Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
424 AM EST Wed Nov 10 2021

Valid 12Z Wed Nov 10 2021 - 12Z Sat Nov 13 2021


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

Increasingly confluent flow south of a low in the Gulf of Alaska
will shed weak vorticity impulses into the Pacific Northwest today
and Thursday in tandem with increasing moist advection. At the
same time, a potent upper jet streak reaching 130kts will surge
onshore accompanied by periods of enhanced diffluence, and
increasing deep layer ascent. This Pacific moisture plume surging
onshore will characterize an AR reaching IVT of 750 kg/m/s, and
will result in widespread precipitation across the West. On Day 1
/through early Thursday morning/ there will be two areas of
precipitation. The first is with residual moisture and ascent
cresting the Intermountain West, leading to pockets of heavy
snowfall in the terrain of the CO Rockies including the Park
Range, the ranges of NW Wyoming, and the Absarokas, where WPC
probabilities for 4 inches are as high as 40%. The other region of
heavy snowfall is likely in the WA Cascades where WPC
probabilities for 4 inches are above 40%, but with snow levels
climbing at the nose of the AR from 3000 ft to 7000 ft.

As the AR continues to push onshore and peak on Thursday, snow
levels will continue to climb towards 9000 ft, limiting heavy snow
to only the highest peaks of the Northern Cascades and Mt Rainier
where WPC probabilities for 4 inches are 20-30%. However, some
trapped cold air east of the Cascades will lead to an overrunning
precipitation event Thursday morning/aftn before warm air can
scour out into the valleys. The guidance has increased the
potential for some freezing rain accretion during this period, and
WPC probabilities now show a 10-30% chance for 0.1" east of the
Cascades, including Blewett Pass and Stevens Pass.


...Northern Plains and Upper Midwest...
Days 1-3...

A Pacific shortwave moving out of the Central Rockies this aftn
will phase with a potent closed low dropping out of Alberta,
Canada early on Thursday to create rapidly strengthening
cyclogenesis across the region. As this low lifts northward and
deepens near Lake Superior, it will slow, and likely retrograde
briefly as it gets captured and becomes vertically stacked, before
shifting rapidly northeast Friday. Impressive moist advection out
of the Gulf of Mexico will surge northward tonight into Thursday
as WAA occurs from the south. This WAA will quickly erode the
marginal thermal structure all the way into Canada, but some
impressive ascent associated with the leading edge of the fgen
could produce some light snow accumulations D1 along the
ND/MN/Canada border.

By D2 as the low deepens quickly, ascent will become more
widespread leading to more significant snowfall. An impressive
TROWAL is progged to rotate cyclonically around the low and pivot
westward into ND at the same time that CAA begins in earnest in
the low levels behind the low and its cold front. The rapidly
cooling column will allow precipitation to change to snow from
north to south, while at the same time WAA within the TROWAL will
help to deepen the DGZ and provide locally enhanced ascent
coincident with the sharpening deformation axis. Total forcing is
likely to be modest except locally across northern ND and MN which
will experience the steepest lapse rates beneath the closed low
and best ascent within the WCB, but the deepening lapse rates and
cooling column should produce higher SLRs to enhance snow
accumulations even in regions of modest ascent. Snow is likely to
become widespread D2 from ND and MN into the higher terrain of the
Coteau Des Prairies of SD, and WPC probabilities for 4 inches are
above 80% on the international border, and as high as 50% in
northeast SD. By D3 the primary low occludes and shifts eastward,
leading to a rapid weakening of the TROWAL and advection east of
the best forcing. The exception will be as a shortwave pivoting
around the closed low sharpens and drops southeast across IA/IL/WI
coincident with some higher instability beneath the cold pool.
This should produce pockets of heavier snowfall rates which can
overcome above-freezing surface temperatures leading to light
accumulations as far east as Lake Michigan.


Weiss