Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
319 PM EST Mon Feb 08 2021

Valid 00Z Tue Feb 09 2021 - 00Z Fri Feb 12 2021

...Ohio Valley, Great Lakes, to Northeast...
Days 1-2...

A fast moving shortwave trough embedded within broad cyclonic flow
will race from the Ohio Valley tonight through the Northeast on
Tuesday and then off into the Canadian Martimes Tuesday night.
Weak surface low development is likely off southern New England
late Tuesday, but additional ascent from that development is
likely to be confined to New England late. Primary forcing for
precipitation will be WAA ahead of the shortwave combined with
modest mid-level divergence, along with a band of mid-level
frontogenesis stretching west to east from Ohio into New England.
It is likely that this fgen, which has shown an intensifying trend
in recent guidance, will produce the heaviest snow rates. However,
the most robust omega looks to be centered just below the DGZ, so
snow rates may remain modest and less than 1"/hr. However, a
several hour period of moderate snow across the Ohio Valley into
the Mid-Atlantic, combined with surface low development off New
England, and some upslope enhancement into the terrain of the
Northeast has led to an increase in WPC snowfall probabilities.
There exists a low chance for 4" across parts of IN, OH, and
western PA within the best band of fgen, and a high chance in the
terrain of the Catskills, Poconos, and Berkshires which will have
the support of the additional ascent. Local maxima above 6" are
possible in these latter locations.

Behind the system, continued CAA will produce some LES in the
favored NW snow belts. However, due to the exceedingly cold air,
the DGZ is actually at the surface (or below) so efficient crystal
growth is unlikely. Still, there exists a high chance for 4" in
the Keweenaw Peninsula of the U.P. of Michigan, with 10" possible
in isolated locations.


...Mid-Atlantic...
Day 3...

A weakening shortwave moving across the Southeast will interact
with strengthening upper diffluence in the RRQ of a departing jet
streak to produce a wave of low pressure along a stalled boundary
over the Deep South. Prolonged and robust WAA ahead of this
feature will spread precipitation northeastward into the
Mid-Atlantic late Wednesday night and Thursday. There remains
considerable spread into the thermal structure of this event as
precipitation overspreads the region, and there is likely to be a
rain, mix, snow event on Thursday. At this point the best
potential for significant snow (4" or more) looks to be in the
higher terrain of WV/MD, and also into Pennsylvania which will be
colder. There has been a noted shift southward in the axis of
heaviest snowfall with the guidance today, and WPC probabilities
have accordingly shifted such that there is now a moderate risk
for 4" in far western MD and adjacent WV, stretching eastward
along the PA/MD border towards Philadelphia.

Further south and west, there is likely to be a transition zone
with some light freezing rain accretions in the Central
Appalachians and adjacent foothills into central/SW Virginia. WPC
probabilities here are as high as 30% for 0.1" of ice.


...The West...
Days 1-3...

The shortwave trough ejecting out of the Southwest D1-2 leads to
confluent and moist mid-level flow converging across the CO
Rockies, atop a cold front that will become stationary and banked
against the terrain. This will lead to several rounds of moderate
to heavy snow tonight through Wednesday. Snow levels east of the
Front Range will be at the surface behind this front, but snowfall
is expected to remain confined to the terrain where snow levels
will generally hold steady at 4000-6000 ft. WPC probabilities on
D1 and D2 are moderate for 6 inches, with 2-day maxima exceeding
12 inches likely.

Wednesday night into Thursday, the above shortwave lifts east
shutting off the forcing across CO, but another shortwave will
round the trough and dig out of the Gulf of Alaska moving into the
Pacific Northwest. This will bring height falls and renewed
Pacific Jet energy, while also driving snow levels rapidly
downward. The heaviest snow is likely D3 in the WA and OR
Cascades, the Olympics, and the ranges of Idaho where WPC
probabilities are moderate for 6 inches. More notably, snow levels
crashing to the surface could bring some light snowfall to the
Columbia Gorge and lowlands around Portland, OR and Seattle WA by
Thursday.


...Southern Plains eastward through the Mid-Atlantic...
Days 1-3...

...Significant freezing rain event likely Wednesday through
Thursday...

Broad cyclonic flow across the CONUS will persist through
late-week, while shortwave energy moves through the flow to
produce periods of wintry precipitation. The first of these is a
weak impulse progged to race across Missouri and through the Ohio
Valley tonight into Tuesday with ascent aided by RRQ diffluence
and modest WAA ahead of the impulse. Cold high pressure will hold
in place keeping surface temperatures below freezing, and a period
of overrunning precipitation as snow or sleet/freezing rain is
likely. The total duration of forcing and intensity of moist
advection is modest so accumulations are likely to be light.
However, WPC probabilities indicate a low chance for 0.1 inches of
freezing rain across southern Missouri and far northern Arkansas.

A much more significant system will begin Wednesday, and guidance
is beginning to show more agreement in a significant freezing rain
event Wednesday and Thursday. A shortwave moving onshore CA
Tuesday will eject into the Southwest Wednesday and then weaken as
it becomes embedded in the westerlies into Thursday. Ahead of this
feature, confluent mid-level flow will help to reinforce very cold
high pressure centered over Canada behind a cold front which will
be sagged across the Southern Plains and Deep South. As this
shortwave interacts with the low-level baroclinic zone, an
intensifying jet streak across the Great Lakes will place the
increasingly favorable diffluent RRQ atop the region, and the
combination of these features is likely to produce surface
cyclogenesis moving along the boundary to the south. As this low
moves eastward, WAA from the south will intensify downstream, with
robust 290K isentropic ascent lifting atop the reinforced cold
high pressure. The guidance is in generally good agreement in the
overall setup, however, the NAM appears a bit too warm with the
warm nose, while the CMC is on the cold edge of the envelope. The
ECMWF/ECmean/GFS all are in close agreement and used heavily for
the forecast - which suggest a narrow but impressive band of
freezing rain will develop Wednesday and then expand northeast
from Missouri, through the lower Ohio Valley, and into the
Mid-Atlantic.
There has been a southern push in the guidance this aftn,
suggesting some areas on the north side may get more sleet, and
the axis of heaviest freezing rain may shift a little further
south.

Using the preferred blend, the heaviest freezing rain is currently
most likely in the Ohio Valley where a long duration of modest
rates due to condensation pressure deficits falling towards 0C
combine with efficient accretion as surface wet bulbs remain in
the 20s. WPC probabilities for 0.25" are as high as 40% from
northern Arkansas through far western Tennessee, and much of
Kentucky. While guidance continues to differ on the SW extent of
the heaviest icing potential, some locally backed flow as a wave
of low pressure develops should isentropically ascend atop the
cold front back into AR/OK, leading to some higher probabilities
there as well. However, the most significant freezing rain
accretions, which could exceed 0.5" in places, appear most likely
across Kentucky.


Weiss