Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 PM EST Wed Feb 10 2021

Valid 00Z Thu Feb 11 2021 - 00Z Sun Feb 14 2021

...Central Appalachians and Mid-Atlantic...
Days 1-3...

Multiple systems will impact this region during the next 3 days,
each with a distinct impact.

The first is a wave of low pressure which will race from KY to off
the NJ coast tonight through early Thursday. This wave will move
along a decaying baroclinic gradient, driven by a weak shortwave
and modest RRQ diffluence within an ejecting jet streak. While
synoptic ascent will be modest, increasing WAA ahead of the system
will spread expanding precipitation northward, and there has been
an increase in QPF/snowfall with today's model runs. A cold column
favoring slightly above climo SLRs will help increase
accumulations north of the Potomac River as well, and WPC
probabilities feature a 40-60% chance for 4" from far western MD,
along the MD/PA line, into southern NJ. In addition, some light
freezing rain may occur across northern VA in the foothills of the
terrain, where WPC probabilities indicate up to a 30% chance for
0.1" of accretion.

System #2 follows quickly on the heels of the first wave, as yet
another weak low pressure moves along the stalled front to spread
precipitation northeastward from the Ohio Valley. The guidance has
trended south with this impulse today, as well as weaker, likely
due to a strong surface high pressure and dry air advecting down
into the wedge from the north. However, strong upper diffluence
within the RRQ of a strengthening jet streak, modest WAA ahead of
the low, and persistent moist confluent mid-level flow will
produce an axis of heavier wintry precipitation generally south of
Washington D.C. WPC probabilities for 4" of snow are very low
across VA due to the weaker system, but intensifying WAA atop the
cold wedge could produce a swath of moderate freezing rain from
far northern NC into central VA. WPC probabilities for 0.1" of
accretion are as high as 40% in this area.

Finally, as this second system moves away, and even more
substantial low pressure is likely to develop in the Deep South
and then move offshore the Carolinas by Saturday night. Increasing
height falls, and potent upper diffluence as a secondary jet
streak strengthens downwind of a longwave trough axis should lead
to deepening of this wave of low pressure. At the same time,
isentropic ascent transporting moisture from the Gulf of Mexico
will drive expanding precipitation northward, with a significant
overrunning event becoming more likely. A swath of sleet and
freezing rain is likely to overspread central NC northeastward
towards NJ on Saturday, and while there remains considerable
uncertainty into the exact placement of sleet/freezing rain, WPC
probabilities indicate more than 50% chance for 0.1" of accretion
across much of VA and northern NC.

...Southern Plains, Mississippi Valley, Tennessee Valley...
Days 1-2...

...Damaging freezing rain event continues through Thursday...

A stalled boundary draped from TX northeast to KY will waver in
place through Thursday as waves of low pressure move along it. At
the same time, a 130kt jet streak remains angled across the
Northeast leaving the favorable RRQ over the front, with a
longwave positively tilted trough continuing to drive moist
confluent flow to the NE and into the region. Low-level southerly
flow out of the Gulf of Mexico will continue to lift
isentropically atop the front and into the arctic high pressure,
creating a prolonged and intense overrunning event with freezing
rain and sleet the primary weather type. A long duration freezing
rain event will continue most of Thursday before gradually winding
down to the northeast as forcing wanes and shifts eastward.
However, favorable surface wet bulbs temps with moderate rain
rates suggests freezing rain will accrete efficiently to produce
significant accumulations of ice. The exception to this may be
across TX and into OK/AR where there is a signal for some negative
theta-e lapse rates indicative of elevated convection, and it is
possible some thunder sleet or thunder freezing rain will occur
overnight into Thursday morning there. During this time, rain
could runoff more than accrete, but this should be the exception
rather than the rule. Additionally, the southern trend has
continued once again today such that the heaviest axis of freezing
rain has shifted slightly south/southwest, with more sleet
possible further north. The heaviest freezing rain is likely to
exceed 0.25" and may exceed 0.5" in places, producing scattered to
widespread damage. WPC probabilities indicate the best chance for
this will be in a stripe from south central Arkansas northeast
into eastern Kentucky, including western Tennessee. Lighter but
still significant icing of up to 0.25" could occur as far west as
central Texas near San Antonio. By Thursday night, most of the
freezing rain should be winding down, with just limited additional
accumulations possible in Kentucky.


...The West onto the Central Plains...
Days 1-3...

Persistent longwave troughing across the CONUS around a central
gyre positioned near Minnesota will drive periods of shortwave
energy southeast from the Gulf of Alaska and into the Pacific
Northwest, then advecting southeast into the Central Rockies. The
first of these will drop southeast into northern CA early Friday
morning, followed by a more impressive vorticity lobe stretching
into WA/OR Saturday morning. Each of these waves will be
accompanied by a surface low and complex frontal structure, as
well as Pacific jet streaks arcing across the West with associated
divergence maxima.

On D1, snowfall across the West will be somewhat limited as
shortwave ridging early in the period takes some time to get
displaced. However, by late D1, precipitation is likely to spread
into the Cascades and WPC probabilities for 6 inches of snow are
low to moderate, highest along the OR/WA border. During D2, the
more robust deep layer ascent drops onshore with mid-level
divergence and WAA spreading precipitation across the Sierra,
Cascades, and points southeast as far as the CO Rockies. Snow
levels will vary considerably D2 as an arctic front lingers banked
against the N/E sides of the terrain, leading to additional
localized ascent as well. WPC probabilities for 8 inches are high
in the Sierra, CO Rockies, and ranges of NW Wyoming, generally
above 4000-5000 ft. Further north WPC probabilities are moderate
for 8 inches in the Sawtooth, Absarokas, and Cascades but with
snow levels down to 1000 ft or less. By D3, the first wave ejects
quickly to the east to be replaced almost immediately by an event
stronger wave, jet, and surface low with associated deep layer
forcing. 700-500mb RH and omega rise dramatically late Friday, and
intense moist advection is likely to produce heavy snow from
Sierra northward into the Olympics and Cascades, and east to the
Blue Mtns and Sawtooth. WPC probabilities are high for 12 inches
in the Cascades on D3.

With snow levels crashing to the surface D2 and D3, and a wedge of
high pressure banking against the Cascades, cold easterly flow
into the lowlands of WA and OR should allow precipitation to fall
as ice or snow. The models continue to shuffle their low
placements both with the Thursday-Friday system, and the stronger
Friday-Saturday low. However, it is becoming more likely that
moderate to heavy snow will impact both the Seattle and Portland
metro areas, with the heaviest snow likely late Friday into
Saturday as some enhanced low-level fgen collocated with intense
overrunning WAA occurs. WPC probabilities for 4 inches are low on
D2 in the lowlands, but high on D3 and an impactful snow event is
likely. Southwest of Portland, OR, a significant icing event is
becoming more likely as well, and WPC probabilities are as high as
30% for 0.25" on D2, lingering with additional freezing rain
likely on D3.

Weiss