Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
413 AM EST Tue Feb 02 2021

Valid 12Z Tue Feb 02 2021 - 12Z Fri Feb 05 2021


...Northeast...
Days 1-2...

...Major nor'easter will continue to spread heavy snow in Upstate
New York and Northern New England...

The potent surface low currently analyzed south of Long Island
will pivot slowly, almost in place, as the anomalously strong
upper trough becomes aligned vertically atop it, with spokes of
vorticity rotating through its base. This will yield to another
low pressure developing near the Gulf of Maine and lifting slowly
northeast through Wednesday. The combination of these two lows
will continue to spread heavy snow well northward, aided by
steepening lapse rates aloft. There are likely to be two areas of
heaviest snow remaining with this nor'easter. The first is on
robust WAA spreading into Maine ahead of the secondary low, where
persistent isentropic ascent and some banding potential will
produce heavy snow through the day and WPC probabilities are high
for 8 inches, with local maxima above 12 inches likely. As the low
departs, increasing NW flow will develop to produce upslope snow
in the Adirondacks, Greens, and Whites, and WPC probabilities are
high for 6 inches, with the most snow likely in the Adirondacks. 
Additionally, as the flow becomes N/NW, some LES enhancement will
develop southeast of Lake Ontario and towards the Finger Lakes
region of NY where WPC probabilities also indicate a chance for 6
inches today.

While less accumulation is expected elsewhere, the strong low
offshore and steep lapse rates aloft could produce brief bursts of
snow across much of the Mid-Atlantic and Northeast through this
evening. Accumulations within any of these bands should be light,
but brief restrictions in visibility are likely in the heavier
snow showers.

Light freezing rain and sleet 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. Day 1 WPC probabilities for 0.1" of ice
accumulation are 10-20% in Downeast Maine.


...Upper Midwest...
Days 2-3...

A shortwave moving out of the Pacific Northwest Wednesday will
move rapidly to the east through Thursday before deepening and
taking on a negative tilt/closing off over the Great Lakes
Thursday night. As this occurs, a subtropical jet streak downwind
of the amplifying longwave trough will eject to the east, placing
the favorable LFQ for diffluence atop the Midwest. The combination
of mid-level divergence and height falls with the upper diffluence
atop a low-level front/baroclinic zone will drive a deepening
surface low moving from the Central Plains into the Great Lakes.
Ahead of this feature, intensifying WAA and moist isentropic
ascent at 300K atop a surface high will drive expanding
precipitation northward, with snow and freezing rain blossoming
from SW to NE. While the freezing rain is expected to be generally
light, less than 0.1", the guidance has shown an increasing trend
in snowfall. While models still differ in the track of this low
and hence the amount and footprint of the heaviest snow, it is
becoming more likely that heavy WAA snow will spread from eastern
MN through WI and MI late D2 into D3, with wraparound snow and LES
occurring as the low ejects east late D3. WPC probabilities for 4
inches have climbed to as high as 60-80% D3, with the heaviest
snow in the U.P. where LES will enhance totals to local maxima
above 10". Elsewhere, a general swath of 4+ is likely.


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

A potent and positively tilted mid-level trough will persist off
the Pac NW coast through Tuesday before a reinforcing trough
arrives Tuesday night and ejects the long wave trough southeast
across the Intermountain West, crossing the central and northern
Rockies Wednesday night. Ahead of the trough, persistent backed
but confluent mid-level flow overlapped with a Pacific jet streak
will continue to drive a modest atmospheric river (AR) into
northern CA and the Pacific Northwest, with residual moisture
spilling over into the Northern Rockies. This moist airmass will
be warm, driving snow levels above 5000 ft. On D1, the persistent
moist flow will produce heavy snow in many of the ranges above
this level, and WPC probabilities for 8 inches are high in the
Sierra, Cascades, Northern CA ranges, Olympics, and into the
Sawtooth of Idaho. During D2, the trough axis finally pivots
eastward while slowly weakening. This will help shed forcing and
moisture southeastward, and the highest probabilities for 8 inches
of snowfall Wednesday shift to the Wasatch, San Juans, and CO
Rockies. After a brief respite late D2 into D3 as shortwave
ridging builds across the West, another shortwave will drop into
the Pacific Northwest embedded in confluent NW flow. This flow
direction is not ideal for intense moist advection, but enough
will transport onshore that heavy snow is likely to redevelop in
the WA Cascades with WPC probabilities for 8 inches as high as 50%
D3.


Weiss