Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 AM EST Tue Jan 19 2021

Valid 12Z Tue Jan 19 2021 - 12Z Fri Jan 22 2021


...Great Lakes...
Days 1-3...
Cold westerly flow will continue to support lake effect snows east
of Lake Erie, with additional heavy accumulations possible across
the western Southern Tier of New York on Tuesday, with WPC PWPF
indicating high probabilities for additional accumulations of
6-inches or more for the region during the Day 1 period.  A
redeveloping band farther to the south may reintroduce the threat
for heavy amounts back into Northwest Pennsylvania Tuesday night. 
Meanwhile, lake effect snows are expected to develop and intensify
east of Lake Ontario into the Tug Hill Region on Tuesday, where
WPC PWPF indicates locally heavy amounts of 8-inches or more are
likely.  The threat for heavy snow is expected to diminish on
Wednesday, with snow bands becoming more transient as the veering
winds direct them farther to the south before diminishing as a
shortwave ridge moves across the region.  Snows are expected to
return to portions of the region on Thursday as a low moves north
of the lakes across southern Ontario into Quebec.  Westerly winds
behind the trailing cold front are expected to help reignite lake
effect snows east of Ontario, with locally heavy amounts possible
once again across the Tug Hill Region.

...Southern Rockies...
Day 1...
Favorable upper forcing along with low level easterly flow will
continue to support snows through the morning hours across the
southern Colorado and northern New Mexico mountains, with
additional heavy accumulations possible across the San Juan and
Sangre de Cristo mountains.  As the supporting upper low continues
to retrograde west across Southern California, expect snow to
quickly diminish during the afternoon.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira