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

Valid 00Z Thu Nov 11 2021 - 00Z Sun Nov 14 2021


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

Onshore flow south low pressure in the Gulf of Alaska and behind a
leading wave currently spanning the High Plains will continue to
spread across the Pacific Northwest through Friday. The next wave
to affect the northern/CO Rockies is Thursday/Thursday night. Day
1.5 WPC probabilities for 6 or more inches are as high as 70% in
the Park Range in northern CO and southern Medicine Bow Mtns of
south-central WY. In the WA Cascades, snow levels quickly rise to
8000ft tonight where they persist through Friday. WPC
probabilities for 6 inches on Days 1 and 2 are around 20% for just
the highest WA Cascades.

As the AR continues to push onshore and peak on Thursday, some
trapped cold air east of the WA Cascades will lead to an
overrunning precipitation event Thursday morning/aftn before warm
air can scour out into the valleys. Day 1.5 WPC probabilities for
a tenth inch of ice or more are 10-30% east of the Cascades,
including east of Blewett Pass and Stevens Pass.


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

Rapid intensification is expected for the low currently over
southern Saskatchewan as it shifts southeast to southern MN
through Thursday before slowly shifting east to MI through Friday
before shifting north Friday night. Impressive moist advection out
of the Gulf of Mexico will surge northward ahead of the low and
across the Great Lakes tonight into Thursday. This warm air
advection will quickly erode the marginally cold 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 tonight over far northern MN.

On Thursday, as the low quickly deepens, ascent will become more
widespread through the developing comma head with snow developing
on the northwest side. An impressive TROWAL is progged to rotate
cyclonically around the low and pivot westward into ND at the same
time that cold air advection 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 warm air advection within the TROWAL will help to
deepen the DGZ and provide locally enhanced ascent coincident with
the sharpening deformation axis. 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 Thursday afternoon from northeastern ND across
far northern MN before spreading south Thursday night. Day 1.5 WPC
probabilities for 6 or more inches are above 50% over these areas
Thursday night into Friday, the primary low occludes and shifts
eastward, leading to a 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 southern
MN/northern IA into WI/IL. Day 2 WPC probabilities for 2 or more
inches are above 30% over northeastern SD and southwest MN. The
cold core of the low will promote at least light snow over WI and
the UP of MI with lake enhancement off Lake Superior expected over
northern WI/western UP of MI starting Friday afternoon and
continuing through at least Friday night where Day 2.5 WPC
probabilities for 2 or more inches are over 30%.


Jackson