandle of MD, and into the Laurel Highlands of PA.

Weiss

$$
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FOUS11 KWBC 140817
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 AM EST Sat Dec 14 2024

Valid 12Z Sat Dec 14 2024 - 12Z Tue Dec 17 2024

...Western U.S...

An active period will bring widespread winter weather to much of
the West through early next week.

A large trough just offshore the Pacific Coast to start the period
will shed two distinct compact vort maxima/shortwaves onshore on
D1. One of these will pivot northward into British Columbia, while
the second one ejects into central CA. Although the northern
impulse is a bit deeper relative to the CFSR climatology (500mb
heights below the bottom 2.5 percentile according to NAEFS), the
greater moisture/IVT will pivot into CA associated with the
southern system as the northern moisture channel lifts into Canada. Additionally, there will be a modestly coupled jet streak
downstream of the primary trough axis and in the vicinity of this
southern impulse, helping to enhance ascent to spread more
widespread moisture northeast. On D1, this will cause widespread
heavy snow from the Sierra and Shasta/Trinity region eastward into
the Salmon River/Sawtooth Ranges, Blue Mountains, and as far east
as the Tetons and Big Horns, WPC probabilities are high (>70%) for
above 12 inches in the CA ranges and into ID thanks to higher
moisture, strong synoptic lift, and impressive upslope flow driving
snowfall rates of 2+"/hr at times. 1-2 feet of snow is possible D1
across these areas.

More modest snowfall across the WA/OR Cascades and Olympics where
both ascent and moisture are more limited. Still, WPC probabilities
in the Cascades and Olympics are moderate to high (50-90%) for
more than 8 inches. Snow levels D1 will climb briefly to as high as
5000-6000 ft in the core of the max IVT/WAA/moisture plume, but
will fall gradually most of the day to as low as 2000 ft in the
Cascades and 3500 ft elsewhere. This will allow at least modest
accumulations below pass levels, but most of the accumulating snow
is likely during the period of higher snow levels.

On D2, the pattern evolves quickly as a narrow but amplified
longwave ridge builds over the Pacific Coast, forced between the
departing trough now moving into the northern High Plains and a
renewed closed low back over the Pacific Ocean. This will force a
respite in precipitation across the coast and into the Great Basin,
but wintry precipitation will continue into the Northern Rockies,
but in a slowly waning fashion. Snow levels will continue to fall
steadily D2, reaching below 1000 ft in the Northern Rockies, which
coincident with continuing (but easing) snow will result in heavy
snow accumulations across the NW WY ranges, with some light
accumulations down into most of the valleys. WPC probabilities D2
are moderate (50-70%) for an additional 6+ inches, highest in the Tetons.

Late D2 and then more substantially into D3, the large closed low
west of WA/OR will spin two distinct vorticity maxima/shortwaves
onshore, leading to increasing waves of precipitation spilling
eastward once again. With this next impulse, snow levels will
initially be low, only around 1500 ft, with modest rising
occurring in the axis of greatest IVT. Despite IVT progged to
remain only around 250 kg/m/s, this will lift snow levels to around
3500 ft west of the Cascades in WA/OR, and then potentially as
high as 4500 ft later D3 with the second wave. With impressive
ascent increasing, and upslope flow amplifying into the terrain,
heavy snowfall will result, and WPC probabilities D3 are moderate
(50-70%) for 6+ inches from the Shasta/Trinity region northward
along the Cascades.

Moisture spilling east into the interior Northwest will encounter
some trapped cold air and modest easterly flow ahead of the
accompanying WAA/IVT, such that some light freezing rain could
result on the eastern side of the Cascades. Potentially more
impressively, the accompanying WAA and associated fgen could result
in heavier snow rates farther east which could reach the valley
floors of interior WA and OR. Confidence is low this far out, but
WPC probabilities do indicate at least a low chance for 1" of snow
across much of WA east of the Cascades, with heavier snow likely
moving back into the Salmon River/Sawtooth Ranges before the end of
the period.

...Central Plains into the Upper Midwest... Day 1...

A mixed snow, sleet, and freezing rain event will be ongoing to
start the forecast period as a wave of low pressure moves steadily
from Kansas into the western Great Lakes. This low will be driven
by a compact but potent closed low moving over-top the surface
low, with downstream warm and moist advection expanding the
precipitation shield northeast through the region. As this
vertically stacked system moves east, it will run into some cold
and dry antecedent air as an expansive high pressure gradually
retreats towards New England. As WAA intensifies, it will push a
warm nose above 0C, while surface wet-bulb temperatures will remain
below freezing. The guidance has narrowed the corridor of
significant freezing rain, and despite what could be moderate
precip rates (supported by soundings) and a lack of dry advection
to offset latent heat release of freezing (and to prevent
substantial warming), there is high confidence in a stripe of
significant ice accretion, especially in eastern and central IA.
Here, WPC probabilities are moderate (50-70%) for an additional
0.1+ inches of ice, leading to local event totals exceeding 0.25".

...Central Appalachians... Days 1-2...

Sprawling high pressure with impressive central pressure
approaching 1050mb will gradually pivot east through New England
during the weekend. While this feature will retreat, it will extend
down the coast east of the Appalachians as a cold-air damming
scenario, at least initially reinforced by mid-level confluence
ahead of an approaching shortwave. This impulse will lift from the
Corn Belt late Saturday to off the New England coast, with the
associated moisture and ascent producing a swath of precipitation
extending from the Southern Appalachians through southern New
England. The WAA overrunning the cold surface layer will likely
result in an axis of moderate to heavy freezing rain in the higher
elevations from southern WV through the MD Panhandle and into the
Laurel Highlands where WPC probabilities for more than 0.1 inches
of ice reach as high as 70%, and locally in excess of 0.25" is
possible near the MD Panhandle and into the southern Laurel
Highlands which is where the greatest duration of freezing rain is
expected. Surrounding this, light freezing rain could result in
accumulating ice exceeding 0.01 inches as far south as the northern
escarpment of SC, and as far north as some of the higher terrain of
Upstate NY.

While the most significant winter impacts are likely due to
freezing rain, many areas from PA through MA will experience
precipitation at least starting a snow with some light
accumulations likely in the higher elevations of the Poconos,
Catskills, and Upstate NY/western New England. At this time
however, WPC probabilities are only 50-70% for 2+ inches in these
areas. In places that receive mostly rain, a brief mix of snow and
sleet is possible during precipitation onset even as far southeast
as the I-95 corridor, but no accumulation is expected.

...Northern Plains... Days 2-3...

Vorticity maxima leftover from a weakening shortwave will lift
northeast from the Great Basin, with secondary intensification of
this feature likely over eastern MT Sunday. The guidance has
continued to trend a bit deeper and farther south with this
secondary development, suggesting a higher potential for some
moderate to heavy precipitation from MT into ND.

As the lead shortwave weakens and pushes northeast, some leading
WAA will expand light precipitation into the northern High Plains.
Forecast soundings suggest considerable dry air to overcome, but
as this occurs it will cause some wet-bulb effects to keep temps
below freezing and an axis of light freezing rain is likely from
eastern MT into western ND Saturday night into Sunday morning.
Accumulations are likely to be modest, however, as reflected by WPC probabilities for 0.1+" of ice peaking around 30% in eastern MT on
D2. Still, any freezing rain can cause hazardous travel
conditions.

Thereafter, the secondary impulse deepens into a closed low which
should cause some enhanced deformation to the north and west of the
system. This deformation acting upon some higher theta-e air
wrapping cyclonically around the low (emerging from intensifying
290K isentropic ascent downstream) will support some banded snow
rotating through ND and then into MN Sunday night and Monday. At
this time total snowfall is still expected to be light however,
with WPC probabilities for 2+ inches of snow reaching just around
50% from eastern ND into northern MN.

Weiss
$$
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FOUS11 KWBC 150727
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 AM EST Sun Dec 15 2024

Valid 12Z Sun Dec 15 2024 - 12Z Wed Dec 18 2024

...Pacific Northwest through the Northern Rockies... Days 1-3...

An active period of weather will bring periods of of heavy snow to
much of the region through the middle of next week.

