Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Sun Oct 31 2021

Valid 00Z Mon Nov 01 2021 - 00Z Thu Nov 04 2021


...Central Rockies to the central High Plains...
Days 1-3...

Several waves of precipitation including moderate to heavy snow
will impact the Central Rockies and central High Plains through
mid-week. These waves of snowfall will be driven by periodic
shortwaves rotating within broad cyclonic flow, interacting with a
low-level baroclinic gradient, especially D1, with additional
snowfall D2 and D3 as more broad but deep layer ascent overlap
across the region.

On D1 /Tonight through Monday/ the heaviest snow will occur in a
swath from the CO Rockies through central Nebraska. In the
Rockies, especially the Front Range and Park Range, a shortwave
producing height falls for ascent at the edge of merging upper jet
streaks will produce heavy snowfall for which WPC probabilities
indicate a moderate chance for 4 inches. Further to the east into
the High Plains, the setup continues to look favorable for a band
of moderate to at times heavy snow, which the snowband tool
suggests has a high probability to include rates exceeding 1"/hr.
There is still uncertainty into the exact placement of this heavy
band of snow, but it does appear likely it will develop, with
potentially convective rates as robust fgen occurs within a
saturated DGZ. WPC probabilities have increased from west to east
with this band, and where the heaviest snow occurs more than 4
inches is possible, with more than 2 inches possible as far east
as 100W longitude.

After a quiet Monday night, a secondary shortwave will eject from
the Pacific and across the Great Basin into the Central Rockies
within the broad cyclonic flow. This trough will undercut a ridge
across western Canada and produce height falls and modest PVA to
drive ascent within a plume of Pacific moisture reaching +1 to
+1.5 standard deviations above the climo mean for PW. WPC
probabilities for snow reach as high as 50% D2.5 above 7000ft in
the Flat Tops, with slightly lower probabilities in to the Front
Range.


...Great Lakes...
Days 1-3...

Lake effect precip begins behind the cold frontal passage off Lake
Superior continues D1 with higher elevations in the U.P. having
the best potential for accumulating snow, lowering closer to lake
level by late Monday. Water temperatures near +10C will drive
steepening lapse rates and a deepening inversion to above 9000 ft.
This should allow for lake enhanced precipitation to expand across
the U.P. of Michigan, but marginal low-level thermals may lead to
a rain/snow mix outside of the higher terrain such as the Huron
and Porcupine Mountains. WPC probabilities for 4 inches peak on
Day 1.5 in the Huron Mtns just south of the Keweenaw Peninsula,
but are still less than 20%.

The cold northwesterly flow spreads across the rest of the Great
Lakes Tuesday with LES for all typical Great Lake snow-belts by
Tuesday night. However, modest duration of forcing and marginal
low-level thermals within the column suggest a rain/snow mix with
accumulations occurring only in the higher terrain. WPC
probabilities for 2 inches are generally less than 10% outside of
the U.P.


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

Weiss