Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Thu Nov 11 2021

Valid 12Z Thu Nov 11 2021 - 12Z Sun Nov 14 2021


...Pacific Northwest and Northern through Central Rockies...
Days 1-2...

Minor shortwaves will continue to push through the Pac NW and
British Columbia Thu/Fri as ridging tries to build from the south,
maintaining an impressive moisture plume into Washington today
into early Friday. With snow levels quite high near 8000-9000 ft,
accumulations will be confined to well above pass level at the
highest peaks where several feet may accumulate via the
significant atmospheric river event.

...Colorado...
Day 1...

Intensifying upper jet streak will bring generally light snow to
the northern Colorado Rockies (Park Range) into southern Wyoming
(Medicine Bow mountains) today on the southwest side of the Upper
Midwest cyclone. WPC probabilities for at least 4 inches of snow
are around 30-60% generally above 9000 ft.

...Northern Plains and Upper Midwest...
Days 1-2...

An upper low will move southeastward through the Dakotas today as
a surface frontal system pivots on its eastern side, eventually
wrapping around in a cyclonic loop over WI/MI/MN as the system
becomes vertically stacked on Friday as it pulls into Ontario. The
low will reach peak intensity this evening as the system matures
and occlusion wraps its warm conveyor belt inflow around its
northwest side, promoting snow (and rain changing to snow) in a
cooling environment from northwest to southeast. Gulf moisture
will funnel northward through the day (PW 0.50-0.75") ahead of the
cold/occluded front into the TROWAL on the northwest side of the
surface low that will wrap around and eventually into the stacked
circulation. Ascent will become more widespread through the
developing comma head as warm air advection within the TROWAL will
help to deepen the DGZ and provide locally-enhanced ascent
coincident with the sharpening deformation axis. The deepening
lapse rates and cooling column should produce locally higher SLRs
to enhance snow accumulations even in regions of modest ascent.
The system will elongate along the occlusion toward Hudson Bay
late Friday as the trailing sharper upper trough takes over for
forcing across the Midwest in a more marginal thermal environment
on Saturday. 

Day 1 WPC probabilities for 6 or more inches are above 50% over
northern Minnesota and northeastern North Dakota through Friday
morning and above 30% along the Canadian border near Lake of the
Woods. The snow will wind down and push east on day 2 as WPC
probabilities for 4 inches drops to near 10 percent over the
western U.P. of MI. On day 2 into 3, trailing upper shortwave and
cold core aloft will promote at least light snow across much of
the Great Lakes and into the central/northern Appalachians
including southwestern NY, Tug Hill, and Adirondacks, but
probabilities of at least 4 inches remain mostly less than 10
percent as of now except over parts of the highest elevations of
the Adirondacks.


Fracasso