Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
220 PM EST Wed Nov 11 2020

Valid 00Z Thu Nov 12 2020 - 00Z Sun Nov 15 2020


...Western U.S....
A warm front associated with an approaching upper level shortwave
will bring organized heavier precipitation, including mountain
snows, back into the Olympics and northern Cascades by late
Thursday.  While this system is expected to weaken and move
progressively downstream, it will be quickly followed a second
system.  Models show an amplifying shortwave along the leading
edge of fast, Pacific flow moving into the Northwest by late
Friday, with another plume of deeper moisture spreading from the
Northwest into the northern Rockies.  Areas impacted by
significant snow through late Friday are likely to include the
Olympics, Washington and Oregon Cascades, the Wallowa Mountains in
northeastern Oregon and the central Idaho ranges. 

This second system is expected to move rapidly across the West as
well, with a well-defined cold front dropping south across the
Rockies and Great Basin on Saturday.  However, upstream onshore
flow with embedded energy aloft, will support periods of high
elevation snow across the Northwest into the northern Rockies
through Saturday.  Additional heavy snows are likely for portions
of the Cascades on Saturday, as well as the central Idaho ranges,
including the Sawtooth.  Areas of heavy snow are also expected to
develop farther north across the northern Idaho and northwestern
Montana ranges, with favorable upper forcing helping to enhance
precpitation rates across the region Friday night into early
Saturday.  Locally heavy accumulations are also possible across
the western Wyoming and northern Utah ranges as the aforementioned
front pushes through the region on Saturday.  

WPC PWPF suggests many of the previously noted areas are likely to
see accumulations of a foot or more by late Saturday.  The
Washington Cascades are expected to see the heaviest three day
totals, with WPC PWPF indicating widespread probabilities of 50
percent or higher for amounts of two feet or more.

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

Pereira