The period begins with a broad longwave mid-level trough centered
over the Rockies. Within this trough, a modest shortwave impulse
and accompanying vorticity maxima will be shedding northeast into
the Northern Rockies, interacting with modest downstream moisture
reflected by near-normal PWs according to NAEFS. This will spread a
swath of precipitation across ID/WY/MT the first half of D1 before
an approaching shortwave ridge brings an end to the precipitation.
Most of this precipitation will be light, with the exception likely
in the vicinity of NW WY/Tetons where upslope flow and some
enhanced fgen will drive heavier rates and greater snowfall
accumulations. WPC probabilities D1 for more than 8 inches of snow
are high, above 70%, in the Tetons and adjacent Absarokas to the north.

Behind this first shortwave, the aforementioned shortwave ridging
will cause a brief respite to precipitation in the Pacific
Northwest, but this will come to an end by the start of D2 as
another amplified closed low drifts eastward across the Pacific
Ocean. Downstream of this low, mid-level divergence will approach
the coast Monday morning, with periodic PVA through shedding
vorticity lobes helping to enhance ascent. The downstream SW/WAA
ahead of this feature will surge moisture back onshore as well,
reflected by IVT approaching the 97th percentile as both GEFS and
ECENS probabilities for IVT of 500 kg/m/s reach 20-30%. This will
spread heavier precipitation back onshore the Pacific Northwest,
with snow falling above generally 3000 ft in the Cascades, and
spilling into the interior Northwest where snow levels will be even
lower, around 1500 ft. This will result in moderate to heavy snow
from the Shasta/Siskiyou/Klamath region of CA northward along the
Cascades and as far inland as the Blue Mountains and Salmon River
range. Across these areas, WPC probabilities are moderate to high
(50-70%) for at least 8 inches of snow, and locally more than 12
inches is likely (>70%) in the highest terrain of the OR Cascades
and near Mt. Shasta.

During D3, a warm front just offshore will lift northward, and the
accompanying precipitation will follow on enhanced WAA lifting
across OR/ID/WA. The intensity of this precipitation should
generally be lighter in the snow areas than on D2, but additional
accumulations exceeding 6 inches are possible (30-50%), highest in
the WA Cascades.

...Central Appalachians... Day 1...

Weakening surface low pressure moving across the OH VLY will spread
downstream moisture northward into the Central Appalachians and
Mid-Atlantic States. This moisture will surge northward on
intensifying 295K isentropic ascent, lifting atop a retreating but
still expansive high pressure centered over New England and its
resultant wedge east of the Appalachians. As the high retreats and
the surface low weakens while moving eastward, low-level flow will
veer to become primarily S/SE, not ideal for locking in any low-
level cold air. This suggests that while precipitation will begin
wintry (snow/sleet/freezing rain) in most areas from the highest
elevations of SC northward, it will quickly turn to rain outside of
the higher terrain. Even in the Mid-Atlantic and Northeast as far
southeast as the I-95 corridor, precipitation may start as a brief
period of sleet or snow before turning to rain. This will limit
wintry accumulation in many areas.

However, in the higher terrain of WV, through the MD Panhandle, and
into the Laurel Highlands, a prolonged period of freezing rain is
likely, which will accrete to moderate to substantial ice amounts
through Sunday night. Some modest conditional instability reflected
by theta-e lapse rates around 0C/km will support at times briefly
heavy rain rates, somewhat limiting the accretion potential
(especially without any dry-bulbing affects), but prolonged
duration of freezing rain has still bumped WPC probabilities to
moderate (30-50%) for 0.25" centered near the MD Panhandle, with
high probabilities (>70%) for at least 0.01" stretched from NC
northward along the Appalachians into Upstate NY.

...Northern Plains... Days 1-2...

A wave of low pressure moving from MT through MN Sunday into Monday
will deepen in response to a mid-level shortwave closing off
overhead, collocated with the LFQ of a modest but poleward arcing
jet streak. As this low slowly intensifies, the downstream plume of rich theta-e air spreading northward from the Gulf of Mexico will begin
to wrap cyclonically around the low, potentially supporting modest
TROWAL development pivoting into far northern ND and northern MN
Sunday night. The leading WAA ahead of this developing low will
likely result in some modest freezing rain across far western ND
and eastern MT early D1 due to lack of saturation within the DGZ,
but WPC probabilities on D1 are above 70% for at least some icing,
with a narrow channel of 10-30% near the ND/MT border for 0.1 inches.

More impressive is likely to be the snowfall on the northern edge
of this system as it progresses east, with snow becoming more
intense Monday from eastern ND through northern MN. Here, elevated
instability beneath the TROWAL combined with modest deformation
could result in banded snow structures, offsetting otherwise modest
omega present within the column. There is still some uncertainty
into how much snow may result as precipitation expands and
intensifies Monday, but WPC probabilities have increased and are
50-70% from northern ND across into northern MN for 2+ inches, and
the WSE plumes do suggest at least a low-end potential for 4-5
inches in some areas. Regardless of the intensity and amounts of
snow, hazardous travel is likely as fluffy SLR snow combines with
gusty winds to cause snow covered roads with restricted visibility,
but WSSI-P for moderate impacts remains quite low for this area
suggesting primarily minor impacts.

Weiss

$$
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FOUS11 KWBC 160830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Mon Dec 16 2024

Valid 12Z Mon Dec 16 2024 - 12Z Thu Dec 19 2024

...Pacific Northwest through the Northern Rockies... Days 1-3...

An active period of weather continues across the West as a trough
persists offshore shedding impulses and accompanying IVT onto the
coast. Despite this, the general trends will be for bulging
thicknesses across the West with increasingly quiet weather
forecast through the period.

On D1, a strung out vorticity lobe accompanying a shortwave trough
will lift northeast towards the WA coast, driving a frontal system
eastward towards the shore. As this feature moves to the east, it
will weaken and elongate, but still push increasing moisture into
the region within confluent mid-level flow. The highest
accompanying IVT will move across CA and into the Great Basin, but
a broader surge of elevated PWs will shift into most of the Pacific
and interior Northwest, resulting in a shield of precipitation
lifting across the area. Snow levels west of the Cascade crest will
rise to around 4000 ft, but remain much lower to the east,
especially across interior WA state at just 1500-2500 ft, but then
rise again to around 3000 ft in the Northern Rockies. Where the
best overlap of synoptic lift (through height falls, downstream
divergence, and upper level diffluence) and upslope flow combine,
likely in the Shasta/Siskiyou region, the OR Cascades, and as far
east as the Salmon Rivers and Tetons. In these ranges and above the aforementioned snow levels, WPC probabilities D1 for more than 8
inches are generally 70-90%, with locally as much as 2 feet possible.

As this first wave sheds inland, it will be quickly followed by
another, more impressive surge of IVT (GEFS and ECENS probabilities
for >500 kg/m/s as high as 80%). This secondary plume will traverse
more quickly north as a low-level trough axis and following warm-
front extending from another approaching shortwave pushes from
northern CA into WA state. This surge will drive snow levels
rapidly upward, reaching as high as 8000 ft by the end of D2 west
of the Cascades, but even 5000-7000 ft as far east as the northern
Rockies and the Great Basin. This will cause a rapid decrease in
snow coverage while SLRs also fall to become much heavier (also
reflected by increasing snow load probabilities in the WSSI-P). WPC probabilities D2 are highest in the WA Cascades and Northern
Rockies where they reach 70-90% for an additional 6+ inches, with
lighter additional accumulations expected across the Salmon Rivers,
Sawtooth, and Tetons.

During D3 most of the ascent and accompanying moisture shifts
eastward into the High Plains, bringing a brief reprieve to
snowfall in the Northwest. However, early in the period /Wednesday
morning/ some light snow is likely in the higher terrain of the WA
Cascades, Okanogan Highlands, and Northern Rockies, where WPC
probabilities are moderate (50-70%) for 4+ inches of additional
snowfall. Potentially more problematic early D3 will be the threat
of some freezing rain along and east of the Cascades, including
some of the passes. The deterministic icing amounts from the
guidance are quite low, as are WPC probabilities for even 0.01
inches of ice (just 10-30%), but any light icing could be
problematic for the Wednesday morning commute.

...Northern Plains into the Upper Midwest... Day 2...

A modest mid-level impulse will traverse southeast from the
Northern Rockies to the Western Great Lakes Tuesday, producing
sharp but temporally short height falls, combined with increasing
LFQ diffluence as a jet streak pivots across the region. The trends
in the guidance have been for this jet streak to be more intense
and more strongly poleward arcing, while also being a bit
displaced to the south. Although moisture downstream of this
shortwave will be modest, it will likely be effectively wrung out
as fgen intensifies directly into the deepening DGZ, supporting
snowfall rates in a narrow channel that may exceed 1"/hr. The
system is progressive, but the briefly heavy snow should cause some
moderate accumulations and modest impacts. WPC probabilities are
currently less than 10% for 4+ inches (but 50-70% for 2+ inches),
but it is possible some higher snowfall will occur with this band.

