Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Thu Mar 05 2020

Valid 00Z Fri Mar 06 2020 - 00Z Mon Mar 09 2020


Days 1-3...

...Great Lakes/Central Appalachians/Northeast...
A well-defined shortwave trough dropping through the upper Midwest
will continue to amplify as it moves farther east toward the mid
to northeast Atlantic coast on Friday. Sharpening deformation and
steepening lapse rates within the 700-500 mb layer as the low
deepens will support a period of moderate-heavy lake-enhanced
snows behind the departing surface low across Upper MI into
northern Lower MI. For the Day 1 period, WPC PWPF indicates high
probabilities for additional accumulations of 6+ inches or more
across portions of central Upper MI between Marquette and the
Keweenaw Peninsula.

As the upper trough dips into the Mid Atlantic region on Friday,
it will interact with the southern stream and support a deepening
surface cyclone off the coast. Overall consensus of the
deterministic guidance continues to show the surface low moving
offshore, however it should be noted that the trends at 12Z
Thursday were for a deeper upper system and surface low track a
little farther west (closer to the northeast coast). As a result,
an uptick in QPF was noted across eastern MA (Cape and Islands).
This is where the threat for significant snows would be, where the
WPC PWPF shows some lower probabilities for accumulations of
4-inches or greater. Meanwhile, the initial surface low is
expected to weaken as it moves east of the Great Lakes on Friday.
Overall, heavy accumulations east of the lakes are not expected;
although northerly winds may help to boost totals south of Lake
Ontario. Farther south, deep cyclonic northwesterly flow, with
embedded energy aloft will support snow showers with locally heavy
totals across the favored terrain of central Appalachians in
eastern WV Friday into Saturday.

...Western U.S....
Strong southwesterly inflow and low-to-mid level frontogenesis
associated to a positively-titled trough dropping southeast across
the northeastern Pacific is expected to contribute to some locally
heavy snows over the Olympics and northern Cascades Thursday night
into Friday.  The upper trough is forecast to pivot across the
eastern Pacific, driving the front farther south and east with
light snows moving into the northern Rockies to the Sierra Friday
night and Sat-Sat night. Post-frontal upslope flow, enhanced by a
wave moving east across Montana, may help to enhance totals over
the northwestern Montana ranges on Sat-Sat night.

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

Hurley/Pereira