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

Valid 00Z Mon Dec 06 2021 - 00Z Thu Dec 09 2021


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

Deepening low pressure will lift northeast from Wisconsin, across
Lake Superior, and into Ontario, Canada on D1. This low will be
driven northeast by an amplifying shortwave and increasing upper
diffluence to produce large scale ascent. Additionally, impressive
WAA will persist ahead of the surface low, enhancing lift and
adding moisture into the region. This will manifest as continued
heavy snow, primarily across the northern L.P., and most of the
U.P. of MI where frontogenesis will maximize during the WAA. Where
this WAA overlaps with the most impressive large scale synoptic
lift, snowfall rates may eclipse 1"/hr as noted by the HREF
probabilities, and WPC probabilities are high for additional
snowfall of at least 4 inches in this area. As the low pulls off
to the northeast, robust CAA will follow in its wake, and as winds
increase from the N/NW over Lake Superior and Lake Michigan,
intense ascent should lead to bands of impressive rates within
LES. WPC probabilities across the U.P., especially from the
Keweenaw Peninsula to the Porcupine mountains are high for 8
inches due to this LES on D1.

Thereafter, as the low pulls further east across Quebec,
persistent troughing will plague the Great Lakes leading to rounds
of LES snow bands and snow showers in the W/NW snow belts across
all the lakes. Forcing overall appears to be modest both D2/3, but
still relatively warm lake temps combined with a very cold column
should produce steep lapse rates to drive periodic bursts of
heavier snow in the favored snow bands. WPC probabilities D2 are
moderate for at least 4 inches in the Keweenaw Peninsula and Tug
Hill Plateau. By D3, the highest WPC probabilities for 4 inches or
more are along the immediate lake shore east of Erie and Ontario,
as well in the western Keweenaw Peninsula.


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

Fast and confluent mid-level flow will consistently spread
moisture onshore beneath periodic Pacific jet streaks. Embedded
within this flow, modest shortwaves will dive southeast from the
Gulf of Alaska onto the coast, providing additional ascent to
drive precipitation across the terrain of the West.

The first shortwave will approach the Pacific Northwest D1
accompanied by a moisture surge noted by increasing IVT anomalies
shifting onshore. As this occurs, snow levels will initially be as
low as 500ft allowing for some snowfall in the lowlands, but as
warm advection commences to drive rising snow levels, it will
quickly change to rain and no accumulation is forecast. However,
in the Olympics, Cascades, Bitterroots, and portions of the
Absarokas, WPC probabilities are high for 4 inches, and some very
light accumulations are possible in the Columbia Basin. Moderate
accumulations are likely at Snoqualmie and Stevens Pass on D1 as
well. As this system dives southeast on D2, snowfall will spread
as far southeast as the CO Rockies and into the Sierra as well.
WPC probabilities are moderate for 4 inches in the Bitterroots,
Tetons, CO Rockies including the Park Range and San Juans, and the
Northern Sierra.

On D3 yet another shortwave and accompanying Pacific jet streak
advects into the Northwest as the active pattern continues. This
will spread additional precipitation across the region with
snowfall exceeding 4 inches likely in the WA Cascades and Northern
Rockies, with moderate probabilities in the Olympics, OR Cascades,
Bitterroots, and Tetons. Snow levels D3 will be around 3000-4000
ft early, falling to 2000 ft late.


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

Weiss