Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
411 AM EST Thu Jan 28 2021

Valid 12Z Thu Jan 28 2021 - 12Z Sun Jan 31 2021

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

Ongoing major winter storm due to a prolonged Atmospheric River
(AR) will continue at least one more day across CA and parts of
the Southwest before slowly waning Friday. Persistent moisture
spreading onshore within a Pacific jet streak of 110-130kts
combined with unidirectional SW 850-500mb flow will drive a narrow
corridor of IVT approaching 750 kg/m/s into CA on D1. This will be
accompanied by slowly rising snow levels, reaching as high as 5000
ft, but with the moisture source slowly sinking southward. Late D1
into D2, the pronounced mid-level trough driving this confluent
moisture stream will drop southeast and come onshore CA late
Friday before opening and advecting through the Four Corners into
Saturday. This will finally bring a slow end to the persistent
overlap of forcing and moisture into CA, with less intense and
shorter duration forcing pushing snowfall eastward.

On D1, exceptional snow is once again likely in the Sierra where
WPC probabilities for 12" are greater than 95%, and some locations
will likely receive in excess of 4 ft of snow above 5000 ft due to
snowfall rates which could reach 4"/hr at times. As the moisture
shunts slowly southward, heavy snow will develop in the transverse
ranges of Southern CA as well, and WPC probabilities are moderate
for 12 inches in the San Gabriels and San Bernadinos. Waves of
moderate to heavy snow due to impulses embedded within the moist
flow will also lead to heavy accumulations of 12 inches or more in
the Sawtooth of Idaho.

Forcing and moisture finally advect eastward D2, but lingering
heavy snow is likely the first half of Friday before snowfall
winds down. WPC probabilities are moderate for 8 inches in the San
Bernadinos and southern Wasatch, with some lower probabilities
along the Mogollon Rim and White Mountains of AZ.

By D3, another shortwave will approach the Pacific Northwest,
preceded by a backing of the mid-level flow to drive warm/moist
advection onshore ahead of subsequent height falls. This will
produce a new round of heavy snow, focused in the Olympics and WA
Cascades where WPC probabilities are moderate for 8 inches,
generally above 3000 ft.


...Eastern New England...
Day 2...

A low pressure system deepening well offshore will leave an
inverted trough to its west. This trough is progged to rotate
cyclonically around to the west then south, scraping the immediate
coasts of Maine and Massachusetts. This trough will be accompanied
by strong 925-850mb fgen and 0-2km theta-e lapse rates that are
less than 0. At the same time, a mid-level closed low moving
overhead will help to steeping lapse rates through a deeper
portion of the column, while Atlantic moisture spreads inland
behind the departing system. All of these features together will
likely create a band of heavy snow rotating along or just off the
coast on Friday. Guidance has come into good agreement with this
feature, and while there is some uncertainty as to whether the
heaviest snow will reach the coast, there has been a westward
trend overnight which has caused an uptick in snowfall
probabilities. The highest probabilities for 4" are along the
mid-coast of Maine and then again across Outer Cape Cod where
snowfall rates could exceed 1"/hr at times. There is potential for
higher snow amounts across Cape Cod, but at this time the heaviest
snow is forecast to remain just offshore.


...Midwest to Central Appalachians...
Day 3...

A negatively tilted shortwave trough lifting out of the Four
Corners will move through the Central Plains and into the Midwest
Saturday night into Sunday. Atop this trough, a modest zonal jet
streak will race eastward, placing the favorable LFQ with a
divergence maxima atop the region, and the combination of these
features will drive an intensifying area of surface low pressure
moving northeast out of Oklahoma. Increasing mid-level divergence
downstream of the trough and intensifying WAA funneling Gulf of
Mexico moisture northward will drive pronounced ascent northward,
leading to an expanding area of precipitation from the Central
Plains into the Midwest, with secondary precipitation spreading
into the Central Appalachians. The column will initially be
marginally cold for snow, so there is likely to be a zone of mixed
sleet/freezing rain, although current WPC probabilities for any
significant freezing rain accretions are low. However, the latter
half of D3 /Saturday night/, heavy snow is likely to develop as
the isentropic lift maximizes and the WCB begins to wrap
cyclonically around the low to the NW. There remains considerable
spread in timing of this system as well as the strength of the
warm nose, but there is increasing potential for a swath of heavy
snowfall from Iowa to Ohio. WPC probabilities are high for 4
inches from eastern IA through NW Indiana, including the Chicago
metro area. An additional area of moderate to heavy snow is likely
very late D3, developing across the Central Appalachians where WPC
probabilities are moderate for 4 inches.


Weiss