Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
443 AM EDT Sat Oct 30 2021

Valid 12Z Sat Oct 30 2021 - 12Z Tue Nov 02 2021


...North-Central Rockies to the central High Plains...
Days 1-3...
A deep upper trough spreads over the Canadian Prairies today with
amplifying shortwave energy shifting south into the northern tier
of the CONUS as the whole system shifts east through early next
week. An associated cold front shifts south down the Plains today
through Monday. Post frontal upslope flow and increasing jet
dynamics aid locally heavy snow on the Bighorn Mtns where Day 1
WPC probabilities for 4 or more inches is 40% in the Bighorn Mtns
as snow levels fall to the surface around there.

Tonight through Sunday intensification of a secondary upper jet
streak combined with a shortwave trough dropping out Alberta and
low level convergent upslope flow will lead to locally heavy snow
developing in the Rockies near the WY/CO border by Sunday. Days 2
and 3 WPC probabilities for 4 or more inches are around 30% in the
Park Range and Front Range near Rocky Mountain NP.

Farther east across the High Plains of southeast WY and into far
northern CO and the Panhandle of NE, there remains a signal for
bands of moderate to heavy snowfall Sunday into Monday. The
intensification of the upper jet streak will place the favorable
right entrance for ascent atop this region, while the ageostrophic
response of this evolution will combine with the low-level
baroclinic zone to produce an enhanced band of fgen. While area
soundings continue to suggest a marginal thermal structure during
the period of most intense ascent, overnight Sunday into Monday
accumulations are likely, and if forcing can overcome the warming
into Monday morning, a low-level isothermal layer could support
heavier snowfall. WPC probabilities for 2 or more inches have
increased to around 20% with nonzero potential for 4 or more
inches.


...Lake Superior...
Day 3...

Lake effect precip begins behind the cold frontal passage of Lake
Superior Sunday with higher elevations having an accumulating risk
starting Sunday night, lowering closer to lake level late Monday.
Day 3 snow probabilities for 4 or more inches are around 5 percent
for the higher terrain of the western UP of MI.


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

Jackson