Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Mon Nov 22 2021

Valid 12Z Mon Nov 22 2021 - 12Z Thu Nov 25 2021


...Pacific Northwest to the Northern Rockies...
Days 1-2...
Unsettled weather is expected to return to the northwestern U.S.
later today as a prevailing upper ridge gives way to a
well-defined shortwave trough.  This system is expected to bring
widespread precipitation, including some orographically focused
heavier amounts to the Pacific Northwest beginning tonight before
spreading east into the northern Rockies on Tuesday.  As the upper
trough moves across the Northwest into the northern Rockies, snow
levels are expected to drop below 2000 ft on Tuesday across much
of Washington, the Idaho Panhandle, into northwestern Montana. For
the two-day period ending 12Z Wednesday, WPC probabilities
indicate that accumulations of 6 inches of snow or more are likely
for portions of the Washington Cascades, mainly for areas above
4000 ft.  Probabilities also suggest that local amounts of 6
inches of snow or more are possible for portions of the Idaho
Panhandle and northwestern Montana ranges.  These include the
Clearwater Mountains in northern Idaho and the Swan and Lewis
ranges in Montana.

...Great Lakes...
Days 1-2...
An amplifying upper trough centered over the Great Lakes and Ohio
Valley will continue to drive a strong cold front across the
Eastern Seaboard today.  Northwesterly flow behind the front will
continue to support lake effect snow showers off of Lake Superior
through today, with locally heavy accumulations possible over the
eastern U.P. of Michigan.  Lake effect showers are expected to
develop in the lee of the eastern Great Lakes later today.  While
veering winds are expected to limit the potential for heavy
amounts, increasing moisture attributed to a Lake Huron connection
may bolster the threat for locally heavy amounts over central New
York later today into the overnight.

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

Pereira