Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Sat Dec 04 2021

Valid 12Z Sat Dec 04 2021 - 12Z Tue Dec 07 2021


...Pacific Northwest to the Northern Rockies...
Day 1...
A potent, quick-hitting shortwave trough crosses northern WA and
the northern Rockies today on a 130kt jet streak. Snow levels will
be around 2500ft in western WA and about 3500ft in the northern
Rockies. Day 1 WPC probabilities remain above 50 percent for
accumulations of 8 inches or more across the northern WA Cascades
including 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.

Tonight into Sunday, the upper trough will amplify as it spills
onto the High Plains, 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 Bitterroot 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.

Dry/ridging spreads east across the area Sunday. However, the next
shortwave trough plunges into the region late Sunday night.
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 with Day 3 moderate to high probabilities for 6 or
more inches for higher terrain WA to western WY.

...Northern Plains to the Great Lakes...
Day 1...
Snows are developing and spreading east across MT this morning,
organizing along a northwest-southeast oriented baroclinic zone
extending across northeastern Montana that will soon reach into
the Dakotas. While widespread heavy accumulations are not
expected, locally heavy amounts are expected over far northeastern
Montana, near the U.S.-Canada border this afternoon before the
activity further develops as it shifts east tonight. 
In fact, the threat for heavy snow is expected to expand markedly
across ND and northern MN tonight as the previously noted
shortwave trough moves east of the Rockies and amplifies over the
northern Plains tonight through 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. Day
1 WPC probabilities are moderate for 6 or more inches across
northern ND, though the true mesoscale banding potential is not
realized in the input guidance, so areas of 8 inches or more are
likely in at least a narrow strip. The highest probabilities on
Day 2 remain over the Minnesota Arrowhead, where heavy snow Sunday
is enhanced by Lake Superior and then the system pivots with wrap
around continuing through Sunday night There are moderate
probabilities for a foot or more over the northern North Shore.

The strong surface cyclone is 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 enhanced snow
showers of Superior and Michigan in the comma head through Monday
before slowly tapering off Monday night. Day 2 probabilities for 8
or more inches are moderate over the typical snow belts of the
Porcupines, Keweenaw, Huron Mtns, and southeast of Whitefish
Bay/SSM.  Day 3 probabilities are moderate for 4 or more
additional inches off Superior and Michigan, but strong winds and
decent cold air should allow locally much more. A fairly brief
bout of LES off the eastern Great Lakes brings some low 4 inch
probabilities to the east sides of Lakes Erie and Ontario.

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

Jackson