Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 PM EDT Thu Mar 18 2021

Valid 00Z Fri Mar 19 2021 - 00Z Mon Mar 22 2021


...Northeast...
Day 1...

A band of precipitation sinking southeast behind a cold front will
be chased by cold air approaching from the NW as CAA increases.
While cold air following moisture rarely leads to heavy snowfall,
this will be enhanced by a stripe of strong deformation to aid in
locally intense ascent which will help dynamically cool the column
more rapidly. The guidance has trended downward in its snowfall
this aftn, and it appears the best chance for moderate to heavy
snow will be in the terrain of the Catskills, Berkshires, and
Worcester Hills due to elevation dependency, as well as in a
narrow band just SW of Boston, MA which may have the most
prolonged period of precipitation with temps cold enough for snow.
Generally 1-2" of snow is forecast, but WPC probabilities do
indicate a 10-20% chance for 4 inches, highest in the Worcester
Hills.


...Western U.S....
Days 1-3...

A slow moving closed mid-level low will pivot slowly southeast
towards the WA/OR coast Friday into Saturday, shearing into an
upper trough as it moves inland during the weekend. This feature
weakens substantially by Sunday, but spokes of vorticity rotating
around the mean trough will continue to produce periods of
enhanced ascent across the West. In addition to these height falls
and rounds of PVA, prolonged 700-500mb southwest flow will spread
moisture across the region, but at the same time drive freezing
levels as high as 5000-7000 ft ahead of the trough axis, lowering
rapidly however as the trough swings eastward, especially by D2/D3.

For D1, the heaviest snow is expected in the Sierra, WA Cascades,
Olympics, and Sawtooth ranges which will be most orthogonal to the
moist mid-level flow and guidance indicates omega maxima across
these ranges due to the upslope enhancement. WPC probabilities for
6 inches are high on D1 in these ranges, slightly lower in the
Sawtooth. By D2 heavy snow continues in the Cascades and Olympics
as the upper trough swings onshore steepening the lapse rates
while maintaining moist flow, but also begins to shift east into
the ranges of NW WY and southern MT as diffluence within the LFQ
of an upper jet streak maximizes. WPC probabilities D2 are again
high for 6 inches in these areas, with locally more than 12"
likely in the highest terrain. The primary trough axis swings well
eastward to the Northern Rockies and Great Basin D3, shunting
moisture and forcing near the PacNW, but continuing to drive moist
ascent into the Central Rockies and ranges of WY/MT. WPC
probabilities for 6 inches are moderate to high D3, highest in the
Big Horn range.


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

Weiss