Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Fri Dec 03 2021

Valid 12Z Fri Dec 03 2021 - 12Z Mon Dec 06 2021

...Swath of Heavy Snow expected from northern Great Plains through
the Upper Great Lakes late Saturday through Sunday...

...Great Lakes...
Days 1/2...
A clipper will shift ESE across Ontario today/tonight, brushing
the northern Great Lakes before shearing out to the east across
the St. Lawrence Valley Saturday.  Mostly light amounts are
expected across portions of northern Michigan and east of the
eastern Great Lakes, with Day 1.5/2 WPC probabilities for 4 or
more inches moderate in the eastern UP and on the Tug Hill of NY.

...Pacific Northwest to the Northern Rockies...
Days 1/2...
High pressure building today quickly gives way to a well-defined
shortwave trough that crosses Vancouver Island Saturday morning on
a 130kt jet streak. Sufficient Pacific moisture (PWs 0.75", low
snow levels (dropping below 2000 ft across northern Washington)
raise the threat for notable snow at the passes.  WPC Day 1.5
probabilities (ending 00Z Sun) are now 40-70 percent for
accumulations of 8 or more inches 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 highlight 6 or more inches for most of the ranges
along the northern ID/MT border and the ranges around Glacier NP
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 are likely to see
accumulations of 4 inches or more by late Saturday.

Day 3...
The next trough/low is significant as it brings about a pattern
shift to troughing over the Rockies rather than the warm ridging
that has been present for the past month. This potent trough and
developed surface low reaches western WA Sunday night with a deep
layer of marginally subfreezing air ahead. This brings
accumulating snow levels around Seattle below 1000ft prior to
precip onset late Sunday night. Ample Pacific moisture will allow
moderate to heavy precip rates along with rising snow levels. Day
3 WPC probabilities (which end 12Z Monday) are moderate for 6 or
more inches in the Olympics and northern Cascades with most of
that falling after 06Z Monday.


...Northern Plains to the northern Great Lakes...
Days 2/3...
The potent system racing across the Pacific Northwest early
Saturday will amplify over MT/ND Saturday night before closing
into a mid-level low over Minnesota on Sunday and pivoting
northeast into eastern Ontario Sunday night. Strong low-to-mid
level frontogenesis along with favorable upper forcing and ample
Gulf moisture will support a classically banded and decently long
duration (for a quick moving system) snow setup north of the
surface low as it tracks east across the northern Plains late
Saturday through Sunday. A leading swath of snow shifts east
across MT Saturday as return flow behind a 1030mb high pressure
center shifts southeast over the MN/IA border intensifies into
Saturday night as it crosses ND as moisture influx increases ahead
of the amplifying upper trough. Then on Sunday the strong system
further intensifies over MN as Lake Superior moisture is drawn
west, aiding enhanced snow over the North Shore escarpment.
Wrap-around snow in the comma head then continues Sunday night
over the Upper Great Lakes as the low pivots northeast. 

Guidance has exhibited uncertainty in timing and low track, but
are all in agreement on banded, locally heavy snow forming on the
north side of the low and impacting ND/MN and the UP of MI late
Saturday through Sunday with potential for 8-12" along the track,
locally higher in the enhanced MN North Shore. The 12Z ECMWF had
been a north outlier for the track, but the 00Z run did notably
shift south toward consensus. Day 2 WPC snow probabilities for 6
or more inches are 40 to 70% over northern ND, expanding across
northern MN for Day 2.5 and increasing to 80% for Day 3 over
enhanced areas of the MN North Shore (mainly for Sunday) and the
Keweenaw Peninsula of MI (mainly for wrap around Sunday night).

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

Jackson