Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Sun Nov 21 2021

Valid 12Z Sun Nov 21 2021 - 12Z Wed Nov 24 2021


...Pacific Northwest to the Northern Rockies...
Days 2-3...
Unsettled weather is expected to return to the northwestern U.S.
by late Monday 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 Monday night,
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.  High probabilities for 6 inches of snow or more are also
indicated for portions of the northwestern Montana ranges mainly
along and west of the Continental Divide, with the highest
probabilities centered along the Swan and Lewis ranges.

...Great Lakes...
Days 1-2...
A shortwave trough diving southeast across the Canadian Prairie
this morning will continue to amplify with an upper center
expected to close off north of the Upper Lakes Sunday night before
lifting northeast into northern Quebec on Monday.  Northwesterly
flow behind a strong cold front will support lake effect snow
showers developing off of Lake Superior beginning later today and
continuing through Monday, with WPC probabilities indicating
accumulations of 6 inches or more likely over the eastern U.P. 
Lake effect showers are expected to develop in the lee of the
eastern Great Lakes on Monday.  While veering winds are expected
to limit the potential for heavy amounts, a Lake Huron connection
to the showers developing off Lake Ontario may bolster the threat
for locally heavy amounts over central New York late Monday into
the overnight.

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

Pereira