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

Valid 00Z Sat Dec 04 2021 - 00Z Tue Dec 07 2021


...Pacific Northwest to the Northern Rockies...
Day 1...
Dry weather today is expected to give way to unsettled weather on
Saturday as a well-defined shortwave moves onshore.  Guidance has
trended a little wetter, but has overall remained consistent --
with WPC probabilities remaining above 40 percent for
accumulations of 8 inches or more across a large portion of the
northern Cascades.  With snow levels expected to be relatively
low, this area includes both Snoqualmie and Stevens passes.  Heavy
accumulations also remain likely for portions of the northern
Rockies, with WPC probabilities remaining high for accumulations
of 8 inches or more across the Lewis Range and Glacier National
Park, including Marias Pass.

Day 2...
The upper trough is expected to move east and amplify, pushing a
strong cold front south through the northern Rockies and High
Plains.  This will shift the focus for better organized snows
farther south along the northern Rockies.  Areas impacted include
the Mission Ridge, and the Swan and southern Lewis Ranges in
northwestern Montana; the Little Belt Mountains in central
Montana; and the Bitteroot and Clearwater mountains in northern
Idaho.  WPC probabilities are greater than 40 percent for snow
accumulations of 4 inches or more across those areas.  However,
widespread heavier amounts are not expected, with only low
probabilities indicated for snow accumulations of 8 inches or more.

Day 3...
Late Sunday is expected to be mostly dry as a shortwave ridge
shifts east across the region.  However, stormy weather is
forecast to quickly return overnight as the next shortwave trough
plunges into the region.  Originating over the Gulf of Alaska,
this system is expected to draw colder air into the region,
supporting low elevation snow across western Washington on Sunday.
 The trough is forecast to continue southeast, bringing snows and
the potential for locally heavy amounts into the Blue Mountains
and the northern to central Idaho ranges.

...Northern Plains to the Great Lakes...
Day 1...
As the shortwave system entering the Northwest on Saturday moves
east, snows will begin to develop east of the Rockies Saturday
morning, organizing along a northwest-southeast oriented
baroclinic zone extending across northeastern Montana into the
Dakotas.  While widespread heavy accumulations are not expected,
WPC probabilities suggest some locally heavy amounts are possible
by late Saturday across portions of northeastern Montana, near the
U.S.-Canada border.  

Day 2...
The threat for heavy snow is expected to expand markedly across
the north-central U.S., as the previously noted shortwave trough
moves east of the Rockies and amplifies over the northern Plains
on Sunday.  Models continue to show strong low-to-mid level
frontogenesis and favorable upper forcing supporting a swath of
moderate to locally heavy snow shifting east across North Dakota
and northern Minnesota overnight, before reaching into northern
Wisconsin and Michigan during the day.  Latest WPC probabilities
suggest that widespread accumulations of 4 inches or more are very
likely across the region, with localized accumulations of 8 inches
or more possible.  Some of the highest probabilities remain over
the Minnesota Arrowhead, where lake enhancement is expected to
bolster totals.

Day 3...
Models show the upper trough pivoting across the Great Lakes, with
a strong surface cyclone expected to track across the U.P. of
Michigan and Lake Superior Sunday night.  Additional synoptically
driven snows Sunday night will be followed by lake effect showers
-- contributing to additional heavy accumulations over the U.P. of
Michigan. 

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

Pereira