...Northeast & Eastern Great Lakes... Day 3...

Guidance starting to converge on a solution which will bring a
progressive but impactful winter storm to the interior Northeast
Wednesday night and Thursday. Dual shortwaves, one lifting out of
the Missouri Valley Wednesday, and a second northern stream impulse
diving across the Great Lakes may interact or phase near 12Z
Thursday (end of D3) over Upstate New York. This will occur along
an eastward racing cold front, with downstream and intensifying
warm advection spreading moisture northward as an expanding
precipitation shield from the eastern OH VLY into New England.
It appears through the OH VLY and Mid-Atlantic, most of the
precipitation will be rain as the column dries before cold air can
catch the moisture.

However, across interior PA, NY, and northern New England, the
risk for heavy snow is increasing as the two shortwaves phase and
produce secondary cyclogenesis off the coast. At this time there
continues to be considerable timing and spatial spread of this
secondary low, leading to wide fluctuations in snowfall potential.
However, the setup should result in a pronounced band of heavy snow
through WAA/Fgen, resulting in an axis of heavy accumulations.
Where this is going to occur, and with what intensity, remains very
uncertain as reflected by DESI LREF plumes and very low WSSI-P
probabilities for moderate impacts. However, current WPC
probabilities have increased, and feature a 50-70% chance of at
least 4 inches of snow, greatest across the high terrain of the
Adirondacks, Greens, and Whites.

...Northern Plains... Day 3...

Zonal flow across Western Canada will become suppressed in response
to a sharpening shortwave which will amplify as it digs east of the
Northern Rockies and reaches the Dakotas before the end of the
forecast period. This shortwave is progged to remain progressive,
but should deepen substantially and will work in tandem with a
digging jet streak to drive pronounced ascent Wednesday night into
eastern MT and the Dakotas. This synoptic lift moving atop a
baroclinic gradient along a weak front will help launch
cyclogenesis, and a strong clipper type low is likely to dive into
the region D3. Impressive WAA downstream of this feature will help
deepen the DGZ (SREF probabilities for > 50mb of depth increase to
70%), which should work together with the ageostrophic response to
the jet streak to produce a band of impressive fgen. The column is
quite cold at this time and the best fgen may reside above the
DGZ, but a band of heavy snow is becoming more likely Wednesday
night. A lot of details will still need to be ironed out, but
current WPC probabilities are moderate (50-70%) for 4+ inches of
snow across northern ND, with additional moderate to heavy snow
likely to continue downstream through D4.

Weiss
$$
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FOUS11 KWBC 170746
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 AM EST Tue Dec 17 2024

Valid 12Z Tue Dec 17 2024 - 12Z Fri Dec 20 2024

...Pacific Northwest through the Northern Rockies... Days 1-2...

Shortwave ridging over the Pacific Northwest will quickly be
displaced by an approaching trough moving eastward from the
Pacific leading to height falls along the coast. While the core of
this feature will lift into British Columbia, a secondary vorticity
maxima will pivot onshore NW WA state Tuesday night with enhanced
ascent through PVA, height falls, and concurrent upper level
diffluence. This impulse will move quickly eastward, but will be
accompanied by a lead warm front and trailing cold front, causing
fluctuations in snow levels.

The heaviest precipitation is likely to accompany the warm front,
generally 18Z Tuesday through 12Z Wednesday, with a secondary surge
along the trailing cold front Wednesday morning. During the warm
frontal passage and accompanying WAA/moisture surge, snow levels
are progged to climb to as high as 8000 ft along and west of the
Cascades. This will limit significant snowfall to just the higher
terrain, with rain the primary p-type at the passes. However,
during the transition from cold to warm, a period of freezing rain
is possible even as low as Snoqualmie and Stevens Passes, creating
hazardous travel this evening before changing to all-rain. East of
the Crest, cold air locked in within Canadian high pressure will
allow for more substantial freezing rain/ice accretion through
early Wednesday. The subsequent cold front will produce primarily
rain, as snow levels crash behind it but occur with rapid drying as
well.

For the areas that get snow and ice accumulations, moderate impacts
are expected as reflected by the WSSI-P showing a high chance

80%) for moderate impacts in the WA Cascades and into the

foothills. Here, WPC probabilities for 0.1 inches of ice are
generally 10-30%, and 70-90+% for 6 or more inches of snow. By D2
as the cold front shifts east, additional moderate snowfall
accumulations are likely in the Northern Rockies where WPC
probabilities are 30-50% for 6+ inches.


...Northern Plains into the Upper Midwest... Day 1...

A compact and fast moving shortwave will eject from the Northern
Rockies early Tuesday and then race eastward towards the Great
Lakes. This feature will remain of low amplitude, but be
accompaniedby a potent vorticity streamer to enhance otherwise
modest mid- level ascent. More impressive will be a strengthening
jet streak beginning to arc poleward immediately downstream of the
shortwave trough axis. Together this will produce an narrow
corridor of intense ascent which can support a heavy snow band
moving generally west to east from SD through southern MN and into
WI. The most intense ascent should occur during the daylight hours,
but a deepening DGZ (SREF 100mb of depth probabilities reaching
50%) which is aligned with the greatest ascent through 700-600mb
fgen, will support heavy snow rates that could exceed 1"/hr at
times. The progressive nature of this will limit snowfall totals,
but WPC probabilities for 2+ inches are above 70% in a stripe
across eastern SD into SW MN, with locally 4-6" probable as
reflected by HREF max ensemble output in the most intense snow
banding.

Days 2-3...
After this first wave exits, a brief respite will occur before a
more pronounced system digs out of Canada and dives SW into the
Northern Plains. This will be driven by a potent shortwave dropping
from the Canadian Rockies and into North Dakota by Thursday
morning, with the primary ascent efficiently overlapped with the
LFQ of a strengthening jet streak also digging into the region.
This deep layer ascent will impinge into a low-level baroclinic
boundary as a warm front drapes eastward, resulting in rapid
cyclogenesis in eastern MT, with this low then moving into ND and
then into the Great Lakes by the end of D3.

As this wave moves eastward and deepens, WAA/isentropic ascent will
begin to intensify between 280K-285K on Thursday surging moisture
into and downstream of the system, reflected by an axis of PWs
exceeding the 90th percentile according to NAEFS. At the same time,
this WAA will occur favorably into an extremely cold column to
deepen the DGZ to more than 100mb, and intensifying fgen will drive
pronounced ascent into this DGZ. The deep DGZ, a sub-DGZ isothermal
layer, and modest winds in a very cold column suggest SLRs will be
quite high, and this will rapidly accumulate despite the general
progressive nature of this system. At this time, WPC probabilities
D2 are high (70-90%) for more than 4 inches across much of northern
ND, with a stripe of 30-50% probabilities for 4+ inches extending
as far east as the Door Peninsula of WI. Locally, 6-8" of snow is
possible across ND during the event.


...Northeast & Eastern Great Lakes... Days 2-3...

Surface low pressure will develop near the TN VLY Wednesday morning
in response to an elongated shortwave diving from the Central
Plains across the region. This shortwave will be accompanied by
modest height falls/PVA to drive ascent, and interact with the RRQ
of a distant but still noteworthy jet streak pivoting over the
eastern Great Lakes. This low will then lift northeast along a cold
front, while a secondary, and more intense, northern stream
shortwave move over the Great Lakes and into the Northeast. These
features are likely to interact across the Northeast, leading to
secondary low pressure developing off the New England coast and
deepening as it moves into Canada.

Moisture associated with these waves will surge northward on rich
theta-e advection, resulting in PWs which are progged by NAEFS to
exceed the 90th percentile in the CFSR database. This will allow
for widespread precipitation to become heavy beginning around 00Z
Thursday, with wintry precipitation spreading across interior
portions of the northeast. While there still remains some
longitudinal spread in the placement of this low as it strengthens
near New England, the ensemble clusters have begun to focus a bit
farther to the east. This will create an environment that is colder
and more supportive to wintry precipitation, especially as the low
pulls away Thursday morning. The passes of the front combined with
the isallobaric flow into the surface low will help enhance fgen as
well, which when overlapping the increased deformation NW of the
deepening low/interacting shortwaves will likely lead to some heavy
snow rates across interior New England. However, the column as a
whole still appears generally marginal for snow, so a heavy, wet
snow, with low SLRs is likely except in the highest terrain.

