Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
428 PM EDT Mon Nov 01 2021

Valid 00Z Tue Nov 02 2021 - 00Z Fri Nov 05 2021


...Colorado and Kansas...
Days 1-2...

Broad troughing across the eastern 2/3 of the CONUS will leave
weak cyclonic flow across the Central Rockies Monday night and
Tuesday. A shortwave emerging from the Pacific will push eastward
across the Great Basin and become embedded within the broad
cyclonic flow on Tuesday as it moves across CO and into the
Central Plains producing modest height falls within a confluent
moisture stream. At the same time, a jet streak arcing across the
Upper Midwest will leave the favorable RRQ for ascent atop the
region, and this deep layer ascent in the enhanced moisture will
produce periods of moderate to heavy snow, generally above 7000 ft
in the CO Rockies. The guidance has trended subtly upward with
snowfall today, and WPC probabilities have followed suit, now
showing a moderate risk for 6 inches in the Flat Tops, Park Range,
and Front Range near Rocky Mountain NP. Further east into the High
Plains of CO and into KS, cooling of a marginal column Tuesday
night into Wednesday coincident with modest ascent could produce
some light accumulations of less than 1 inch.


...Great Lakes...
Days 1-2...

Persistent cyclonic flow leading to CAA across the Great Lakes
will produce a favorable environment for Lake Effect precipitation
each of the next three days. The most intense precip is likely D1
/tonight and Tuesday/ as the strongest CAA across the waters leads
to the steepest lapse rates and highest inversion depths.
Additionally, guidance indicates periodic shortwave impulses
rotating within the flow through Wednesday /D2/, before the trough
re-orients itself to the west leading to more neutral advection D3
and a lower probability for lake enhanced precip.

The thermal structure of the column will remain marginal, and
water temps are quite warm at +8C for Superior and +15C for Erie
and Ontario. While this will lead to lapse rates appropriate for
Lake Effect snow, surface temps will be near freezing except
within the terrain. This will likely limit the snowfall
accumulations to areas away from lake level, but WPC probabilities
for 4 inches are above 60% in the U.P. of MI where isolated maxima
to 12" is possible on D1. Otherwise for D1 and D2, WPC
probabilities for 4 inches are 5-20% in the higher terrain of the
northern L.P. of MI, as well as SW of Buffalo, NY east of Lake
Erie and in the Tug Hill Plateau.


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

Weiss