Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 PM EST Sat Feb 20 2021

Valid 00Z Sun Feb 21 2021 - 00Z Wed Feb 24 2021

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

A positively tilted longwave trough will advect eastward from the
Four Corners while shedding shortwave energy to the northeast
across the Central Plains and into the Midwest Sunday. As this
shortwave lifts northeast, it will deepen slowly, and guidance has
begun to converge on an amplitude somewhere in the middle of the
closed/deep NAM/CMC and the shallow/progressive UKMET. As this
feature lifts quickly northeast, it will be accompanied by a 250mb
jet streak which will begin to arc poleward in response to the
amplifying longwave trough across the west, driving increasing LFQ
diffluence atop the Midwest, and working in tandem with height
falls to drive surface low pressure development over the Central
Plains. This low will lift E/NE quickly Sunday into Monday, moving
from Kansas Sunday morning to Michigan Monday morning, and then
progressing as an open wave to off the New England coast by
Tuesday morning. In addition to the deep layer ascent noted above,
increasing WAA ahead of the low and associated warm front will
help expand precipitation northeastward as low-level winds advect
Gulf moisture northward.

On Day 1 /Sunday/ as the low deepens and moves through the
Midwest, deep layer synoptic ascent combined with at least modest
frontogenesis will produce moderate to at times heavy snowfall.
The heavy snowfall will likely be confined to any banding that my
develop N/NW of the surface low as the 700mb low and 500mb trough
interact to the NW. The stronger guidance (NAM CMC) has weakened
today leading to less banding potential, which combined with the
transient nature of the trough suggests total snowfall should be
modest, and WPC probabilities indicate a 20-30% chance for 4
inches across northern and central IA with lower probabilities
extending as far east as the L.P. of MI. However, locally higher
snowfall is possible in any banded structures. Additionally, some
very light freezing rain is possible D1 as precip onset occurs
through WAA overrunning a weak and retreating surface high
pressure.

Further east and on D2, WAA will spread precipitation northward
through the Mid-Atlantic and into the Northeast. The column will
initially be cold enough for snow everywhere, but warming
low-level temperatures in response to WAA, mid-level divergence,
and a retreating high pressure producing return flow across the
east, should quickly change p-type to rain for the Mid-Atlantic
and Southern New England. This suggests that after a brief period
of snow everywhere, any significant accumulations should be
confined to northern PA, including the Poconos, and northward into
the Catskills, Adirondacks, and central New England where WPC
probabilities for 4 inches are 10-30%.

As the wave pulls away late D2, broad cyclonic flow across the
Great Lakes will be reinforced by a shortwave dropping across Lake
Superior Monday evening and then towards New England by Tuesday
morning. This will enhance the LES potential in the favored W/NW
snow belts despite modest 850mb temps falling to around -10C and
mostly ice covered lakes. WPC probabilities on D3 are high for 4
inches downwind of lake Ontario on D3, with maximum amounts
exceeding 8 inches possible.


...Pacific Northwest through the Northern Rockies...
Days 1-3...

Broad cyclonic flow across the CONUS due to a broad but amplified
trough will keep confluent mid-level NW flow angled into the
Pacific Northwest through early next week. Within this mid-level
flow, at least one shortwave trough will drop into WA state on
Monday evening, while persistent upper level jet energy drives
moisture and ascent continuously into the region. The combination
of high Pacific moisture and continued mid-level confluence will
produce an atmospheric river (AR) that is progged by both the GEFS
and ECENS to reach 500 kg/m/ms IVT, indicating a weak 2-day AR is
likely during the period. This AR combined with the synoptic
ascent, aided by modest WAA as winds back ahead of the shortwave,
will drive ascent and precipitation across the terrain from the
Olympics through the Northern Rockies, extending into the Tetons
by D3. D1 and D2 are likely to feature the most QPF and strongest
ascent as the core of the AR moves onshore. During this time, snow
levels are likely to rise on the moist warm flow, starting D1
around 3000-5000 ft, and climbing through D2 to more widespread
4000-6000 ft. Above these levels, snow will be plentiful, and WPC
probabilities are high for 8 inches int he WA Cascades and parts
of the Northern Rockies. Locally, snowfall could be much higher
than that, exceeding 2 ft each day. By D3, the best ascent and
moisture begins to shift to the southeast, so while heavy snow
will likely continue in the WA Cascades and Northern Rockies,
although with slightly less intensity, it will also shift into the
NW WY ranges. WPC probabilities on D3 are high for 8 inches once
again across all these ranges.


The probability of a quarter inch of freezing rain is less than 10
percent Days 1-3.

Weiss