Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 PM EST Wed Dec 18 2019

Valid 00Z Thu Dec 19 2019 - 00Z Sun Dec 22 2019

...Northeast...
Day 1...

Ongoing lake effect snow is expected to continue into tonight in
the wake of an arctic front that passed through the region
Wednesday afternoon.  A potent shortwave will be pivoting around a
closed low, and this will support lift and northwest flow across
the warmer lake waters.  Steep lapse rates should allow for
elevated inversion heights and narrow lake effect bands can be
expected downwind of Lake Erie and Lake Ontario before tapering
off by Thursday morning.  Snow showers are also expected across
portions of central and southern Maine in conjunction with the
upper level shortwave impulse.  Isolated additional snowfall
totals in excess of 4 inches will be possible tonight where lake
effect bands persist. 


...Northwest...
Days 1-3...

An active weather pattern will be in place across the West Coast
region for the end of the week and into Saturday as an atmospheric
river event is expected.  Snow levels will initially be low early
in the event, but are forecast to rise on Thursday with snow
levels expected to reach 6000 feet or greater across northern
California by Thursday night.  The heaviest snowfall this period
is for the Washington Cascades where snow levels briefly rise to
near 4000 feet Thursday night as mid level heights rise ahead of
the next trough, before gradually dropping again.  Snowfall totals
on the order of 1 to 3 feet are likely for the higher elevations
of the Cascades and Olympic Mountains, with isolated higher maxima
across the highest peaks.  A transition from snow to rain is
expected for the lower terrain of the Cascades owing to the warm
air advection, and this will greatly reduce the total snowfall
accumulation.  Heavy snow on the order of 1 to 2 feet also appears
likely for the higher terrain of extreme northern Idaho and
northwest Montana during the Day 2 and 3 period. 

The probability for significant freezing rain is less than 10
percent all 3 days.

Hamrick