FOUS11 KWBC 190943
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
443 AM EST Tue Nov 19 2024

Valid 12Z Tue Nov 19 2024 - 12Z Fri Nov 22 2024

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

***Impressive and long-lasting atmospheric river event to bring
heavy snow to the Northwest***

This morning, a large ridge of high pressure building over Alaska
will help to force the longwave trough that has been parked off the
British Columbia coast for the last 24 hours southward into the
heart of the Northwest Pacific. Rounding the base of the longwave
trough is a potent 500mb vort max that will phase and lead to the
rapid cyclogenesis of a sub 950mb low off the Pacific Northwest
coast by Tuesday afternoon. The NAEFS SAT page is littered with
atmospheric parameters (wind speeds, heights, MSLP, IVT) that are
approaching or reaching maximum (or minimum) climatological
percentiles in the CFSR climatology. Perhaps the most notable
parameters are the IVT (topping 1,000 kg/m/s at its peak off the
northern California coast) and 850-700mb winds that are above the
97.5 climatological percentiles Tuesday night into Wednesday
morning. This same moisture plume will spill over the West Coast
mountain ranges and into the Northern Rockies

These two parameters are vital in producing what should be prolific
snowfall rates in the Olympics, Cascades, Siskiyou, Salmon, and
Shasta Mountains Tuesday night. WPC's Snowband Probability Tracker
(SPT) utilizing the HREF shows the potential for 2-3"/hr snowfall
rates as the heaviest precipitation arrives Tuesday evening with
some potential for localized 4"/hr rates. Snow levels will begin
the forecast period quite low; only around 200 ft east of the
Cascades to around 2000ft in northern CA. However, the exceptional
WAA accompanying the AR will raise snow levels steadily D2 and D3,
reaching as high as 4000ft in northern WA and the Northern Rockies,
and to 8000ft across northern CA. Despite this rise in snow
levels, hazardous snowfall impacts are likely at many passes before
p-type changes over to rain in the Olympics and Cascades first
(both via WAA and dry slotting aloft), then over the northern
California ranges Wednesday evening. This coincides with both a
thermodynamic profile that will lead to snowfall being a heavy/wet
consistency that when paired with strong wind gusts will cause
stress to the trees and power lines.

WPC PWPF shows high chances (>70%) for snowfall totals >12 inches
along the spine of the WA/OR Cascades, as well as the Blue,
Sawtooth, and Bitterroots Mountains that stretch as far north as
the Idaho Panhandle. In the Salmon/Siskiyou/Shasta and Olympics,
WPC PWPF shows high chances (>70%) for >30" with the highest
elevations seeing anywhere from 3-5 feet in some cases. The impacts
will be exceptional in northern California where the WSSI shows
Extreme criteria being met in portions of the
Salmon/Siskiyou/Shasta. The Snow Amount and Snow Load components
are the primary drivers in these impacts that are likely to include
extremely dangerous to impossible driving conditions, as well as
extensive closures and both tree damage and power outages. Major
Impacts are highlighted along the spine of the Cascades above
5,000ft and the Olympics above 3,000ft.

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

The much anticipated storm system responsible for today's winter
storm across puritans of the Northern Plains is tracking north
through the Upper Midwest this morning with a comma-head shaped
precipitation shield wrapping around the northern and western
flanks of the storm. As the morning unfolds, temperatures will
continue to fall across North Dakota and allow for lingering areas
of rain to changeover to snow later this morning. The heaviest
period of snow arrives this afternoon and into tonight as the
TROWAL on the backside of the storm pivots through with a
deformation axis capable of generating 1-2"/hr snowfall rates
across north-central North Dakota. On top of the heavy snowfall
rates, whipping wind gusts topping 40 mph in many cases means
significant visibility reductions that will lead to whiteout
conditions Tuesday evening.

As a complex 500mb evolution unfolds over Minnesota Tuesday night
and into Wednesday morning, an enormous upper level low gyre will
still have a saturated ribbon of 700-300mb moisture over the
Northern Plains that will spill east over the Red River of the
North and as far south as northeast South Dakota. WPC PWPF does
show low-to-moderate chances (30-50%) for snowfall totals >6"
east of the Red River in northwest Minnesota and in northeast South
Dakota. Periods of snow will persist throughout the day on
Wednesday but coverage and intensity will gradually dissipate
throughout the day until the snow finally comes to an end overnight
Wednesday. When all is said and done, portions of north-central
North Dakota are likely looking at 6-12" of snowfall with the
heaviest amounts likely to occur closest to the north-central North Dakota/Canada border. The latest WSSI shows a fairly large
footprint for Moderate impacts over most of central North Dakota
and northeast South Dakota, which suggests there are likely to be
some disruptions to daily life such as hazardous driving conditions
and potential closures. Minor Impacts are most commonly seen as
far west as eastern Montana and to the east across the Red River of the North.

...Northern & Central Appalachians... Day 3...

The aforementioned bowling ball of a 500mb closed low over the
Midwest on Wednesday will aid in the development of a new wave of
low pressure tracking over the northern Great Lakes Wednesday
evening. A strong cold front associated with the storm will push
through the Central Appalachians Wednesday night that will not only
lead to much of thermal profiles in the region being sub-freezing,
but a plume of 850-700mb moisture will reach the Central
Appalachians by Thursday morning. With 300-700mb mean flow
generally out of the west, this is a recipe for the first upslope
snow event of the season for the Central Appalachians. After the
first lobe of dynamical potential vorticity (PVU) swings through
the Mid-Atlantic on Thursday (helping to kick-start the upslope
event), the second PVU lobe will swing through Thursday night. This
will reinforce the ongoing upslope flow into the Central
Appalachians, and with this feature the initial low pressure
system that pushed the strong cold front through Wednesday night,
will introduce another surge in 850-700mb moisture over the region
through Friday morning. WPC PWPF shows high chances (>70%) for
snowfall >4" along the Allegheny Mountains and Laurel Highlands. In
fact, for elevations >3,000ft in the Allegheny Mountains, WPC PWPF
shows some moderate-to-high probabilities (50-70%) for >8" through
early Friday morning with additional snowfall still to come later
in the day on Friday. The latest WSSI-P shows a large swath of
moderate-to-high chances (50-70%) for Minor Impacts from the
Smokey's of North Carolina on north along the central Appalachians
and into the Laurel Highlands through Friday morning.

Farther north, similar to the original low pressure development
evolution over the Great Lakes on Wednesday, another area of low
pressure will form on the nose of the 2PVU rotating through the
Mid-Atlantic Thursday afternoon and deepen over Long Island
Thursday evening. This becomes a highly anomalous closed mid-upper
level low that is placed at the divergent left-exit region of a
strong 250mb jet streak over the Southeast. Geopotential heights
over the Mid-Atlantic (500mb, 700mb, 850mb) are all pegged to be
below the 1st climatological percentile Thursday night. As the
storm deepens, strong frontogenetic forcing on the northwest flank
of the 850mb low will result in a narrow deformation axis that is
currently forecast to set up somewhere from northern PA on east
through Upstate NY, northern NJ, and into southern New England by
Friday morning. This evolution has been present on the EC-AIFS for
over 24 hours. While boundary layer thermals may be questionable
away from the deformation axis, this setup favors snowfall at
elevations above 1,000ft in the Poconos, Catskills, Adirondacks,
and Berkshires. It is worth noting that there remains some
different solutions on snowfall totals and which areas see the
heaviest amounts. Regardless, this would be the first accumulating
snowfall of the season for these higher elevated areas. Should
guidance trend toward a more robust deformation zone, some lower elevated/valley locations in the Northeast could receive measurable
snowfall on Friday. At the moment, the WSSI-P shows moderate
chances (40-60%) for Minor Impacts (generally hazardous driving
conditions) in the Catskills, northern Poconos, and the Adirondacks
through Friday morning.

