Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Tue Oct 27 2020

Valid 00Z Wed Oct 28 2020 - 00Z Sat Oct 31 2020

...Southern Rockies and Plains...
Days 1-2...

A robust closed upper low over New Mexico will gradually begin to
fill and eject to the east tonight into Thursday. Ahead of this
feature, mid-level divergence combined with height falls and a
potent upper jet streak will drive deep layer synoptic ascent,
being enhanced by isentropic lift as mid-level WAA tops the cold
surface temperatures behind a cold front. This setup has already
produced significant snow and ice across the region, with more
likely through Wednesday aftn. The WAA aloft riding atop the cold
surface dome will continue a freezing rain threat through late
morning/early aftn Wednesday before increased mid-level divergence
and increasing low-level WAA erode the wet-bulbs to above
freezing. Persistent lift becoming more intense suggests rainfall
rates within the region of freezing rain will begin to intensify
towards morning, but with heavier rates and subtly warming wet
bulb temperatures, accretion efficiency is likely to wane. Still,
WPC probabilities indicate a moderate risk for an additional
0.25"+ of freezing rain, highest potential in western OK into the
eastern Panhandles of TX/OK.

Further to the NW, cold mid-level temperatures and intense
synoptic ascent will drive rounds of moderate to heavy snowfall
from New Mexico E/NE into the TX/OK Panhandle and far western
KS/eastern OK. The heaviest snow on D1 is likely across eastern NM
where the presence of the upper low will drive better instability
and colder temperatures aloft, while LFQ diffluence and more
robust height falls provide intense ascent. WPC probabilities are
moderate for an additional 6" of snowfall after 00Z this evening.
Further to the east, lighter snows are expected as the low begins
to fill and eject eastward, causing a reduction in both the
strength and temporal duration of forcing. WPC probabilities
indicate a low to moderate risk for an additional 4" of snow in
the Panhandles and spreading into southeast CO and far western KS.

The threat for winter weather is expected to finally come to an
end as the system begins to accelerate rapidly east by early
Thursday.


...Northeast...
Day 3...

The slowly filling upper low moving out of the Desert Southwest
will lift into the Mid-Atlantic and pass south of New England by
Saturday. This mid-level feature will combine with an intense jet
streak strengthening over southeast Canada (190kts, above the
climatological record) to drive lowering pressure, and a wave of
low pressure is likely to move south of New England Friday. Robust
WAA and isentropic lift ahead of this feature combined with strong
synoptic deep layer ascent will spread precipitation in the form
of rainfall late Thursday into Friday across the Mid-Atlantic and
into the Northeast. As the low moves offshore and begins to depart
early Friday, strong CAA from the north will combine with a band
of modest fgen and deformation to cool the column and begin to
chance the precip from rain to snow. While there should be a sharp
cutoff in precip across northern New England, rain may transition
to a period of moderate to heavy snow in the terrain of the
Adirondacks and ranges of northern/central VT/NH, before slowly
shifting southward until precip winds down and ends Friday aftn.
The heaviest snowfall is likely in the terrain of the Southern
Adirondacks, Catskills, Southern Greens, Berkshires, and into the
Worcester Hills and Monadnocks of SW NH where the best overlap of
precip with cooling temperatures exists. WPC probabilities in
these elevated areas are 30-50% for 4 inches. In the lower valleys
and into southeast Maine, some lighter accumulations are likely,
with some slushy accumulation possible approaching the coast of
Southern New England before the event winds down.

Weiss