Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 PM EST Wed Mar 04 2020

Valid 00Z Thu Mar 05 2020 - 00Z Sun Mar 08 2020



...Northwestern CONUS...
Days 1-3...

A low pressure system will track south-southeast from the Gulf of
Alaska tonight through Saturday, reaching Vancouver Island by late
Saturday. Pacific moisture on southwesterly and lift from the
approaching low ends the brief respite in precipitation for
western WA Thursday afternoon. Snow levels will be generally 3000
to 4000ft on the leading edge of the slowly drifting south precip
and closer to 2000ft on the other end which is closer to height
falls and colder air. Day 1 snow probabilities for 6 or more
inches are limited to the highest Olympics. This expands to
include the highest WA Cascades on Day 2. Day 3 the focus has
shifted to far northern CA and across to the northern Rockies with
moderate to high probabilities for 6 or more inches for the CA and
OR Cascades, Blue Mtns, and higher ranges of ID/MT with lesser
probabilities for the WA Cascades and Olympics.


...Northern Great Lakes across the Mid-Atlantic and Northeast...
Days 1-3...

A potent shortwave trough will continue to amplify as it track
across the northern Plains tonight, closing off as it track
southeast across the Great Lakes Thursday afternoon/night. Ahead
of this trough/low is a filling surface low that crosses the
northern Great Lakes Thursday and the eastern Great Lakes Thursday
night. Across the Great Lakes, a brief period of warm air
advection ahead of the low will spread precipitation across far
northern MN/MI, with moderate snow likely only in northern
Arrowhead and the U.P. where thermal profiles are cold enough for
frozen precipitation. As the low digs further SE, a brief period
of northerly flow/cold air advection will encompass the Lakes,
with LES likely in the favored N/NW snow belts south of Lake
Superior in the U.P. Inversion heights are shallow and the fetch
is generally unfavorable for more than multi bands of short
duration, but Day 1 and Day 1.5 WPC probabilities across the U.P.
are moderate to high for 4 or more inches, with the best chance
for 6 or more inches in the eastern portion of the U.P in Day 1.5.

This system trails a particularly strong southern stream trough
that pushes off the NC coast Thursday night, but the two systems
begin to phase off the northern Mid-Atlantic coast Friday as the
mid-level trough/low crosses PA. Wrap around snow from this
mid-upper level feature will be promoted from the instability in
the cold air over the warmer surface air Friday into Friday night.
While low level temperatures are rather marginal for accumulating
snow during the day Friday, continued precip in cold air advection
in the wrap around could allow some light snow accumulations in
the central Mid-Atlantic. Farther north and at higher elevation,
particularly on the west side of the crest of the Appalachians,
colder air (and upslope flow for the mountains) will promote
notable snow. Day 2 WPC probabilities for 4 or more inches of snow
are high over the Allegheny Plateau in WV into westernmost MD,
with scattered low probabilities in the lee of the eastern Lakes
and at higher elevations such as the Poconos, Catskills, and
Adirondacks.

The low tracks off shore with the 12Z consensus keeping heavy snow
from the rapidly developing low offshore. The only area with
potential for heavy snow in Day 3 is for Cape Cod and surrounding
Islands.


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

Jackson