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

Valid 00Z Sun Dec 05 2021 - 00Z Wed Dec 08 2021


...Northern Plains to the Great Lakes...
Days 1-3...

An amplifying shortwave digging out of Alberta, Canada will
amplify into a more robust trough across the Great Lakes while
racing quickly southeast in fast mid and upper level flow. As this
shortwave deepens, a strong upper jet streaking across the
northern tier will begin to arc poleward downstream of the primary
trough axis producing a modestly coupled jet structure. The
overlap of this jet and the shortwave will lead to surface
pressure falls and a surface low pressure is likely to form over
ND early Sunday morning before racing east-northeast into Ontario
by Monday aftn. Impressive dynamics including impressive WAA ahead
of the associated cold front will spread precipitation northward
as IVT reaches +1 to +2 standard deviations above the climo mean
according to NAEFS ensemble tables. While this WAA will likely
lead to a p-type transition from snow to rain in the eastern Great
Lakes, the Northern Plains and western Great Lakes should remain
all snow.

The overlap of the diffluent portion of the jet streak aloft and
the WAA should produce a band of intense snowfall driven by
frontogenesis within a deepening DGZ. Forecast soundings suggest
an increasing isothermal layer within the DGZ which should support
large aggregates, collocated with the most strongly sloped fgen
beneath SEPV <0. This indicates the potential for convective snow
rates, and the recent HREF indicates a high probability for 1"/hr
snowfall within this impressive band. While there remains some
uncertainty into the position of this band, and guidance has
trended a bit southward today, where it occurs there is likely to
be a swath of heavy snowfall as SLRs are progged to be around
15:1. On D1 this swath of snow is likely to be heaviest from
central ND into northern MN, where WPC probabilities are high for
6 inches, with local maxima of 10 inches possible. Along the
Arrowhead of MN near the coast of Lake Superior, enhanced moisture
from the lake and ascent into the Iron Ranges could produce in
excess of 12 inches of snowfall. 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.

During D2 /Sunday night and Monday/ the low will move into Ontario
and the associated synoptically forced snow will lift into Canada
with it. However, 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 WPC probabilities for 6+ inches are high from
the Keweenaw Peninsula southward towards the Porcupine Mountains
of the U.P., with moderate probabilities for more than 4 inches
near Traverse City, MI. Additional snowfall adding up to more than
4 inches in the Arrowhead of MN early on D2 before winds shift and
snow winds down the latter half of the day. By D3 NW flow
continuing across Lake Superior may bring additional light
accumulations to the far NE portion of the U.P. near Whitefish
Bay, while increasing westerly fetch over Lake Ontario produces
moderate LES into the Tug Hill Plateau.


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

A weakening shortwave embedded within confluent mid-level flow
will drape a surface low and cold front across the Pacific
Northwest tonight into Sunday while moisture advection maximizes
into the Northern Rockies. This feature will be fast moving as the
upper jet screeches eastward at 170kts. 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. WPC probabilities are moderate for 6
inches across portions of the Bitterroots, Northern Rockies, and
Absarokas on D1, with several inches of snow likely falling at
Marias Pass.

A brief period of mid-level ridging will follow this lead
shortwave cutting off precipitation, 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 precipitation may
begin as light snow even in Seattle before changing to rain, with
little to no accumulation expected, and light accumulations are
likely in the Columbia Basin. In the Olympics, WA Cascades, and as
far east as the Bitterroots on D2, WPC probabilities are moderate
for 6 inches, with several inches of snowfall likely accumulating
at both Snoqualmie and Stevens Passes.

This second impulse will continue to traverse southeast into
Tuesday spreading snowfall through the terrain of ID and into the
NW WY ranges and parts of the CO Rockies. WPC probabilities on D3
are high for 6 inches in the Tetons and Absarokas near NW WY, as
well as parts of the CO Rockies including the Park Range, with
several inches of snowfall possible at Rabbit Ears Pass.


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

Weiss