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

Mullinax

...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 200808
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Wed Nov 20 2024

Valid 12Z Wed Nov 20 2024 - 12Z Sat Nov 23 2024

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

***Impressive and long-lasting atmospheric river event continues to
produce heavy snow in the Northwest***

The rate of deepening from the storm system off the Pacific
Northwest coast has been breath-taking to watch. The storm's
atmospheric parameters (geopotential heights, wind speeds, moisture
advection) are all approaching NAEFS maximum (or in MSLP cases,
minimum) values in its CFSR record. Over the course of the next
couple days, the powerful storm system will be one in a series of
systems that are embedded within a massive 500mb trough gyre off
the Pacific Northwest coast. This is due to an expansive ridge over
Alaska that has effectively cut this upper low off from the
mean westerly flow pattern. Even as the current massive cyclone
occludes and gradually weakens Wednesday night, the next rapidly
strengthening storm will approach west of California on Thursday
directing yet another anomalous plume of moisture into the Pacific
Northwest and northern California that will persist into Friday.
All this to say that the synoptic scale evolution is favorable to
sustain a prolonged atmospheric river event that will generate
significant impacts across portions of the Pacific Northwest.

In terms of sensible weather, heavy precipitation will engulf much
of the Pacific Northwest and spill over into the Northern Rockies.
Snow levels starting out fairly low overnight will gradually rise
as strong WAA and a dryslot in the 700-300mb layer works its way
across western WA by Wednesday afternoon. Farther inland, colder
air will be harder to scour out in the Columbia Basin and along the
ridge lines of northeast Washington and the Idaho Panhandle.
Snowfall rates initially along the Cascades, Olympics, and on south
to the Shasta/Salmon/Sierra Nevada of northern California will be
on the order of 2-3"/hr. But as snow levels quickly rise through
Wednesday evening and into Thursday, most snow levels will be above
5,000ft in the Cascades and Olympics while the Shasta/Trinity
mountains see snow levels rise to as high as 8,000ft. The heaviest
snowfall by Wednesday night is likely to occur in the Blue and
Sawtooth Mountains. As the next storm approaches late Thursday, the
heaviest snowfall will shift farther north into the northern
Bitterroots and the Lewis Range through Friday.

WPC PWPF shows high chances (>70%) for snowfall >12" in the
Shasta/Trinity of northern California, Olympics, Cascades, Blue,
Sawtooth, and Lewis Ranges. Portions of the northern Idaho
Panhandle also show >70% odds for >12" of snowfall. Snow by Friday
will be primarily confined to the tallest peals of the Pacific
Northwest, making rain the primary precipitation type for the vast
majority of the residents impacted by Friday's storm. The WSSI
maintains Major Impacts in parts of central Washington, the
Cascades and Olympics, and even Extreme Impacts in the
Shasta/Trinity of northern California through Wednesday afternoon.
Only the Shasta/Trinity maintain Major Impacts through Thursday and
into Friday.

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

The winter storm that has produced periods of heavy snow throughout
North Dakota will gradually start to wind down on Wednesday as the
upper low over northwest Minnesota gradually weakens. However, a
500mb vort max pivoting around the northwest flank of the low will
still support modest upper level divergence aloft while a narrow
ribbon of modest 700-300mb moisture remains parked over central
North Dakota and eastern South Dakota. This moisture will spill
over into the Red River of the North and parts of northwest
Minnesota and keep periods of snow in the forecast there through
Wednesday afternoon. In addition, wind gusts of 30-40 mph will
linger throughout the day, leading to drifting snow on roads and
poor visibilities for motorists. Snow should finally taper off
Wednesday evening as the aforementioned 500mb vort max tracks south
into southern Minnesota and the plume of mid-level moisture aloft
dries up. WPC PWPF shows moderate chances (40-60%) for an
additional 4"+ in parts of northeast South Dakota and north-central
North Dakota where the favorable vertical ascent and moisture
profiles aloft are paired with the best 850-700mb frontogenetical
forcing. The WSSI-P still shows moderate chances (40-60%) for Minor
Impacts due to a combination of Snow Amount and Blowing Snow
criteria over central North Dakota, western Minnesota, and
northeast South Dakota.

