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

Valid 12Z Fri Nov 12 2021 - 12Z Mon Nov 15 2021


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

The significant atmospheric river event will weaken later
today/tonight with additional heavy snowfall for the highest peaks
as snow levels near 9000 ft this morning fall to below 5000 ft as
the precipitation comes to an end. 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. WPC
probabilities for 24-hr snow amounts > 4 inches are confined to
the highest elevations above 8000-9000 ft.

...Upper Midwest/Western Great Lakes...
Day 1...

Exiting surface low will complete its cyclonic loop later today
over MN/WI and weaken as wrap-around snow and snow showers
diminish through the day. Amounts will generally be light but
perhaps up to 4 inches in some favored areas of northern Wisconsin
to the U.P. of Michigan where WPC probabilities of exceeding 4
inches of snow are up to about 30-40%.


...Northern Plains and Upper Midwest...
Days 2-3...

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. Colder air
mass in the wake of the departing cyclone on Day 1 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. Accumulations should generally be light (glaze
to perhaps a few hundredths of an inch) as WPC probabilities of at
least 0.10" ice are less than 10 percent. Sharp but progressive
height falls and brief upper diffluence will support a
concentrated swath of snow from North Dakota southeastward just to
the north of the surface low track. Though the guidance remains a
bit dispersive on the axis, consensus has nudged up a bit as have
higher end potential amounts per the NBM. WPC probabilities for at
least 4 inches are around 30-50% from ND into northern MN on day
2. By Day 3, surface low will track through the eastern Great
Lakes as the upper trough digs into the Northeast. Snowfall
amounts are forecast to be an inch of two overall and
elevation/daylight-dependent across upstate/central/western New
York. WPC probabilities of at least 4 inches of snow are around
10-20% only in favored areas of the U.P. and northwestern PA.

Fracasso