The progressive nature and low SLR nature of this system will limit
total snowfall amounts, but heavy accumulations are still possible,
especially in the higher terrain which could result in at least
modest impacts due to snow load. This is reflected by WPC
probabilities that feature a moderate risk (50-70% chance) of more
than 4 inches in the highest terrain of the Adirondacks, Greens,
and Whites, with lesser accumulations extending through much of
northern New England except in the lowest valleys. Locally, 12" of
snow is possible in the highest terrain of the Whites near Mt.
Washington.

Weiss
$$
d
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FOUS11 KWBC 180752
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
252 AM EST Wed Dec 18 2024

Valid 12Z Wed Dec 18 2024 - 12Z Sat Dec 21 2024

...Pacific Northwest through the Northern Rockies...
Day 1...

Compact and fast moving but potent shortwave will lift eastward
from the WA coast into the Northern Rockies today through tonight.
This feature will help drive a surface low along the international
border with Canada, pushing a warm front downstream, and then
dragging a cold front from west to east in its wake. The enhanced
ascent in the vicinity of this frontal system will act upon robust
moisture (PW anomalies +2 sigma according to NAEFS) to wring out
some heavy precipitation from the Olympics through the Cascades
and into the Northern Rockies. Snow levels will steadily rise
behind the warm front, reaching as high as 8000 ft,
limiting significant snowfall to just the highest terrain, band WPC probabilities reach 50-70% for 6+ inches in the northern WA
Cascades and Northern Rockies near Glacier NP. Although snow levels
will crash behind the cold front, this will be coincident with a
rapid drying of the column, so any residual snowfall should be light.


...Mid-Atlantic and Northeast...
Days 1-3...

A wave of low pressure will develop across the TN VLY and then race
northeast along a cold front, with secondary wave development
likely east of Maine Wednesday night. Moisture along and ahead of
this system will increase, with PWs surging towards the 90th
climatological percentile, highest east, as WAA intensifies along a
northward advancing warm front. The antecedent airmass is
marginally conducive for wintry precipitation, and without any
strong cooling from the north, locations along and east of this
low track will likely remain all rain, with just a small temporal
window for changeover back to snow as the low pulls away Thursday.

The exception will be from the Laurel Highlands through the
Adirondacks, and across much of northern New England where, despite
still a marginal atmospheric column, the precipitation should fall
primarily as snow, except in the lower valleys. A heavy and wet
(low SLR) snow is likely, which when combined with the fast
motion of this wave will keep snowfall amounts modest, and WPC
probabilities D1 above 30% for more than 4 inches of snow are
confined to the Adirondacks, Greens, Whites, and mountains of
north/central ME.

As the low moves away Thursday, some modest upslope/lake-effect
snow may occur, but in general precip intensity and coverage will
wane during Thursday. This sets the stage for an interesting
development on Friday. A shortwave digging from the Midwest and
embedded within the larger trough will sharpen and may become
negatively tilted over the Mid- Atlantic Friday, while a surface
low develops well offshore and downstream of this larger scale
trough. The guidance has trended a bit deeper with this shortwave,
leading to greater interaction with the larger low pressure
offshore. A lot of uncertainty remains, but if these systems can
interact, the overlap of moisture and some more intense ascent
due to deformation/height falls, could result in widespread, at
least light, snow across much of the Mid-Atlantic and Northeast.
Current WPC probabilities for more than 4 inches of snow are modest
at just 10-30% across much of Upstate NY, the Poconos, and eastern
MA/ME, but this event will need to be monitored with recent model
runs for the potential for heavier snowfall in some areas.


...Northern Plains to the Midwest...
Days 1-2...

A shortwave amplifying over the Canadian Rockies will dig southeast
on Wednesday moving into the Northern Plains by Thursday morning
and then continue through the Upper Midwest and then reach the
Mid-Atlantic early on Friday. As this shortwave digs and amplifies,
it will be accompanied by a sharpening jet streak to drive deep
layer ascent through overlapping height falls, PVA, and diffluence.
This synoptic lift impinging on a low-level baroclinic gradient
will result in cyclogenesis, with this low diving progressively
southeast through D1 and D2.

Downstream, moisture will begin to enhance through the region as
impressive 280-285K isentropic lift drives rich theta-e northward,
with weak TROWAL development possible on Thursday. This overlap of
moisture and ascent produces an expanding swath of snow, with the
attendant WAA surging north to deepen the DGZ while concurrently
producing a corridor of impressive fgen. The column will be
extremely cold, so the presence of a deepening DGZ with strong fgen
should cause fluffy and above-climo SLRs which will accumulate
rapidly as reflected by the WPC prototype snowband tool suggesting
1"/hr rates moving across ND and into MN. The progressive nature
of the low will somewhat minimize the potential for significant
snowfall amounts, and there remains considerable latitudinal spread
by D2, but WPC probabilities suggest a moderate risk (50-70%) in a
stripe from northern ND into eastern WI for more than 6 inches of snow.

Weiss
$$
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FOUS11 KWBC 190856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Thu Dec 19 2024

Valid 12Z Thu Dec 19 2024 - 12Z Sun Dec 22 2024

...Midwest to Great Lakes...
Days 1-2...

A progressive Alberta Clipper producing a swath of heavy snow over
North Dakota tonight will track across the Upper Midwest today.
Modest 850mb WAA along the warm front and 850mb theta-e wrapping
around the northern flank of the storm will prompt a snow to come
down at 1-2"/hr rates in some cases from eastern ND to central
Minnesota and central Wisconsin. The higher end of those rates will
be harder to come by east of the Mississippi River as 850mb
frontolysis sets in, leading to a reduction in snowfall rates.
Still, the 500mb shortwave trough responsible for upper level
ascent will still preside over the region and allow for periods of moderate-to-heavy snow through Thursday evening. In fact, some
snowfall could be lake-enhanced along the shores of Wisconsin and
Illinois as northerly winds advecting -10C 850mb temps race over
Lake Michigan. By Thursday night, the 850mb low will track through
northern Indiana with the best accumulating snow potential to the
north of the 850mb low track. This favors central and northern
Michigan Friday morning with a chance for the Detroit metro area to
see light accumulations Friday afternoon. The storm system will
gradually weaken into an open wave Friday afternoon as it
approaches the central Appalachians, effectively ending the period
of snowfall related to this Clipper in the Great Lakes by Friday evening.

The WSSI shows a large swath of Minor Impacts that stretch from
North Dakota and far northern South Dakota to as farther east as
northern Michigan. There are some embedded Moderate Impact areas,
primarily in parts of central North Dakota, around the Twin Cities
metro, and in eastern Wisconsin. Moderate Impacts imply these
areas can expect hazardous driving conditions with some potential
closures and disruptions to infrastructure. WPC probabilities sport
high chances (>70%) for snowfall totals >4" from central MN to the
Door Peninsula of Wisconsin. It is worth noting that there are
some localized low chance probabilities (10-30%) for snowfall
totals >8" in central MN and central WI, as well as around the
Green Bay area where lake-enhanced snowfall could result in
localized amounts approaching 10 inches.

...Central Appalachians to Northeast....
Days 2-3...

As the Alberta Clipper tracks through the Lower Great Lakes Friday
morning, a secondary 500mb vort max that raced south from south-
central Canada will begin to phase with the shortwave associated
with the Clipper. This process, starting as early Thursday night,
will lead to an amplification of the 500mb trough as it tracks
towards the Central Appalachians Friday morning. Meanwhile, off the
East Coast, increasing upper level divergence along a frontal
boundary will allow for low pressure to quickly organize and
strengthen Friday afternoon. As WAA increases along the front,
moisture will wrap around the low and lead to broad precipitation
shield just off the Northeast coast Friday night. This could result
in heavy snow along Downeast Maine, but how close to the Maine
coastline the storm gets remain lower in confidence. WPC
probabilities suggest the eastern-most portion of Downeast Maine
has low-to-moderate chances (30-50%) for snowfall >6" of snow
through Saturday evening. In addition, high pressure over Quebec
will work in tandem with the deepening storm system to tighten the
pressure gradient and strengthening low-level northerly winds off
the MA Capes. There is the potential for ocean-effect snow over
southeast MA Friday evening. WPC probabilities show low chances
(10-3) for snowfall >4", which does include the Boston metro area.

