Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 AM EST Mon Jan 18 2021

Valid 12Z Mon Jan 18 2021 - 12Z Thu Jan 21 2021

...Great Lakes...
Days 1-3...
Cold, cyclonic, westerly flow will continue to support lake effect
snow showers off of Lake Erie, with several more inches possible
for portions of New York's Western Southern Tier on Monday into
Tuesday.  By late Tuesday, strengthening westerly winds will
support the redevelopment of lake effect snows east of Lake
Ontario, with locally heavy accumulations possible in the Tug Hill
region.  Veering winds with the passage of a shortwave trough will
push snows farther south on Wednesday, before diminishing late in
the day with the arrival of a shortwave ridge.

...Northern Rockies...
Day 1...
Unstable, northwesterly flow behind a cold front will continue to
support periods of snow, with additional locally heavy
accumulations possible across the central to southwestern Montana
mountains on Monday.  WPC PWPF shows high probabilities for
additional accumulations of 6-inches or more across portions of
the Little Belt, Big Snow and northern Bighorn moutains.

...Southern Rockies/Southwest...
Days 1-2...
A developing mid-to-upper level low dropping south through the
Great Basin into Southern California, along with a cold front
pushing south across the Rockies and High Plains will support
large scale ascent and increasing snowfall across portions of the
region by the late Monday.  As moisture deepens ahead of the low,
favorable upper forcing along with low level easterly flow behind
the front will support developing snows across the region by late
Monday and continuing into Tuesday.  Heaviest accumulations are
expected to center across the San Juan and Sangre de Cristo
mountains, where WPC PWPF shows widespread probabilities for storm
totals of 8-inches or more. 

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Pereira