...Great Lakes, Appalachians, & Northeast... Days 2-3...

A complicated yet highly anomalous upper level closed low will
spawn the first winter storm of the season for parts of the
northern Mid-Atlantic. First off, a potent 500mb low over the
northern most portions of Michigan's Mitten will dive south on
Thursday and lead to a strong surge in both NW winds off Lake
Michigan and strong lift aloft ahead of the 500mb low. Periods of moderate-to-heavy snow are possible from the northern Michigan
Mitten on south to the Michiana region. WPC PWPF shows moderate
probabilities (40-60%) for snowfall totals topping 4" in the
Michiana region through Thursday afternoon.

Meanwhile, a new wave of low pressure will form along a triple
point (occluded front/warm front/cold front convergence) somewhere
near the Jersey Shore early Thursday morning. The storm will be
located beneath the divergent left-exit region of a 120kt jet
streak and at the nose of a vigorous 2PVU lobe that will pivot
north towards the Lower Hudson Valley Thursday evening. As the
850mb low deepens Thursday afternoon, rich 850mb theta-e air off
the Atlantic will be drawn west and wrap around the storm's mid
level circulation Thursday evening. Intense 850-700mb WAA will
prompt the development of a robust deformation zone of heavy
precipitation. Given the strong vertical velocities aloft within
the band, precipitation will changeover to all snow and fall at a
heavy clip Thursday night and into Friday morning from north-
central PA on east to the Poconos and Catskills.

Despite the large scale circulation and more guidance coming into
better agreement on track, a storm like this usually contains an
intense deformation axis that may result in subsidence in areas
neighboring the band of heavy snow. This can either cut back
available QPF, or given the marginal thermals in the boundary
layer, cannot sufficiently cool the atmospheric column enough to
keep precipitation as all snow. Still, the signal is growing
stronger for heavy snowfall beneath the deformation zone with
snowfall rates of 1-2"/hr likely. Also, strong NE-E flow should
provide strong orographic lift into portions of the Catskills and
Poconos, making these locations more likely to receive heavy
snowfall. One spoiler would be any dry slot wrapping in from the
east at mid-upper levels that hinders the atmosphere's ability to
produce dendrites. Given these factors, there is still room for
adjustments in the snow forecast. However, with such a favorable
atmospheric setup and strong dynamics at play, this is likely to
be the first impactful winter storm of the season for parts of the
northern Mid-Atlantic. WPC PWPF shows moderate-to-high chances
(50-70%) for >8" of snowfall in parts of the Poconos. WSSI-P sports moderate-to-high chances (50-70%) for Minor Impacts throughout
much of northeast PA and the southern tier of NY.

Farther south, a multi-day upslope flow event is set to occur from
Thursday and last through Friday night. Thursday's snowfall will
be more light-to-moderate but pick up in intensity on Friday as a
500mb vort max rotating on the western flank of the large upper low
brings both an increase in synoptic scale lift, as well as stronger
850-700mb winds and an increase in moisture at those same levels.
Snowfall rates on Friday could range between 1-2"/hr at their peak
with rapid accumulations possible on roads. Areas most heavily
affected are likely to be the Laurel Highlands, the Allegheny
Mountains, and into portions of the Potomac Highlands. WPC PWPF
shows high chances (>70%) for >8" of snowfall in these ranges
through Friday night with some moderate-to-high odds (50-70%) for

12" in east-central WV above 2,000ft in elevation.

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

Mullinax

...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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