Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EST Sun Dec 05 2021

Valid 12Z Sun Dec 05 2021 - 12Z Wed Dec 08 2021


...Great Lakes...
Days 1-3...

An amplifying shortwave trough shifting east across the Dakotas
this morning continue to develop over northern MN this afternoon
before turning north over Lake Superior tonight. The surface low
occludes today with a large comma head developing over northern MN
tonight and a notable dry slot over southern WI/northern IL. The
combination of convergence in the warm/cold conveyor belt followed
by wrap around comma head precip will make for an extended snow
event over northern MN, particularly along the North Shore in MN
which will have Lake Superior enhancement in easterly flow today. 

Forecast soundings suggest an increasing isothermal layer within
the DGZ which should support large aggregates, collocated with the
most strongly sloped frontogenesis. Convective snow is expected
and the 00Z HREF indicates a good probabilities for 1"+/hr
snowfall within this impressive band from northeast MN this
morning across the UP this afternoon and the northern LP this
evening. Along this swath, Day 1 WPC snow probabilities are high
for 6 inches, with local maxima of 10" expected. Along the North
Shore of MN, enhanced moisture from Lake Superior and ascent into
the Iron Ranges should produce snowfall in excess of a foot a
locally 18". Additionally on D1, WPC probabilities for at least 6
inches are high within the pronounced WAA band lifting through
northern WI and into the U.P. of MI, with a weak secondary jet
maxima potentially enhancing snowfall in this area, aided by
moisture from Lake Michigan and Huron.

As the low lifts northeast into Ontario Monday, strong CAA in the
wake of this system will lead to bands of LES in the favored N and
NW snow belts. High SLRs in the cold column should allow snowfall
to pile up quickly where bands persist, and Day 2 WPC
probabilities for 6+ inches are high from the Keweenaw Peninsula
south through the Porcupine Mountains, with moderate probabilities
for more than 4 inches near Traverse City, MI and the eastern
portion of the U.P. near Whitefish Bay. As the flow backs to
westerly Monday night and Tuesday, the increasing fetch over Lakes
Erie Ontario produces moderate LES into the Tug Hill Plateau where
Day 2.5 snow probs for 4" or more are moderate with lesser amounts
off Erie and over western Michigan.


...Pacific Northwest to the Colorado Rockies...
Days 1-3...

A reinforcing shortwave trough will push a cold front south across
the Northern Rockies today. The overlap of modest height falls,
upper diffluence, and modest low-level convergence will produce a
swath of moderate to heavy snow across the Northern Rockies, with
snow levels falling from around 5000 ft to 1500 ft during the
heaviest snowfall. Day 1 WPC probabilities are moderate for 6 or
more inches of snow for the south-central MT terrain including the
northern Absarokas.

A brief period of mid-level ridging spreads east over the Pacific
Northwest today, but cold surface high pressure will allow snow
levels to drop to near the surface as far south as Seattle, WA and
the Columbia Basin. This will be in advance of the next impulse
racing southeast Monday morning accompanied by a modest upper jet
streak and moisture advection noted by IVT of +1 standard
deviation above the climo mean shifting into WA and OR. WAA
spreading precip eastward will gradually lift snow levels during
the day, but snow levels will start near 800ft around Seattle
before changing to rain, with little to no accumulation expected
in higher suburbs, and light accumulations are likely in the
Columbia Basin. The fast moving system will spread locally heavy
mountain snows. In the Olympics, WA Cascades, the Blue Mtns,
Bitterroots, and the Tetons, Day 2 WPC probabilities are moderate
for 6 inches, with several inches of snowfall likely accumulating
at both Snoqualmie and Stevens Passes. This then spreads into the
western CO Rockies Monday night, reaching as far as the Park
Range, with several inches of snowfall possible at Rabbit Ears
Pass.


For Days 1-3, the probability of ice accretion over 0.1" is less
than 10 percent.

Jackson