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

Valid 12Z Thu Mar 05 2020 - 12Z Sun Mar 08 2020


Days 1-3...

...Great Lakes/Central Appalachians/Northeast...
A well-defined shortwave trough dropping into the northern Plains
will continue to amplify as it moves farther east across the Upper
Midwest-Great Lakes on Thursday.  Low-to-mid level frontogenesis
in addition to strong upper forcing will support a period of light
to moderate snows that will move east from northern Minnesota to
northern Michigan.  As the low moves across the Upper Peninsula
into northern Lower Michigan late Thursday into the overnight,
strong northeasterly flow on the backside of the system will
support lake-enhanced snows, which are expected to contribute to
locally heavy totals across the Upper Peninsula.  For the Day 1
period, WPC PWPF indicates high probabilities for accumulations of
6-inches or more across much of the northern Upper Peninsula. 

The upper trough is forecast to drop east-southeast from the Great
Lakes into the Mid Atlantic region on Friday where it will
interact with the southern stream -- supporting a deepening
surface cyclone off the coast.  Overall consensus of the
deterministic guidance continues to show the low moving offshore
with little precipitation impacting the northeastern seaboard. 
Any threat for significant snows is limited to southeastern
Massachusetts, 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 on Friday.

...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.  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
Saturday.  Post-frontal upslope flow, enhanced by a wave moving
east across Montana, may help to enhance totals over the
northwestern Montana ranges on Saturday.

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

Pereira