Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
301 AM EST Tue Nov 23 2021

Valid 12Z Tue Nov 23 2021 - 12Z Fri Nov 26 2021


...Pacific Northwest to the Northern Rockies...
Days 1-3...
Precipitation associated with a shortwave upper trough and its
surface cold front will continue to spread east across the Pacific
Northwest and into the northern Rockies today.  As snow levels
drop below 3000 ft, significant snow accumulations are expected
for portions of the northern Cascades, with WPC probabilities
indicating that accumulations of 4 inches or more are likely at
Snoqualmie and Stevens passes.  Locally heavy snows are also
expected to develop over the Blue Mountains and portions of the
northern Rockies beginning later this morning and continuing into
late in the day.  Areas impacted include the ranges in the Idaho
Panhandle and northwestern Montana.  WPC probabilities continue to
indicate that widespread accumulations of 4 inches or more are
likely for these areas.

Deep ridging will support dry conditions across much of the region
on Wednesday into early Thursday.  Then on Thursday and continuing
into Friday, a series of shortwaves will begin to flatten the
ridge, bringing another frontal system and onshore flow back into
the Northwest.  However, snow levels will be high across the
northern Cascades, with significant snow accumulations remaining
above pass level through early Friday.

...Great Lakes...
Days 1-3...
Ongoing lake effect snow showers are expected to diminish today as
an amplified upper trough continues to move east of the region,
replaced by deep ridging shifting east across the Midwest/Great
Lakes.  Dry conditions are forecast to continue into late
Wednesday before the system currently impacting the Northwest
moves across the Upper Midwest.  General consensus of the models
show the system amplifying, with a closed upper low developing as
it moves across the upper Great Lakes Thursday night.  This will
bring a return of lake effect snow showers developing off of Lake
Superior, with probabilities for significant accumulations
beginning to increase once again across the U.P. Friday morning.


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

Pereira