Throughout the rest of the Northeast, western NY and northern PA
will see the clipper's remnant 850mb circulation track overhead
with modest 850-700mb moisture and weak 850mb WAA will supply the
ingredients necessary for periods of snow late Thursday night and
into Friday. By Friday afternoon, the forecast over parts of the
northern Mid-Atlantic will hinge upon the strength of low-level
easterly flow and a convergence axis over the region, while
sufficient upper level divergence from the left-exit region of a
500mb jet streak arrives. Should these factors mesh together in
just the right way, a pivoting band of snow may organize some where
between northeast MD or the northern DelMarVa Peninsula on
northeast through eastern PA, the Poconos, and into the Tri-State
area. The latter is most favored for accumulating snowfall as WPC
probabilities show moderate chances (40-60%) for snowfall totals
surpassing 2" between Friday afternoon and early Saturday morning.

If there is one mountain range that is favored to witness heavier
snowfall, it is the central Appalachians in eastern WV and western
MD. As the remnant surface low of the Clipper heads east Friday
evening, its moisture source will track into the mountains at the
same time as northwesterly winds begin to increase. The end result
is upslope flow that causes periods of snow as far north as PA's
Laurel Highlands. WPC probabilities favor those windward slopes of
eastern WV with moderate chances (40-60%) for snowfall >4",
especially at elevations above 2000ft between 18Z Fri - 18Z Sat.
For all these areas mentioned in this section, the WSSI shows Minor
Impacts for impacted areas that would primarily lead to locally
hazardous travel conditions through Saturday morning.


The probabilities for significant ice accumulations are less than
10 percent.

Mullinax
$$
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FOUS11 KWBC 200803
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Fri Dec 20 2024

Valid 12Z Fri Dec 20 2024 - 12Z Mon Dec 23 2024

...Great Lakes...
Day 1...

The gradually weakening clipper system will track across the Lower
Great Lakes this morning with lingering periods of snow across
much of the region, although most totals will be on the light side.
As the storm tracks east towards the Mid-Atlantic, northerly winds
will accelerate over Lakes Superior, Michigan, and Huron and lead
to a handful of lake-effect streamers containing occasional bursts
of heavy snow on Friday. Latest WPC probabilities show low chances
(10-20%) for snowfall totals >4" in parts of northwest Indiana,
near Traverse City, MI, and along the coastal areas of Michigan's
thumb. Otherwise, additional snowfall totals will generally hover
around 1-3" in parts of Michigan, Indiana, and western Ohio.

...Central Appalachians to Northeast....
Days 1-2...

As the remnant circulation of the weakening clipper system
approaches the central Appalachians, its residual moisture will
flow directly into the mountain range Friday evening. By Friday
night, as low pressure off the East Coast deepens and high pressure
builds in from the north, low-level NW winds will be favorably
oriented into the Central Appalachians with some periods of snow as
far south as the Smokeys. Upslope ascent will continue into the
central Appalachians through Friday night and gradually taper off
by Saturday afternoon. Snowfall totals in east-central WV will
likely range between 1-4" with localized totals up to 6" possible
in the tallest peaks of the WV Appalachians and the Smokeys.

Farther east, an area of low pressure will organize along a
strengthening frontal boundary off the East Coast this morning.
This low will strengthen as low-level WAA to its north and a jet
streak couplet takes shape over the Gulf Stream. Meanwhile, broad
upper level divergence out ahead of the 250-500mb mean trough axis
will supply sufficient ascent aloft in tandem with the residual
moisture from the clipper system to support periods of snow over
northern PA and western NY. Colder temperatures aloft supporting
higher SLRs will also support As the low off the East Coast
deepens, easterly flow is expected to strengthen and a low level
convergence trough will setup over the northern Mid-Atlantic coast.
Latest CAMs guidance is keying on this convergence boundary
setting up over the Delaware Valley and stretching north into the
Poconos. WPC probabilities do show moderate chances (40-60%) for
snowfall >4" in parts of the Poconos, while there are low-to-
moderate chances (30-50%) for snowfall >2" in parts of the Lehigh
Valley, Delaware Valley, and into northern New Jersey. Localized
amounts in the highest elevations of the Poconos have low chances
(10-20%) for snowfall amounts topping 6". There is a chance for
minor accumulations (coating-2") along I-95 from the Philadelphia
metro on north through the NYC and Tri-State metro regions this afternoon

Coastal New England sports the higher "boom" scenarios given the
combination of ocean-enhanced snowfall over eastern MA Friday
afternoon and the deformation axis of the coastal storm extending
far enough west to produce heavy snow over Downeast Maine Friday
night into early morning Saturday. Latest guidance is starting to
come into better agreement with a swath of 1-3" of snowfall over
eastern MA with some CAMs suggesting higher end totals (>4") within
the range of possibilities. Downeast Maine could feature the
heaviest snowfall for the event in coastal New England with WPC
probabilities showing moderate-to-high chances (50-70%) for
snowfall >4" through Saturday morning.

...Pacific Northwest...
Days 2-3...

A pair of Pacific storm systems will bring rounds of high
elevation mountain snow to the Olympics, Cascades, Blue, and
Sawtooth Mountains this weekend. Snow levels will generally be as
low as 4,000ft in the Cascades, but the heavier snowfall totals

6") will likely be confined to elevations >5,000ft in the

Olympics and Cascades. In the Blue and Sawtooth Mountains, >6"
snowfall totals will likely be confined to elevations >6,000ft.
These heavier totals will generally be above pass level in these
mountain ranges, so expect the more hazardous travel impacts to be
in the more remote and complex terrain of these mountain ranges
this weekend.


The probabilities for significant ice accumulations are less than
10 percent.

Mullinax
$$
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FOUS11 KWBC 210737
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 AM EST Sat Dec 21 2024

Valid 12Z Sat Dec 21 2024 - 12Z Tue Dec 24 2024

...Northeast...
Day 1...

Periods of snow on the backside of a departing winter storm
heading for Nova Scotia is expected to linger over Downeast Maine through Saturday morning before finally concluding Saturday afternoon. WPC probabilities show low chances (10-30%) for additional snowfall
totals >2" through Saturday morning. Farther west, cyclonic flow
over the Great Lakes will keep some lake-enhanced bands lingering
through the day with additional snowfall amounts of 1-3" expected
in parts of northeast OH, near the Finger Lakes of NY, and as far
south as the central Appalachians. Snow showers will taper off by
Saturday evening as high pressure builds in from the west.

...Pacific Northwest & Northern Rockies...
Days 1-3...

A trio of Pacific storm systems will escort rounds of Pacific
moisture into the Olympics and Cascades into early next week. Some
of this moisture will spill over into the Northern Rockies. Snow
will generally be confined to the higher elevations of these
mountain ranges due to flood of Pacific air infiltrating much of
western North America and keeping any frigid Canadian air-mass
intrusions at bay. For the Olympics and Cascades, the heaviest
snowfall is likely to be located at/above 4,500ft in elevation. The
first storm system arrives Saturday morning with locally heavy
snowfall possible as far south as the tallest peaks of northern
California. The heaviest snowfall from this event for the Cascades,
Olympics, and Blue Mountains comes Saturday afternoon and tapers
off Saturday night thanks to the storm's progressive movement.
Following a brief break Sunday morning, the next round of snowfall
arrives Sunday afternoon in the Cascades with the heaviest
snowfall occurring Sunday night. Then, following another break
during the day Monday as high pressure briefly builds in, snow
returns to the Olympics and Cascades Monday night. Guidance
suggests sharper height falls aloft and a weak CAD signature in
the Columbia Basin that may result in lower elevations snow/ice
east of the Cascades. Through 12Z Tuesday, WPC probabilities show
high chances (>70%) for snowfall >8" in elevations >5,000ft in
parts of the Olympics, Cascades, and as far east as the Blue Mountains.

Some of this Pacific moisture will also result in high elevation
snow in parts of the Bitterroots, Sawtooth, Boise, Absaroka, Lewis, and
Teton Mountains. Of the ranges referenced, the Sawtooth and Teton
Ranges above 7,000ft feature moderate-to-high chances (50-70%) for
snowfall totals >8" through early Tuesday morning.


...Northern Plains to the Great Lakes...
Days 2-3...

The first in a series of Pacific Northwest storm systems will make
its way into Montana while the divergent left-exit region of a
110kt 250mb jet streak moves in over the Northern Plains. Southerly
flow at the lowest levels of atmosphere will intersect a frontal
boundary that gives way to weak vertical ascent over parts of North
Dakota and central Minnesota. The moisture source and DGZ aloft is
marginal, while 850mb temps rise >0C despite surface temps
remaining below freezing. This should lead to light icing
accumulations from northeast Montana and northern North Dakota to
central Minnesota Sunday afternoon and into Sunday night.

