Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 PM EST Fri Nov 12 2021

Valid 00Z Sat Nov 13 2021 - 00Z Tue Nov 16 2021


...Pacific Northwest...
Days 1-3...

A significant atmospheric river event will weaken later tonight
with additional heavy snowfall for the highest peaks as snow
levels fall to below 5000 ft as precipitation comes to an end.  
The next system Saturday afternoon will push its moisture and warm
front into Washington from the southwest, again raising snow
levels toward 9000 ft after a brief period of warm advection snow
as temperatures rise. Levels are expected to remain high until
Monday when an amplified upper shortwave and its associated cold
front push into the region.


...Northern Plains, Upper Midwest, and Great Lakes...
Days 1-3...

A fast-moving shortwave trough will track east-southeastward out
of southwestern Canada late tonight into early Saturday then
deepen as it moves through the Dakotas by early Sunday. A colder
air mass in the wake of a departing cyclone -- lifting across the
upper Great Lakes tonight -- will erode aloft first, resulting in
a brief period of freezing rain as precipitation moves in from the
west before changing to rain south of the low track.   Ice
accumulations should generally be light (glaze to perhaps a few
hundredths of an inch) as WPC probabilities of at least 0.10" ice
remain less than 10 percent.

Sharp, but progressive height falls and brief upper diffluence
will support a concentrated swath of snow, just to the north of
the surface low track, from North Dakota southeastward into
northern Minnesota, on Saturday.   Subtle difference in the
amplitude and timing of the trough appear to be contributing to
more discernable differences in QPF/snow amounts, even in the
short term -- with the some of the guidance, including the
ECMWF/UKMET/Canadian notably heavier than the GFS, GEFS, NAM and
several CAMs members.  Given the spread, WPC probabilities for
accumulations of 4 inches or more have come down a little from
eastern North Dakota into northern Minnesota; but given the
potential for greater dynamics, cannot rule out the threat for a
narrow band of locally heavy amounts setting up across the region.
 Snow is expected to shift from northern Minnesota into upper
Great Lakes region Saturday night and Sunday, with the better
forcing expected to center the heavier amounts from northwestern
Wisconsin to central and southern Lower Michigan.  WPC
probabilities indicate than at least an inch or two is likely,
with a small threat for 4 inches or more.    

Prior to the shortwave, cyclonic flow/cold air advection in the
wake of a departing low is expected support lake effect showers,
producing at least an inch or two across portions of the U.P. and
east of Lake Michigan, away from the lakeshore, Friday night into
Satuday.  An amplified shortwave pivoting south of the low is also
expected to help ignite lake effect activity as it moves across
the lower Lakes late Saturday into early Sunday.

Pereira/Fracasso