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

Valid 00Z Tue Nov 23 2021 - 00Z Fri Nov 26 2021


...Pacific Northwest to the Northern Rockies...
Days 1-2...
A well-defined shortwave trough and attendant cold front will push
into/through Washington tonight, bringing widespread precipitation
to the area, including some orographically-focused heavier amounts
to the Cascades. This will spread east into the northern Rockies
tomorrow as the front continues southeastward. 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.
Ridging will move into the Pac NW by early Wednesday, helping to
diminish snowfall.

For the two-day period ending 00Z Thursday, 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.

Day 3...
Minor height falls will push into Washington on day 3 as another
cold front comes ashore. Snow levels will rise toward 7000-8000ft
by late Thu with accumulating snow generally limited to above pass
level.


...Great Lakes...
Day 1...
An amplifying upper trough centered over the Great Lakes and Ohio
Valley will continue to drive a strong cold front off the Eastern
Seaboard tonight. Northwesterly flow behind the front will subside
tonight as ridging moves in from the west, shutting off any lake
effect snow showers off of Lake Superior. Lake effect showers in
the lee of the eastern Great Lakes will maintain an Ontario band
across central New York this evening/overnight until winds veer
and the band sinks southward, decays into weaker multi-bands, and
then diminishes tomorrow afternoon. Several inches may accumulate
in a more focused band this evening.

Day 3...
Low pressure moving across Lake Superior late Wed into Thu will
bring in colder air on day 3 on NNW to NW winds, restarting lake
effect snow for the U.P. of Michigan. Several inches are possible
over western and central portions of the U.P.

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

Fracasso/Pereira