Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 PM EDT Sat Mar 14 2020

Valid 00Z Sun Mar 15 2020 - 00Z Wed Mar 18 2020


...Oregon and California...
Days 1-3...

...Prolonged Heavy Snow with Several Feet Expected for the
Northern and Central Sierra Nevada through Monday night...

A cold core low drifts south off the OR coast this evening and
then off the northern CA coast Sun evening and the central CA
coast Monday evening, arriving off the south central CA coast Tue
evening. The upper level jet slowly moves across the ranges of
northern and central CA. This directs Pacific moisture ahead of
the low across northern/central CA through Monday night into Tue.
Prolonged synoptic ascent through upper divergence/low level
convergence, with aid from upslope flow in windward terrain will
produce widespread heavy snow in the ranges of northern to central
CA, with the heaviest amounts in the northern to central CA Sierra
Nevada mountains, with multiple feet of snow are expected through
Mon. Both day 1 and Day 2 have high risks for heavy snow in the
northern to central Serra Nevada.  The risks for heavy snow in the
Shasta/Siskiyous are centered on day 1, and drop on Day 2.

The filling low progresses south on Monday into Tuesday, so the
axis of snow shifts south out of central CA into the southern CA
Sierra Nevada.  Falling temperature aloft allow snow to develop in
the Transverse ranges of southern CA on Tue. Day 3 (Tue) snow
probabilities for 12 or more inches are lower in the central and
southern Sierra Nevada as the weaker low results in weaker inland
moisture transport and lift.


...Northern Rockies...Northern Plains...Upper MS Valley...Lake
Superior...
Days 1-2...

Ongoing heavy snow in north-central MT winds down this evening as
the 700 mb warm front that provided lift moves north across the
border with Canada.  The upper jet continues to drift slowly east
across central to eastern MT, but 700 mb lift is weaker late
tonight and Sunday, keeping amounts in eastern MT light.  The
moisture plume from the closed low off CA continues inland across
NV and then southeast ID and northwest WY. A prolonged period of
lift results in potential for significant snowfalls
for the Absarokas/Tetons/Wind River ranges.  There is potential
for locally a foot of snow in favored upslope areas for the 
Tetons and around Yellowstone National Park.  The models forecast
lower amounts Sun night-Mon as ascent wanes, and drier air aloft
advects across the region Mon.

A 700 mb shortwave impulse moves east from MT across ND to
northern MN Sunday into Monday, allowing generally light snow to
develop in northern ND late tonight and Sunday, followed by
eastern ND late Sun to northern MN Sun night to early Mon.
The steady progression of the wave plus modest ascent should keep
amounts light. Light lake enhanced snow are indicated over the
Keweenaw Peninsula, but again event potential is limited as the
system move east of the region and drying aloft occurs Tue. 


...Central Appalachians...
Day 1...

An area of mixed precipitation moving east from the Ohio Valley
continues across the central Appalachians tonight in response to
low-mid level moisture advection and corresponding 850-700 mb
lift, further supported by an approaching jet streak aloft.
Higher elevations of the central Appalachians should see snow
accumulating tonight before the wave causing the ascent moves east
of the region on Sun, with the event ending. Snow probabilities
are moderately high over the northern Allegheny Plateau centered
on western MD, and adjacent southwest PA/northern WV, tonight
where terrain enhances snow.  The Models also show dry air aloft
advecting into the region when the snow tapers, with the
possibility of a brief period of freezing drizzle in northeast WV.

...Northern New England Day 3...

The system which impacted the upper lakes Day 2 continues across
northern New York and northern New England on Day 3.
The mid level warm advection changes precip over to rain quickly
in most of northern NY and VT, with cold air lasting for a longer
period when it is cold enough for snow in interior Maine.  Several
inches of snow are possible in northern Maine on Tue. 
The models are in good agreement on temperatures remaining too
warm for snow in downeast ME.

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

Petersen