Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 PM EST Sat Feb 12 2022

Valid 00Z Sun Feb 13 2022 - 00Z Wed Feb 16 2022


...Great Lakes...
Days 1-3...

Persistent mid-level troughing across the Great Lakes will
maintain favorable CAA for LES across the lakes, with enhanced
ascent possible at times through weak PVA as shortwaves rotate
through the mean flow. On D1, mean northerly flow combined with
some land breeze confluence across Lake Michigan should pivot a
band of snow along the eastern shore of the lake into parts of SW
Michigan. The DGZ is quite shallow and forcing is modest, but WPC
probabilities suggest a moderate risk for 4+ inches before flow
becomes more westerly and dries out to bring an end to the LES.
Late D2 into D3, a potent shortwave moving across the eastern
Great Lakes will be followed by more impressive CAA and
unidirectional westerly flow across Lake Ontario and Lake Erie.
With deepening inversion depths and an impressive DGZ, snowfall
rates may become intense, especially east of Lake Ontario, with
upslope enhancement into the Tug Hill aiding in snow
accumulations. WPC probabilities for 4+ inches are already above
80% in the Tug Hill, with high probabilities also present for more
than 8 inches. Lake Erie is almost entirely ice covered according
to GLERL data, so LES will be much more muted east of that lake on
D3.


...Mid-Atlantic/Northeast...
Days 1-2...

Shortwave pivoting through a longwave trough across the eastern
CONUS will produce height falls and PVA across the Mid-Atlantic
and Northeast. At the same time, a poleward arcing jet streak
intensifies into New England placing favorable RRQ diffluence
across the east coast, with a secondary jet streak producing some
coupled ascent late on Sunday. The overlap of these forcings will
drive modest cyclogenesis along the coast from VA to off Cape Cod,
with moist advection producing an expanding area of precipitation
NW of this feature. For D1, the primary ascent mechanism across
the Mid-Atlantic will be enhanced fgen through the sharpening
baroclinic gradient behind a surface cold front, combined with the
ageostrophic response of the upper jet streak. While the best
ascent may be just beneath the DGZ, it still appears favorable for
some brief heavy snow rates approaching 1"/hr, with the best
chance in the higher terrain near Shenandoah NP and the Allegheny
Front. Here, WPC probabilities for 4+ inches are more than 50%,
with local maxima above 6" likely. Further to the northeast along
the I-95 corridor from Washington, D.C. to Baltimore and
Philadelphia, 1-2" with locally heavier amounts are possible.

Into D2, as the low pressure deepens modestly off the coast and
lifts northeast, precipitation will spread into New York and New
England. This wave of low pressure will help enhance ascent which
will already be robust through the coupled jet streak and enhanced
fgen, but the rapid motion of the system will somewhat limit
accumulations. However, guidance suggests an inverted trough will
try to linger back into southeast MA, with some OES possible into
the Cape and Islands as well. WPC probabilities for 4+ inches are
more than 50% southeast of Boston near Plymouth, MA, but are
generally 10-30% elsewhere from eastern Long Island through the
immediate coast of Maine.


...Pacific Northwest through Northern Rockies...
Days 2-3...

A potent closed low will dive out of the Gulf of Alaska D2 and
shift across the Pacific Northwest before sharpening over the
Great Basin Tuesday evening. This feature will create robust
height falls and drive a cold front and at least subtle waves of
low pressure southeast across the area. The elongating upper
trough will drive coupling of dual jet streaks across the Great
Basin by D3, further enhancing ascent as moisture begins to
increase into the region. Ascent on D2 is likely to be modest, but
should increase dramatically as the synoptic forcing begins to
overlap on D3. For D2, the heaviest snowfall is likely to be
confined to the WA and OR Cascades, and generally above 5000 ft,
although lowering with time as the front digs southeastward. This
should prevent any significant accumulations at the Cascade
Passes, with WPC probabilities for 6 inches only above 20% in the
far northern Cascades and around Mt. Rainier. For D3, the moderate
to heavy snow will shift eastward into the Absarokas, Northern
Rockies, and ranges of NW WY. Once again, snow levels will
initially be around 5000 ft but will fall to around 3000 ft behind
the cold front, and WPC probabilities for 6 inches are confined to
the higher terrain of these ranges.

For Days 1-3, the probability of significant icing is less than 10
percent.


Weiss