Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 PM EST Thu Dec 02 2021

Valid 00Z Fri Dec 03 2021 - 00Z Mon Dec 06 2021


...Great Lakes...
Day 1...
Another clipper system is forecast to dive southeast across
central Canada, brushing the northern Great Lakes Friday night
into Saturday morning, before shearing out to the east across the
St. Lawrence Valley.  Mostly light amounts are expected across
portions of northern Michigan and east of the eastern Great Lakes,
with WPC probabilities showing little to no threat for widespread
heavy amounts.

...Pacific Northwest to the Northern Rockies and High Plains...
Day 2...
Following a dry day on Friday, shortwave ridging is expected to
give way to a well-defined shortwave trough moving onshore
Saturday morning.  While the available moisture is expected to be
much more limited than previous systems, snow levels will be quite
low -- dropping below 2000 ft across northern Washington --
raising the threat for notable accumulations at the passes.  WPC
Day 2 probabilities -- ending 00Z Sun -- are around between 40-70
percent for accumulations of 4 inches or more at both Stevens and
Snoqualmie passes.  Farther east, upslope flow developing on the
backside of a low developing east of the Divide is forecast to
bolster amounts across portions of the northwestern Montana
ranges.  WPC Day 2 probabilities suggest that accumulations of 4
inches or more are almost certain, with 8 inches or more likely
for portions of the Lewis Range/Glacier National Park.  Meanwhile,
snow is expected to develop ahead of the wave, along a
northwest-southeast low-to-mid level baroclinic zone extending
across northeastern Montana into North Dakota on Saturday. 
Although significantly lower than in areas farther to the west,
WPC probabilities do suggest that at least some areas in
northeastern Montana near the Canada border could see
accumulations of 4 inches or more by late Saturday.

...Northern Plains to the northern Great Lakes...
Day 3...
The previously noted system entering the Northwest on Day 2 is
forecast to amplify over the Dakotas and Minnesota on Sunday. 
Strong low-to-mid level frontogenesis along with favorable upper
forcing will support a solid swath of snow advancing east from
North Dakota into northern Minnesota Saturday night, with the
leading edge spreading across northern Wisconsin into northern
Michigan on Sunday.  For the Day 3 period -- ending 00Z Monday --
WPC probabilities are 40 percent or greater for 4 inches or more
from central North Dakota and across much of northern Minnesota,
parts of northern Wisconsin and the entire U.P. of Michigan. 
Higher probabilities are noted in the Minnesota Arrowhead and
portions of the U.P., where some lake enhanced amounts may occur.

For Days 1-3, the probability of significant icing is less than 10
percent.

Pereira