Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
341 PM EST Fri Nov 19 2021

Valid 00Z Sat Nov 20 2021 - 00Z Tue Nov 23 2021


...Pacific Northwest to the Northern Rockies...
Day 1...

The combination of a positively tilted shortwave and pacific jet
energy will drive a weakening surface low across the Northern
Rockies tonight into Saturday before dissipating. As this wave
shifts southeast, precipitation will spread from the Northern
Rockies of MT through the Central Rockies of WY and CO. As the
moisture and forcing shifts east, snow levels will fall from
6000ft to around 3000ft in MT, and to 4000-5000ft WY/CO. This will
confine the heaviest snowfall above these levels, and with a
primarily transient event, WPC probabilities for at least 6 inches
are above 30% only in the Tetons of WY and Park Range of CO.

...Great Lakes...
Day 3...

A deepening low pressure moving across Ontario Province north of
the Great Lakes will be followed by a cold front, and then a
reinforcing shot of cold air Sunday and Monday. Intense CAA will
follow in the wake of these fronts, and a robust lake enhanced
snow event may develop across the U.P. and northern L.P. of
Michigan before spreading to east of Lake Erie and Lake Ontario
late in the forecast period. Lake temperatures remain warm, +8C
across Superior to as warm as +15C on Lake Erie. With 850mb temps
progged to fall to -8C to -12C, this will drive steep lapse rates
to produce 1000-1500 J/kg of CAPE over the Lakes, and push
inversion depths to 10,000ft or higher. With little directional
shear showing up in profiles, this setup should support intense
bands of LES in the favored NW snow belts. For D3, the heaviest
snowfall is likely in the U.P. of MI where the most intense ascent
within the deepest inversion is progged, and WPC probabilities for
at least 6 inches are 30-50%, highest in the eastern U.P.
Elsewhere, WPC probabilities for 6 inches or more are 20-30% in
the NW L.P. of MI where some effective fetch from Superior aids,
and SW of Buffalo, NY east of Lake Erie. Despite the expected
intensity of these bands, shortwave ridging begins to build in
quickly late D3, so the temporal extent of any band through the
end of the period should be limited, which is reflected by PWPF
and NBM 90th percentile snowfall of 8-10 inches.

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

Weiss