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/La...

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