By Monday morning, a more consolidated surface low will form in
the Upper Mississippi Valley at the same time 925-850mb moisture
(with origins out of the Gulf of Mexico) streams into the Great
Lakes. 850mb WAA over a boundary layer air-mass that is cold and
quite dry will support wet-bulb temperatures that support mostly
snow from the Michigan U.P. to the northern half of Michigan's
Mitten. Broad 250-500mb troughing will also provide adequate lift
atop the atmosphere, giving rise to a more solid shield of
snowfall over the region. WPC probabilities show moderate chances
(40-60%) for snowfall totals >4" in the far northern portions of
Michigan's Mitten and in the eastern-most areas of Michigan's U.P.
In fact, there are some low-chance probabilities (10-30%) for
localized amounts >8" in the eastern Michigan U.P.

This same moisture source, synoptic-scale forcing, and isentropic
glide will translate over the eastern Great Lakes Monday night
with potentially 1-3" of snowfall along the Chautauqua Ridge and
along the Tug Hill. Localized amounts could top 4" along the Tug
Hill where some upslope enhancement would allow for slightly
heavier snowfall rates and thus higher totals than their neighbors
in western NY through Tuesday AM.


The probabilities for significant ice accumulations are less than
10 percent.

Mullinax
$$
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FOUS11 KWBC 220800
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Sun Dec 22 2024

Valid 12Z Sun Dec 22 2024 - 12Z Wed Dec 25 2024

...California, Pacific Northwest, & Intermountain West...
Days 1-3...

A pair of atmospheric rivers (AR) are going to make for an active
multi-day stretch of weather from California on north through the
Northwest and into the Rockies. The first AR arrives Sunday
afternoon, which NAEFS shows will top 750 kg/m/s off the Oregon
coast Sunday morning, will weaken on approach but still deliver

90th climatological percentile precipitable water values into the

northwestern U.S.. This AR will neither be accompanied by an
unusually cold air-mass, nor will it be directed at a continental
polar air-mass over the Northwest. Snow levels will initially be
as low as 4,000ft in the Cascades and Olympics, but rise to above
5,000ft by Sunday night as WAA increases aloft. This same streams
of moisture will advance inland through the Northern Rockies Sunday
night and into Monday with locally heavier snowfall possible in the

5,000ft peaks of the Blue Mountains, above 7,000ft in the Salmon

River and Sawtooth Mountains of Idaho, and the Tetons in Wyoming.
The peaks of the Tetons above 8,000ft sport moderate-to-high
chances (50-70%) for snowfall totals >6", while the Blue, Salmon
River, and Sawtooth show similar probabilities for >4" through
Monday evening.

By Monday afternoon, the next AR is already getting ready to strike
the Pacific Northwest coastline, but this AR is noticeably stronger
than the one arriving late Sunday. NAEFS shows a larger 750 kg/m/s
fetch (above the 99th climatological percentile off the northern
CA coast) with origins stemming out of the subtropical Pacific. The
initial round of precipitation arriving Monday evening will be
primarily rain in western OR and northern CA (snow levels as high
as 7,000ft), but snow will be most likely in the Cascades and Blue
Mountains. By Tuesday morning, sharper height falls from the
approaching upper trough will force snow levels to drop to as low
as 4,000ft in the Cascades, Olympics, and Blue, while farther
south, snow levels fall to ~5,000ft in the CA ranges and northern
Great Basin. Unlike Sunday's AR, Tuesday's AR will have a higher
longevity with >90th climatological percentile IVTs extending as
far inland as the Wasatch and on south to the Mogollon Rim Tuesday
night. Mountain snow will be common throughout much of the
Intermountain West Tuesday night with heavy snow advancing into the
Tetons, Uinta, and Wasatch by early Wednesday morning.

WPC probabilities depict high chances (>70%) for snowfall >6" for
the central Sierra Nevada above 7,000ft. Farther north, moderate
chances (40-60%) for snowfall >4" are shown in the higher terrain
of the Blue, Salmon River, and Sawtooth ranges. Lastly, the Wasatch
and Tetons show low chances (10-30%) for snowfall >4" above
8,000ft through 12Z Wednesday, but additional snowfall is possible
Wednesday afternoon. In total through 12Z Wednesday, there are high
chances (>70%) for snowfall totals >12" for elevations above
5,000ft in the Cascades, while similar high chance probabilities
for >8" of snow are present in the Blue, Salmon River, Sawtooth,
Tetons, and central Sierra Nevada.


...Northern Plains, Great Lakes, to Northeast...
Days 1-3...

A progressive 500mb shortwave trough over western Montana this
morning will be the catalyst for the next winter storm to track
through the Midwest today, the Great Lakes on Monday, and the
Northeast Monday night into Tuesday morning. The initial wave of
low pressure will be quite weak on Sunday, but modest upper level
ascent aided by a 110kt 250mb jet streak's left-exit region over
the Northern Plains. In addition, the moisture source and DGZ are
marginal while low level temperatures above the surface are >0C.
This is an icy wintry mix setup from northeast Montana through
central North Dakota and into central Minnesota Sunday and into
Sunday night. WPC probabilities shows moderate-to-high chances for
ice accumulations >0.01" for much of central North Dakota. The
potential for light icing extends as far east as eastern Wisconsin,
northern Illinois, southern Michigan, and northern Indiana by
Monday morning.

Monday morning is when the storm system tracking through the Great
Lakes becomes more organized, while at the same time generating a

300 kg/m/s IVT (>90th climatological percentile values via NAEFS)

oriented at Michigan and the eastern Great Lakes. An expansive
area of 850-700mb WAA over a boundary layer air-mass that remains
quite cold and dry allows wet- bulb temperatures to support snow
being the primary precipitation type for much of Michigan's Mitten
and the eastern Michigan U.P.. Combined with excellent upper level
divergence courtesy of a broad 250-500mb trough to the west, and
there is likely to be a swath of heavy snow over northern Michigan
and the far eastern Michigan U.P.. Residents in souther Michigan
should anticipate some wintry precipitation, although it remains
unclear if snow or a wintry mix (causing light ice accumulations
in the process) will be dominant precipitation types. Latest WPC
probabilities show moderate-to-high chances (50-70%) for snowfall
totals >4"in northern Michigan and over the thumb of Michigan,
while some parts of northern Michigan also have moderate chances
(40-60%) for localized totals surpassing 6" through Monday afternoon.

This same storm system will direct its anomalous moisture source
and modest low-level WAA into the Northeast Monday night.
Sufficient upper-level divergence provides support for the healthy
shield of precipitation throughout the region, while the air-mass
remains quite cold and dry ahead of the storm (temps in the teens,
dew points in the single digits). Higher SLRs of 12-16:1 are
anticipated in these areas and the ground is very cold, which
combined with the event occurring at night, will maximize snowfall
accumulation potential. The storm races off over Nova Scotia by
midday Tuesday and snowfall should taper off by Tuesday afternoon.
WPC probabilities do show moderate-chance probabilities (40-60%)
for snowfall >4" in parts of northern NY (including the Tug Hill)
and in the tallest peaks of the Green mountains. Should guidance
continue to trend wetter, it is possible for an increase in
snowfall amounts in subsequent forecast. Downeast Maine also sports
similar moderate-chance probabilities for >4" of snowfall through
Tuesday morning. For those affected areas listed above, the WSSI-P
does sport moderate-chance probabilities (40-60%) for Minor Impacts
from northern Michigan and northern NY to Downeast Maine.

Mullinax
$$
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FOUS11 KWBC 222036
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Sun Dec 22 2024

Valid 00Z Mon Dec 23 2024 - 00Z Thu Dec 26 2024

...California, Pacific Northwest, & Intermountain West...
Days 1-3...

