Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
435 PM EDT Thu Apr 22 2021

Valid 00Z Fri Apr 23 2021 - 00Z Mon Apr 26 2021

...Far Northern Maine...
Day 1...

A strong area of low pressure will continue to slowly lift
northeast away from New England and into the Canadian Maritimes
through early on Friday. The potent mid-level closed low
associated with this feature will also drift near northern Maine
through the evening, and the combination of cold advection and NW
flow will lead to periods of moderate to heavy snowfall until the
column dries out Friday morning. Snowfall accumulations should be
confined to the northern tip of Maine, with the highest
accumulations across far northwestern Maine. This region is where
WPC snowfall probabilities for 4 inches are as high as 80%.


...Northern Rockies...
Day 1...

Shortwave energy rotating around a larger upper-level low located
across north-central Canada will continue to swing across the
Northern Rockies tonight, plunging a cold front southward into
Wyoming by Friday morning. Modest height falls will accompany the
shortwave, but briefly intense low-level convergence/frontogenesis
combined with intensifying upslope flow behind the front will
produce a round of moderate to heavy snow across parts of MT and
WY on D1. The heaviest snow is expected where upslope enhancement
occurs, and this is likely on the eastern side of the Northern
Rockies, Absarokas, and Big Horns. WPC probabilities for 4 inches
are as high as 70% in these areas.


...Northern Rockies, Pacific Northwest, and California...
Days 2/3...

By Saturday night, a strengthening closed upper-level low will
swing just off the coast of OR as it drops southeast from the Gulf
of Alaska. As this trough sharpens, a downstream jet streak will
intensify and stretch from Northern CA to the Northern Rockies.
Within this jet streak and south of the mid-level closed low,
moist advection will enhance from the Pacific and provide ample
moisture up to +1 standard deviations above the climo mean.  The
result will be an increasing area of precipitation from CA to WA,
eastward into MT. Snow levels are expected to rise initially on
Saturday night as the warm moist air floods inland, but a cold
front ahead of the mid-level wave will help bring down snow levels
to around 4000 ft by Sunday morning. Much of the heaviest precip
should occur on Sunday through the end of the forecast period
across northern California and the Sierra Nevada. WPC
probabilities for 6 inches are over 70% on D3 in portions of the
Northern Sierra Nevada, with up to 50% for 12 or more inches.


For Days 1-3, the probability of significant icing (0.25 inch or
more) is less than 5 percent.

Snell