Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 AM EST Wed Jan 20 2021

Valid 12Z Wed Jan 20 2021 - 12Z Sat Jan 23 2021


...Great Lakes...
Days 1-3...
The potential for single band, heavy snows in the lee of the
eastern Great Lakes will continue to lessen as winds begin to veer
to the northwest this morning.  Snow showers spreading south of
the lakes are expected to diminish with the passage of a shortwave
ridge later today.

The threat for significant accumulations is expected to return on
Thursday as low pressure moves from southern Ontario into Quebec. 
Westerly winds to the south across Lake Ontario will once again
support lake effect snow showers into the Tug Hill region, while a
mid level shortwave moving through the base of the trough supports
larger scale ascent across the region.  Meanwhile, strong
northwesterly flow developing across Lake Superior is expected to
support lake effect snows, with locally heavy accumulations
possible across the lake effect belts in the U.P. of Michigan on
Thursday continuing into Friday.

...Central Rockies...
Day 3...
An upper low dropping southeast from the Gulf of Alaska on
Wednesday, to the Washington and Oregon coasts by late Thursday,
is expected to transition to an open wave as it continues farther
southeast into Northern California and Nevada on Friday. 
Widespread snows are expected from the Sierra to the central
Rockies on Friday into early Saturday, although overall
accumulations are expected to be light -- with WPC PWPF well under
50 percent across most areas for accumulations of 4-inches or
more.  A notable exception is the Utah mountains, where a mid
level shortwave moving east across the state, along with a low
level front dropping south, is expected to raise the potential for
locally heavier amounts.  WPC PWPF shows higher probabilities for
accumulations of 6-inches or more across the Wasatch, Uinta, as
well as the southwestern Utah mountains on Friday and Friday night.

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

Pereira