As the first of a pair of atmospheric rivers (AR) causing active
weather continues moving ashore in northern CA, OR, and WA,
significant precipitation associated therewith should diminish into
tonight as the moisture plume shears apart over the interior
Pacific Northwest. GEFS integrated vapor transport (IVT) analysis
shows this first AR has topped at about 600 kg/m/s off the Oregon
coast this morning. This AR will neither be accompanied by an
unusually cold air-mass, nor will it be directed at a continental
polar air-mass over the Northwest. Snow levels are currently over
7,000ft as per 18Z NBM analysis from the Cascades west. As the
precipitation moves inland, snow levels will remain above 6,000 ft
into Idaho and the interior Northwest. These very high snow levels
will confine any significant snowfall through Monday evening to the
highest peaks of the northern Washington Cascades and the Blue,
Salmon River, Tetons, and Sawtooth Ranges. The peaks of the Tetons
above 8,000ft sport moderate- to- high chances (50-70%) for
snowfall totals >6", while the Blue, Salmon River, and Sawtooth
show similar probabilities for >4" through Monday evening.

By Monday afternoon, the next AR will be moving into the Pacific
Northwest coastline, especially northern California. This AR is
noticeably stronger than the one moving ashore now. NAEFS shows a
larger 750 kg/m/s fetch (above the 99th climatological percentile
off the northern CA coast) with origins stemming out of the
subtropical Pacific. The initial round of precipitation arriving
Monday evening will be primarily rain in western OR and northern CA
(snow levels as high as 7,000ft), but snow will be most likely in
the Cascades and Blue Mountains. By Tuesday morning, sharper height
falls from the approaching upper trough will force snow levels to
drop to as low as 4,000ft in the Cascades, Olympics, and Blue,
while farther south, snow levels fall to ~5,000ft in the CA ranges
and northern Great Basin. Unlike today's AR, Tuesday's AR will
have a higher longevity with >90th climatological percentile IVTs
extending as far inland as the Wasatch and on south to the Mogollon
Rim Tuesday night. Mountain snow will be common throughout much of
the Intermountain West Tuesday night with heavy snow advancing
into the Tetons, Uinta, and Wasatch by early Wednesday morning.

WPC probabilities depict high chances (>70%) for snowfall >6" for
the central Sierra Nevada above 7,000ft. Farther north, moderate
chances (40-60%) for snowfall >4" are shown in the higher terrain
of the Blue, Salmon River, and Sawtooth ranges. Lastly, the Wasatch
and Tetons show low chances (10-30%) for snowfall >4" above
8,000ft through 12Z Wednesday, but additional snowfall is possible
Wednesday afternoon. In total through 12Z Wednesday, there are high
chances (>70%) for snowfall totals >12" for elevations above
5,000ft in the Cascades, while similar high chance probabilities
for >8" of snow are present in the Blue, Salmon River, Sawtooth,
Tetons, and central Sierra Nevada. Snow will continue into Utah and
Colorado through the day on Wednesday with a 30-50% chance of 3-6
inches of snow through Wednesday night.


...Northern Plains, Great Lakes, to Northeast...
Days 1-3...

A progressive 500mb shortwave trough over western Montana this
morning will be the catalyst for the next winter storm to track
through the Great Lakes on Monday and the Northeast Monday night
into Tuesday morning. The initial wave of low pressure is quite
weak, with modest upper level ascent aided by a 110kt 250mb jet
streak's left-exit region over the Northern Plains. In addition,
the moisture source and DGZ are marginal while low level
temperatures above the surface are above freezing. This is an icy
wintry mix setup from northeast Montana through central North
Dakota and into central Minnesota into tonight. WPC probabilities
shows moderate-to-high chances for ice accumulations >0.01" for
much of west-central North Dakota. The potential for light icing
extends as far east as eastern Wisconsin, northern Illinois,
southern Michigan, and northern Indiana by Monday morning.

Monday morning is when the storm system tracking through the Great
Lakes becomes more organized, while at the same time generating a

300 kg/m/s IVT (>90th climatological percentile values via NAEFS)

oriented at Michigan and the eastern Great Lakes. An expansive
area of 850-700mb WAA over a boundary layer air-mass that remains
quite cold and dry allows wet- bulb temperatures to support snow
being the primary precipitation type for much of Michigan's Mitten
and the eastern Michigan U.P.. Combined with excellent upper level
divergence courtesy of a broad 250-500mb trough to the west, and
there is likely to be a swath of heavy snow over northern Michigan
and the far eastern Michigan U.P.. Residents in southern Michigan
should anticipate some wintry precipitation, although it remains
unclear if snow or a wintry mix (causing light ice accumulations
in the process) will be dominant precipitation types. The latest
WPC probabilities show high chances (70-90%) for snowfall totals

4" in northern Michigan and over the thumb of Michigan, while some

parts of northern Michigan also have moderate chances (40-60%) for
localized totals surpassing 6" through Monday afternoon.

This same storm system will direct its anomalous moisture source
and modest low-level WAA into the Northeast Monday night.
Sufficient upper-level divergence provides support for the healthy
shield of precipitation throughout the region, while the air-mass
remains quite cold and dry ahead of the storm (temps in the teens,
dew points in the single digits). Higher SLRs of 12-16:1 are
anticipated in these areas and the ground is very cold, which
combined with the event occurring at night, will maximize snowfall
accumulation potential. The storm races off over Nova Scotia by
midday Tuesday and snowfall should taper off by Tuesday afternoon.
WPC probabilities show moderate-chance probabilities (40-60%) for
snowfall >4" in parts of northern NY (including the Tug Hill) and
in the tallest peaks of the Green mountains. Should guidance
continue to trend wetter, it is possible for an increase in
snowfall amounts in subsequent forecast. Downeast Maine also sports
similar moderate-chance probabilities for >4" of snowfall through
Tuesday morning. For those affected areas listed above, the WSSI-P
does sport moderate-chance probabilities (40-60%) for Minor Impacts
from northern Michigan and northern NY to Downeast Maine.

Wegman/Mullinax
$$
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FOUS11 KWBC 230757
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Mon Dec 23 2024

Valid 12Z Mon Dec 23 2024 - 12Z Thu Dec 26 2024

...The West...
Days 1-3...

An active pattern continues this week with mountain snow expected
from the West Coast mountain ranges to both the Northern and
Central Rockies. This stormy pattern in the short range is due to
three atmospheric rivers (ARs) bringing plume after plume of
Pacific moisture into the western U.S.. The first AR will gradually
weaken throughout the remainder of the day, but residual Pacific
moisture and a lack of a meaningfully cold air will keep most
heavy snowfall in the Northern Rockies above 7,000ft. Snow should
taper off over the Northern Rockies by Monday evening.

The second AR arrives Monday evening with IVT values topping 1,000
kg/m/s off the Oregon coast and moisture origins stemming out of
the subtropical Pacific. The initial round of precipitation
arriving Monday afternoon will be primarily rain in northern CA,
western OR, and western WA as snow levels in the OR Cascades are as
high as 8,000ft and even topping 9,000ft in northern CA. The
Cascades and Blue Mountains will have the better odds of staying
mostly snow above 5,000ft. By Tuesday morning, sharper 700-500mb
height falls from the approaching upper trough will force snow
levels to drop as low as 3,000ft at pass level in the WA Cascades
and below 6,000ft in the Sierra Nevada by Tuesday afternoon. The
Sierra Nevada have the best odds for an impactful snowfall event
with the Winter Storm Outlooks now sporting 50% odds of snowfall
exceeding warning criteria for elevations >7,000ft. Periods of
high elevation snow in the Great Basin and Wasatch are likely
Tuesday night but heavier accumulations (>4") will be mostly
confined to elevations at/above 8,000ft. Through 12Z Wednesday, WPC probabilities show moderate-to-high chances (50-70%) for snowfall

8" above 7,000ft in the Sierra Nevada, above 5,000ft in the Cascades/Olympics/Blue Mountains, and above 8,000ft in the Tetons.

The third and more expansive AR arrives Wednesday as a powerful
sub-960mb low tracks towards the British Columbia coast. NAEFS
shows this AR with moisture origins northwest of Hawaii that will
be directed at the Pacific Northwest, including IVT values topping
the 97.5 climatological percentile. Unlike the first pair of ARs,
this one will have a slightly colder air-mass to work with at the
onset while mean 700-300mb winds out of the WSW are better aligned
orthogonally to enhance upslope ascent into the Olympics and
Cascades. This is a recipe for heavy/wet snow in the Olympics and
Cascades Wednesday afternoon and into Wednesday night. Latest WPC
probabilities show high chances (>70%) for >8" of snowfall for
elevations >3,000ft, and similar chances for >12" of snowfall
above 5,000ft. The WSSI-P does show 50-70% odds for Moderate
Impacts in these ranges through Thursday AM, which would include
some of the WA Cascade passes.


...Northern Plains, Great Lakes, to Northeast...
Days 1-2...

A positively tilted 250-500mb trough over the Midwest this morning
is providing sufficient upper-level ascent over the Great Lakes and
supporting a weak area of low pressure tracking towards southern
Wisconsin. Broad 850mb WAA and 290K isentropic glide via SWrly flow
will introduce a slug of Gulf of Mexico moisture (embedded within a

300 kg/m/s IVT) into the region which maintains sub-freezing

boundary layer temperatures as the precipitation arrives. This
will result in periods of snow on the north side of the low that
stretches from southeast MN and central WI to northern MI today and
into tonight. WPC probabilities show moderate-to-high chances
(50-70%) for snowfall totals >4" across the northern tier of
Michigan's Mitten with low chances (10-30%) or >6". Some light
icing is possible in parts of southeast MN, western WI, southern
MI, and northern IN where there are moderate-to-high chances
(50-70%) for ice accumulations >0.01". With ground temperatures so
cold, even minor amounts <0.1" can cause slick conditions on roadways.

As the storm heads east Monday night, storm system will direct its
anomalous moisture source and modest low-level WAA into the
Northeast. Modest upper-level divergence in the form of a
strengthening 250mb jet streak aloft provides support for the
healthy shield of precipitation throughout the region, while the
air-mass remains quite cold and dry ahead of the storm (temps in
the teens,single-digit dew points). Higher SLRs of 12-16:1 are
anticipated in these areas and soil temperatures are near freezing,
which combined with the event occurring at night, will maximize
snowfall accumulation potential. The storm races off over Nova
Scotia by midday Tuesday and snowfall should taper off by Tuesday
afternoon. WPC probabilities show moderate-chance probabilities
(40-60%) for snowfall >6" in parts of northern NY (including the
Tug Hill) and in the tallest peaks of the Green and White
Mountains. These mountain ranges are favored for the heaviest
snowfall given the added help of upslope flow. Downeast Maine
sports moderate-chance probabilities for >6" of snowfall through
Tuesday morning. For those affected areas listed above, the WSSI-P
has increased its chances for Minor Impacts to moderate-to-high
chances (50-70%) from northern MI on east through northern NY and
into northern New England.

...Mid-Atlantic...
Day 2...

Residents in the Mid-Atlantic will want to monitor the forecast
closely Tuesday morning as the same tongue of moisture bringing
snow to the Northeast could bring a swath of light snow and/or
wintry mix from the central Appalachians to the New Jersey Shore.
Soils temperatures are close to freezing and light ice or snow
accumulations could make for slick travel conditions Christmas Eve
morning. WPC probabilities do show some low chance probabilities
(10-30%) for ice accumulations >0.01" in parts of the DC,
Baltimore, and Philadelphia metropolitan areas Tuesday morning.
Odds of >0.01" ice accumulations (low-to-moderate chances, or
30-50%) are greater to the west of these cities in parts of
northern MD, southeast PA, and the central Appalachians.

Mullinax
$$
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FOUS11 KWBC 261945
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Thu Dec 26 2024

Valid 00Z Fri Dec 27 2024 - 00Z Mon Dec 30 2024

...The West... Days 1-3...

Widespread active weather to impact the Pacific and Interior
Northwest through the weekend.

Broad troughing centered south of Alaska will shed periodic
shortwave energy eastward towards the Pacific Coast through the
weekend, although accompanying surface lows are progged to remain
north into Canada. This will result in a pattern which features
impressively convergent flow coming eastward from the Pacific,
leading to rounds of elevated IVT as period atmospheric rivers (AR)
surge onshore. This moisture combined with strong jet streaks aloft
will result in widespread heavy precipitation across much of the
Pacific and Interior Northwest, with precipitation spreading as far
south as the Sierra, and as far east as the Central Rockies, at times.

The first wave will move onshore late Thursday night into Friday
morning as the low pressure lifting into British Columbia pushes a
warm front eastward and into OR/WA. The accompanying moist
advection will push IVT above the 99th percentile according to the
CFRS climatology, highest into the Great Basin, but the northern
periphery of >90th percentile IVT will lift into OR/ID, and GEFS
IVT probabilities for 500 kg/m/s peak above 50% even as far east as
the Foothills. The overlap of moisture with ascent (aided by
transient LFQ jet level diffluence and periods of upslope in N-S or
NW/SE terrain features) will result in widespread precipitation D1
from the Sierra and coastal OR through the Great Basin and into the
Central Rockies. Snow levels in the highest IVT core will reach
5000-6000 ft, but remain around 3000-4000 ft farther north, leading
to at least modest winter impacts at the Passes. WPC probabilities
D1 for more than 8 inches of snow are high (70-90%) for parts of
the Sierra, OR Cascades, Blue Mountains, Salmon River/Sawtooth
region, Uintas, Tetons, and into the Park Range of CO. Locally 1-2
feet is likely in the highest terrain.

A second, somewhat weaker, impulse will follow immediately in the
wake of this first wave and lift northeast into British Columbia
once again, with the attendant warm front and accompanying
warm/moist advection lifting into WA/OR late Friday night into
Saturday. Once again, enhanced ascent through jet streak will help
expand moisture, with upslope flow driving the most intense
precipitation into the Cascades and farther east in the Salmon
River/Sawtooth Ranges. Snow levels with this second wave will be
slightly higher than the first, around 8000 ft across CA/Great
Basin, and 4000-6000 ft as far north as the WA Cascades. Heavy snow
is likely once again above these levels, and in some places farther
east into ID/MT/WY snow intensity may decrease between the two
waves, but will never really shut off. Current D2 WPC probabilities
are high (70-90%) for more than 8 inches again from the WA
Cascades, into the Northern Rockies, and across the Tetons.

Yet a third wave in this persistent confluent onshore flow will
push onshore Saturday night and Sunday as a more pronounced
shortwave trough digs eastward and comes onshore near the OR/WA
coast at the end of the forecast period. This will again be
accompanied by onshore flow and enhanced IVT, but mesoscale forcing
for ascent may be a bit more robust Sunday as a warm front stalls
in the vicinity beneath the favorable LFQ of a more potent stream.
The axis of higher moisture is more restrictive this day as NAEFS
PWs above the 97.5 percentile are in a narrow channel from OR to
UT, but this is also where some enhanced fgen may occur through the
favorable overlap of low-level WAA and upper level diffluence.
Uncertainty is considerable in the placement of this corridor, but
another round of heavy snow is likely above generally 4000-6000 ft
on Sunday. WPC probabilities D3 are above 70% for 8 inches D3
across the spine of the Cascades, into the Salmon River/Sawtooth
region, Blue Mountains, and continue in the vicinity of
Yellowstone NP. 3-day total snowfall of 3-5 feet is possible in the
hied terrain of the Cascades and Tetons.

...Interior Mid-Atlantic and Northeast... Days 2-3...

A shortwave rotating through the base of an elongated trough
positioned from the Northern Rockies through the Ohio Valley will
lift northeast into the Great Lakes Friday night into Saturday
morning while a surface high pressure sits south of New England.
The confluent flow between these two features will surge a plume of
moisture northward on 300K isentropic ascent leading to PW
anomalies as high as +2 sigma according to NAEFS from the Central
Appalachians through the northern Mid-Atlantic. The surface high
will slowly retreat during this period, but cold air at the surface
will at least initially be trapped leading to some light freezing
rain accumulations. WPC probabilities for 0.1" or more are 10-30%,
focused in the Catskills and southern Adirondacks, but light icing
accumulating to above 0.01" may impact much of PA, Upstate NY, and
southern New England.

After a brief break in precipitation, a more significant wave of
low pressure will develop across the Ozarks Saturday evening in
response to a potent shortwave trough digging through the primary
trough axis from the Central Plains. This shortwave will gradually
weaken into D3 as it encounters mid-level ridging across the
eastern CONUS, but a favorably placed jet streak will leave
sufficient diffluence through the RRQ to allow slow deepening of
the surface wave as it moves into the eastern Great Lakes and then
Ontario Province by the end of the forecast period. To the east of
this low, precipitation will expand on renewed WAA, while subtle
wedging of the retreating high clings across northern PA, Upstate
NY, and northern New England. Eventually all of the precipitation
should turn to rain as WAA overwhelms the column, but a period of
icing is likely before that time, leading to at least modest
accumulations of ice. Some of the recent WSE plumes are quite
aggressive across NH/ME, so there is potential for heavier icing,
but at this time the WPC probabilities for 0.1" or more are capped
at 30-50% in central ME.

Weiss
$$
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