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

Valid 00Z Fri Nov 12 2021 - 00Z Mon Nov 15 2021


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

A significant atmospheric river event pushing into the Pacific
Northwest and British Columbia through Friday will bring
widespread moderate to locally heavy precipitation to the region.
A warm front lifting through the area later today will lift snow
levels to or above 9 kft, confining any snowfall accumulations
above pass level. The highest peaks of the WA Cascades could pick
up in excess of 1 foot with local maximum above 2 feet.

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

Shortwave energy and intensifying upper jet streak will spread
snow across the northern Colorado Rockies and southern Wyoming
through early Friday morning. Light snow accumulations are
possible for the favored terrain areas with the highest peaks of
northern CO and southern WY likely to see 4-6" above 8-9 kft. WPC
probabilities for 4 inches are above 60 percent.


...Northern Plains and Upper Midwest...
Day 1...

Surface low deepening over Wisconsin later today occludes and
wraps cyclonically back toward the Minnesota North Shore tonight
while its associated upper low drops southeastward through the
Dakotas. A plume of higher Gulf moisture (PWs 0.5" to near 0.75")
wraps ahead of the surface fronts into the TROWAL side and as the
environment becomes more supportive of snow (steepening lapse
rates, favorable light within DGZ, increasing SLRs), an axis of
enhanced snow accumulations should setup across the eastern
Dakotas and northern/northwest Minnesota. Day 1 WPC probabilities
for 6 inches are highest along the Canadian border and max out
between 40-50 percent.

Days 2-3...

A fast-moving shortwave trough will track across the northern
Plains, Upper Midwest, and Great Lakes region late Friday through
Sunday. Favorable upper level diffluence combined with a
moistening low level environment aided by southerly flow ahead of
the deepening surface low will lead to a narrow axis of
precipitation. Mixed precipitation is expected across portions of
North Dakota (potential for a glaze of ice accumulation) where
soundings show a pronounced warm nose developing over the colder
near surface temperatures while soundings suggest all snow further
north/northeast. There remains some uncertainty in the track of
this clipper but guidance has trended a bit wetter such that the
WPC snow probabilities of 4 inches reach 20-40 percent in the Days
2-3 timeframe.

Taylor