Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 AM EST Mon Feb 01 2021

Valid 12Z Mon Feb 01 2021 - 12Z Thu Feb 04 2021


...Mid-Atlantic through the Northeast...
Days 1-3...

...Major nor'easter will spread heavy snow from eastern
Pennsylvania through Maine through Tuesday...

Closed-mid level low over the lower OH VLY is clearly evident on
GOES-E WV imagery this morning digging southeast towards the
Southern Appalachians. Beneath this feature, a surface low has
occluded and is weakening, while energy transfers to a new low
pressure developing east of the Delmarva and also notable as a
baroclinic leaf blossoming south of New England. As this second
low strengthens and lifts slowly northeastward, a major winter
storm will bring heavy snow from Pennsylvania to Maine.

Snow has already blanketed much of the Mid-Atlantic through WAA
processes, but exceedingly heavy snow is likely to develop shortly
and spread northward. As the surface low blossoms, a robust TROWAL
is progged to develop as the WCB wraps northward around the low,
and a potent CCB drives westward towards New York City. The potent
RRQ of a strengthening jet streak, and at least a subtle coupled
jet structure, will help intensify the surface low while also
sharpening the low-level fgen through the ageostrophic response of
this evolution. The overlap of the CCB with this fgen lifting
northward will create an intense band of snowfall, likely from
near NYC westward into the Poconos, and then shifting slowly
northeast with time, or potentially pivoting eastward before
tertiary low pressure develops near the Gulf of Maine on Tuesday.
Within this potent band, snowfall rates in excess of 2"/hr are
likely, with moderate to at times heavy snow spreading as far
northward as the Canadian Border. Additionally, as the low pivots
northeast slowly, the deformation/comma snows will try to pivot
southward as far as the Mason-Dixon line, but again guidance has
trended a bit north tonight so only light accums are forecast
along I-95 from DC to PHL.

Further northeast into New England, robust WAA overtopping a
coastal front will lead to heavy snow from eastern CT through ME,
with upslope enhancement likely into the Worcester Hills. However,
the dry slot which is clearly evident across MD and PA is progged
to expand northward, and may move as far NW as NYC, and into RI
and much of eastern MA. Where this occurs, p-type may change from
heavy snow to light rain, limiting accumulations, but heavy snow
is still expected across much of the region. WPC probabilities for
8 inches are high from central PA through NYC, much of CT, RI, MA,
and and into southwest ME. The heaviest snow is likely in
mesoscale bands and where upslope enhancement can occur, with
lollipops of more than 20" likely in northern NJ, northeast PA,
and into the Catskills. On D2, the heaviest snow will shift
northeast as the new low develops off the coast of Maine, and WPC
probabilities are again high for 8 inches in NH and much of ME.
Additionally, heavy accumulations of snow are likely on NW
flow/upslope into the Adirondacks and Greens on D2 with more than
8 inches of snow possible before the entire system pulls away
Wednesday leaving just lingering light snow.

Some light freezing rain is also likely across eastern ME as the
surface low gets pulled inland allowing warm air to wrap northward
atop the continued cold surface temperatures within Canadian high
pressure. WPC probabilities for 0.1" of ice accumulation are as
high as 20% near Downeast Maine.


...NC/TN Mountains...
Day 1...

The significant nor'easter near New England will leave pronounced
NW flow in its wake across the Ohio Valley and into the Southern
Appalachians. At the same time, an expansive mid-level low
pressure will drop southward and then eastward Monday into
Tuesday, with lobes of vorticity spinning around it. The first
half of D1 will feature pronounced and prolonged upslope flow on
the NW winds, with modest inversion heights and forcing leading to
several inches of snowfall, generally above 3000 ft. The latter
half of D1 will likely feature an uptick in accumulations and
snowfall intensity as the upslope flow gets enhanced by intense
synoptic ascent beneath steepening lapse rates and a wave of PVA.
As the upper low pulls away into D2 the threat for heavy snow will
rapidly wane. WPC probabilities are high for 6 inches in a narrow
corridor of the Smokies primarily on the west /upwind/ side, with
local maxima of more than 12 inches likely.


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

A potent and positively tilted mid-level trough will arc slowly
southward from the Gulf of Alaska through Tuesday before finally
moving onshore the Pacific Northwest on Wednesday, driving a
modest low pressure and frontal system inland. Ahead of this
system, D1 and D2, persistent backed but confluent mid-level flow
overlapped with a Pacific jet streak will drive a modest
atmospheric river (AR) into northern CA and the Pacific Northwest,
with residual moisture spilling over into the Northern Rockies.
While this moist airmass will be warm, driving snow levels above
5000 ft, it will be accompanied by heavy snow above this level.
WPC probabilities on D1 and D2 are high for 8 inches in the
Olympics, WA Cascades, and Shasta/Siskiyou/Trinity ranges of
Northern CA, shifting into the Sierra on D2. As the trough axis
moves onshore D3, bringing the wave of low pressure and associated
front with it, the forcing and moisture will shift to the
southeast. While ascent intensifies beneath rapid height falls,
the temporal duration of forcing shortens, so WPC probabilities
moderate for 8 inches to 30-50% while shifting to the Wasatch and
CO Rockies.

Weiss