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

Valid 00Z Mon Nov 22 2021 - 00Z Thu Nov 25 2021

...Pacific Northwest to the Northern Rockies...
Unsettled weather is expected to return to the northwestern U.S.
by late Monday as a prevailing upper-level ridge gives way to a
well-defined shortwave trough coming in from the Pacific. This
system is expected to bring widespread precipitation, including
some orographically focused heavier amounts to the Pacific
Northwest beginning Monday night, and then spreading east into the
northern Rockies on Tuesday. As the upper-level trough moves
across the Pacific Northwest and into the northern Rockies, much
colder air aloft will be arriving and the snow levels are forecast
to drop to as low as 2000 to 3000 feet across much of Washington,
the Idaho Panhandle, and northwest Montana. The high terrain of
the northern Washington Cascades are expected to see as much as 1
to 2 feet of snow going through Tuesday. Heavy totals are also
expected locally across the Bitterroots and the high terrain of
northwest Montana along and west of the Continental Divide. The
Swan and Lewis ranges are forecast to see totals locally over a
foot for the period.

...Great Lakes...
A shortwave trough currently amplifying across southwest Ontario
and the upper Great Lakes region will advance steadily off to the
east overnight before lifting up across western Quebec on Monday.
This will occur as a broader upper trough moves into the eastern
U.S. over the next couple of days. The strong energy with the
shortwave and deeper layer cyclonic flow over the Great Lakes will
couple with strong cold air advection behind a cold front to
produce heavy lake-effect snow showers and squalls downwind of the
lakes. Going through tonight, some of the heaviest snowfall should
be off Lake Superior involving the U.P. of Michigan and snowfall
amounts locally exceeding 6 inches are expected across this area
going through early Monday. Gradually the lake-effect snow threat
will extend downstream to areas downwind of Lake Erie and Lake
Ontario on Monday and this will continue through Monday night.
Portions of northwest Pennsylvania and southwest New York downwind
of Lake Erie should see locally several inches of snow. However,
somewhat heavier totals that could locally exceed 6 inches are
expected for areas immediately downwind of Lake Ontario to the
southwest of the Tug Hill plateau where a multi-lake fetch from
Lake Huron through Lake Ontario may favor more organized and
intense snow shower/squall activity.

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

Orrison