Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
235 PM EST Sat Nov 20 2021

Valid 00Z Sun Nov 21 2021 - 00Z Wed Nov 24 2021

...Great Lakes...
Days 1-3...

A shortwave trough embedded within progressive northwest flow over
Alberta this afternoon is expected to dig across central Canada
tonight, amplifying as it approaches the Upper Midwest Sunday
morning. This feature is forecast to swing east, with model
guidance indicating a compact upper low developing as it moves
across the northern Great Lakes Sunday night. A surface low is
expected to quickly develop north of the lakes and move northeast
from northern Ontario into Quebec. Strong cold air advection
provided by deep northwesterly flow on the backside of the system
will support lake effect snow showers developing initially off of
Lake Superior into the U.P. and northern Lower Michigan, where
locally heavy amounts may accumulate beginning Sunday and
continuing into Monday. Day 2 WPC snow probabilities for 4 or more
inches are focused over the eastern U.P. with a max of around 70%
near Whitefish Bay. By Monday afternoon and continuing into early
Tuesday, flow will support lake effect showers east and southeast
of lakes Erie and Ontario. Day 2.5 WPC snow probabilities for 4 or
more inches are limited to about 10% south of Buffalo and on the
Tug Hill. Activity is expected to wane as deep ridging shifts east
across the Lakes on Tuesday.

...Pacific Northwest...
Day 3...

Upper-level ridging across the Pacific Northwest is expected to
shift eastward into the central U.S. on Monday, ahead of a compact
shortwave trough approaching coastal Washington/Oregon. This
shortwave will usher in a narrow, but notable (+1 SD) atmospheric
river into portions of the Pacific Northwest Monday night into
Tuesday. A short period of upslope flow is anticipated for the
Olympics and Cascades with initially relatively low snow levels
(~3000-4000 feet) falling to as low as 2500 feet during the day on
Tuesday (behind the system) coinciding with heavy snowfall
tapering off. Day 3 WPC snow probabilities for 6 or more inches
are moderately high (40-70%) for much of the Washington Cascades.

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

Pereira/Churchill