Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
400 PM EDT Tue Mar 16 2021

Valid 00Z Wed Mar 17 2021 - 00Z Sat Mar 20 2021

...Colorado and New Mexico to southeastern Kansas and the
Oklahoma/Texas Panhandles...
Day 1-2...

An anomalously strong closed mid-level low approaching -3 standard
deviations from the climo mean according to NAEFS ensemble tables
will roll eastward out of New Mexico and into the Texas Panhandle
early Wednesday before shifting east towards the Arklatex by the
end of D1. As this low moves eastward, it will be joined by weakly
coupled jet streaks and a divergence maxima to drive surface
pressure falls and surface cyclogenesis across eastern NM Tuesday
night with subtle deepening likely as the low shifts eastward into
the Missouri Valley D2. Mid-level divergence ahead of the closed
low, combined with WAA on SW flow will spread precipitation across
the High Plains of NM and CO, and then eastward into the Panhandle
of TX, OK, and into KS. Initially, the precipitation will likely
be all rain outside of the terrain of CO/NM, but a strengthening
deformation axis N of the wave combined with intensifying fgen as
850mb temps collapse southward should allow for a rapid changeover
from rain to snow. The timing at which this occurs, as well as the
intensity of the snowfall after changeover will determine how much
accumulation can occur. While uncertainty continues which is
reflected by a large spread in the WSE plumes, there has been a
noted increase in the guidance this afternoon despite hostile
antecedent conditions. WPC probabilities now indicate a 20-40%
chance for 4 inches in the northern TX Panhandle and OK Panhandle,
with lower probabilities into KS and points east. Across the
higher terrain of southern CO and northern NM, WPC probabilities
are high for 4 inches, with local maxima in excess of 8 inches
possible.


...Upstate New York and New England...
Day 3...

Low pressure emerging from the Ohio Valley will spread a warm
front northeastward towards New England, with secondary
redevelopment possible along this front Friday morning. At the
same time, a cold front will race southeast from Canada.
Precipitation spreading northward with the WAA ahead of the OH VLY
will be primarily rainfall Thursday night, but as CAA intensifies
behind the cold front a changeover from rain to snow will occur.
As this front drops southward and interacts with the wave of low
pressure, low-level fgen and a deformation axis will intensify in
a band SW to NE across Southern New England. This should drive a
rapid changeover to snow with heavy rates possible. However, the
antecedent warm column and rain changing to snow implies it will
be a struggle to achieve heavy accumulations. WPC probabilities
have increased, and now show a 10-20% chance for 4", highest in
the Catskills, Berkshires, Worcester Hills, and Monadnocks.


...Western U.S....
Days 2-3...

A closed low dropping out of the Gulf of Alaska will move SE
towards the Pacific Northwest Thursday, reaching the coast on
Friday. This feature will be accompanied by modest diffluence
within the LFQ of a Pacific jet streak, and locally backed
500-700mb flow to drive warm advection into the West. These
features together will push a cold front draped from a surface low
onshore, and spread precipitation from CA to WA, and eastward into
the Rockies. In the terrain, this will lead to an extended period
of heavy snowfall beginning Thursday morning and persisting
through Friday evening. The heaviest snow is likely in the
Shastas/Siskiyous/Trinities where WPC probabilities on D2 and D3
are moderate to high for 8 inches, and locally more than 2 ft of
snow is possible. Elsewhere, heavy snow is likely confined to just
D3, with high WPC probabilities for 8 inches occurring in the
Sierra, Cascades, and Olympics.


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

Weiss