Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Fri Jan 31 2020

Valid 12Z Fri Jan 31 2020 - 12Z Mon Feb 03 2020


...Pacific Northwest through the Great Basin and Northern
Rockies...
Days 1-3...

A prolonged fetch of Pacific moisture will intensify today into
Washington State as PWs over 1 inch advect onshore accompanied by
50 kts of 850mb flow behind a warm front lifting into British
Columbia. While moisture is expected to be copious, very warm air
flooding inland will raise snow levels above 7000 ft on D1, and
WPC probabilities for 4 inches of snow are only moderate, and
confined to above 7000 ft. The highest peaks of the Cascades and
Northern Rockies may see 12 inches however.

During D2 /Saturday/ a cold front will move across the PacNW
draped from a surface low moving across western Canada. Continued
moist advection from the Pacific will transport high PWs of +1 to
+1.5 standard deviations above the climatological mean into much
of the NW, while snow levels crash quickly behind the cold front
and associated strong upper trough. Ascent will remain robust
through height falls and jet level diffluence, and snowfall will
spread slowly southwest through the weekend, becoming heavy at
times in parts of the NW, Northern Rockies, and even into the
Great Basin. WPC probabilities for 12 inches feature a high risk
on D2 across the Washington Cascades, and on D3 into the ranges of
southern MT and northern WY. With snow levels forecast to fall to
less than 500 ft across much of the Great Basin and Northwest,
light accumulations are possible even into the valleys of
ID/MT/NV/UT by day 3, but the heaviest snows, where WPC
probabilities indicate a good chance for 4 inches or more, will
remain confined to the higher terrain.


...Northeast...
Days 2-3...

Low pressure developing along the North Carolina Coast on Saturday
is forecast to track offshore toward the Canadian Maritimes. Low
end probabilities for snow accumulations of 4-inches or more exist
across portions of eastern Maine as a reflection of a few ensemble
guidance members which track the system farther west.  However,
the consensus of the deterministic guidance continues to keep the
system too far to the east for any significant snow accumulation.

On Sunday, a northern stream piece of shortwave energy will race
southeastward across the Great Lakes and New England. Modest
forcing associated with this feature will likely produce a period
of light to moderate snowfall, with upslope enhancement likely in
the Adirondacks. Additionally, westerly CAA across Lake Ontario
will spread some modest LES into the Tug Hill Plateau and
Adirondacks. The combination of these two forcings should produce
some minor accumulations which may exceed 4 inches in the Tug
Hill, Adirondacks, and northern Greens before the energy shifts
quickly to the east and is replaced by shortwave ridging Sunday
night.

The probability of significant icing (0.25-inch or greater) is
less than 10 percent.

Weiss