Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 AM EST Sun Jan 31 2021

Valid 12Z Sun Jan 31 2021 - 12Z Wed Feb 03 2021


...Southern and Central Appalachians through the Mid-Atlantic and
New England...
Days 1-3...

...Major nor'easter developing this morning will bring heavy snow
from Virginia to Maine...

A potent closed mid-level low evident on GOES-E WV imagery this
morning is rotating over Indiana with a significant moisture
shield blossoming to the east. This closed low will waver eastward
slowly before diving southward in response to vort energy swinging
through the associated longwave trough to its south. At the same
time, this longwave trough amplification will drive downstream jet
streak strengthening and potentially a modest coupled jet
structure to drive intense upper diffluence, and guidance indicate
a robust divergence maxima will move off the coast of VA tonight
before lifting northward. The interaction of these features will
drive secondary surface cyclogenesis off the Delmarva, and this
low will meander slowly northeast through Monday, before
elongating and lifting towards the Gulf of Maine on Tuesday.

Today through tonight, most of the precipitation will be driven by
intense WAA on southerly flow ahead of the main upper low, with
moisture being advected northward from the Gulf of Mexico. A cold
and dry high pressure centered over Canada will maintain a wedge
down through the Appalachians, and as precipitation overruns this
air mass, snowfall will overspread the region from SW to NE. This
first burst of snow could be moderate to heavy at times as
isentropic ascent maximizes into the moistening DGZ, and WPC
probabilities are high for 4 inches from western MD through
central PA and into central NJ. One caveat to this snowfall is a
rapidly accelerating dry slot noted in RH fields and the DGZ which
will result in a loss of cloud ice for much of northern VA,
eastern MD, and into DE/southern NJ. The guidance has become more
aggressive with this dry slot, and in response there is likely to
be a prolonged period of light freezing rain or freezing drizzle
which could accrete to more than 0.1" along this I-95 corridor
Sunday night into Monday morning. Further south into the Foothills
and Piedmont of NC/VA, more significant freezing rain is likely as
the warm nose surges above 0C, and WPC probabilities are high for
0.1", and as high as 40% for 0.25" in a few locations.

On Monday, the secondary low is likely to deepen rapidly, and
guidance has continued to show a more northern track of this low
tonight. As this low deepens, it will enhance the precipitation
shield through WAA, but also develop into an environment favorable
for intense banding. The setup looks favorable for a
laterally-quasi stationary deformation band Monday into Tuesday,
before pivoting northeastward on the edge of the dry slot. Where
this band sets up is still somewhat uncertain, but the most likely
location at this time is from central PA northeastward to just
north of NYC where the CCB is persistent. While much of the
northeast on Monday is likely to see heavy snow, and WPC
probabilities for 6" are high from south-central PA through
Albany, NY, Boston, MA and towards Portland, ME, the heaviest snow
is likely in this band where local maxima in excess of 18" are
possible. There is some uncertainty for the immediate I-95
corridor due to potential warm air wrapping into the coast causing
a changeover to a mix or rain, but just inland from the major
cities there could be widespread 12" of snowfall. Additionally, as
the low pulls away, the pivoting deformation band may try to surge
southward as the 700mb trough digs to the southeast. While
guidance has significantly backed off on the southward extent of
this banding, it is possible some snow could rotate back as far
south as Washington, D.C. for additional light accumulations, but
confidence is increasing that the heaviest snow on Monday will
remain NW of the I-95 corridor from D.C. to PHL.

By D3, the low finally begins to kick out into the Gulf of Maine,
and may get pulled northward very close to the coast. This could
produce some light freezing rain in parts of coastal Maine, but
again the predominant p-type D3 should be snow from upstate New
York through Northern New England. WAA snow at onset followed by
upslope snow as the low departs has caused an increase in WPC
probabilities for upstate NY, VT, and NH where there is a high
likelihood for at least 6 inches of snowfall. Across Maine,
persistent and robust ascent could produce an expansive area of
more than 12" of snow on D3 just inland from the coast.


...Ohio Valley...
Day 1...

Lingering moderate to heavy snow will persist across IN and OH as
an upper low moves overhead and a surface low weakens due to
energy transfer to the coast. A band of deformation north of the
surface low is likely to enhance snowfall at least for several
hours in OH before the forcing wanes to the east. Otherwise, snow
will gradually lessen from west to east through the day. WPC
probabilities on D1 are high for 4 inches in eastern OH and parts
of western PA.


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

A slow moving mid-level low dropping out of the Gulf of Alaska
will move onshore WA and OR Tuesday night with height falls and
PVA. Ahead of this feature, however, prolonged confluent and moist
flow will drive moisture onshore from northern CA into WA state
and the Northern Rockies, aided by a modest but persistent Pacific
jet streak. The persistent warm/moist flow will keep snow levels
elevated to 4000-5000 ft, but above these levels heavy snow is
likely all 3 days of the forecast period. WPC probabilities on D1
and 2 are each high for 8 inches in the Olympics and WA Cascades,
as well as the Shasta/Trinity/Siskiyou ranges of northern CA. By
D3, as the trough finally shifts onshore, the best forcing and
moisture begin to shunt southward such that probabilities for 8
inches become high in the Sierra while remaining high in the
Cascades. 3-day totals may exceed 3 ft in the highest terrain.


Weiss