Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
322 AM EST Thu Nov 25 2021

Valid 12Z Thu Nov 25 2021 - 12Z Sun Nov 28 2021


...Pacific Northwest and the Northern Rockies...
Day 1...

Upper level ridging will flatten out today into tonight as weak
shortwave trough energy pushes into the Pacific Northwest and
British Columbia. A cold front progressing through and an
associated atmospheric river will bring widespread precipitation
to the region. Snow levels will be quite high today, mostly above
pass level, confining the bulk of snowfall accumulations above
5000 ft. The highest peaks will see some higher snow accumulations
and the WPC probabilities for 6 inches are moderate to high for
the northern WA Cascades. 


...Great Lakes and the Northeast...
Days 2-3...

Positively to neutrally tilted troughing will become increasingly
negatively titled and compact as the shortwave energy approaches
the eastern Great Lakes and Northeast tonight into Friday. Surface
low pressure will likely weaken on Friday north of Vermont as a
wrapped-up oceanic system retrogrades northwestward, eventually
merging with and overtaking the approaching system from the west.
This will result in favored upslope snow across northern NY and
the northern/interior sections of New England (VT/NH/ME) where the
models continue to depict favored lift and saturation within the
DGZ. As the core of the closed low moves overhead, the crashing
cold temperatures will likely support higher SLRs and better lift
within the DGZ. The model spread remains quite high with the ECMWF
and its ensemble members on the higher/wetter end outlier though
the mean/consensus has trended wetter/snowier compared to the
previous cycle. The higher elevations of the NY Adirondacks,
northern VT/NH, and northwestern Maine show the greatest potential
for 4"+ where WPC probabilities are 80+ percent. There is an
increasing signal for heavy snow accumulations of 8-12" across the
extreme northern areas along the Canadian border, where WPC
probabilities for at least 8 inches is now up to 50-60 percent.
Behind the departing system, lake effect snows will increase off
of Lake Erie and Ontario where WPC probabilities for at least 4
inches are locally 40-50 percent off Lake Erie and above 70
percent for Lake Ontario / Tug Hill area.


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

Taylor