Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
421 PM EST Thu Feb 11 2021

Valid 00Z Fri Feb 12 2021 - 00Z Mon Feb 15 2021

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

Two low pressure systems moving into the Pacific Northwest will
spread heavy wintry precipitation across much of the West the next
3 days.

The first is a modest low which will move near the OR coast early
Friday while weakening. This low will be accompanied by low-level
convergence along the associated warm front, as well as height
falls and PVA from a sharp but filling shortwave trough, and
modest jet level diffluence to aid in ascent. WAA ahead of the low
will spread precipitation as snowfall into the terrain of the
Sierra, Cascades, and other ranges as far east as the CO Rockies,
generally above 4000 ft, but much lower in the Cascades. This
impulse will interact with a stalled arctic front banked against
the east side of the terrain, providing some enhanced ascent, and
also driving much colder temperatures sinking southward and oozing
through the gaps on easterly winds into WA/OR. This cold surface
air is critical as the overrunning will likely lead to a period of
freezing rain from Portland, OR to the coast both SW and W,
including some of the coastal range. While the heaviest snow is
likely in the high terrain, where WPC probabilities are high for 8
inches or more in the Sierra, CO Rockies, Tetons, and OR/WA
Cascades, other significant snow is likely as well, with moderate
probabilities for 2 inches in Portland and Seattle D1.
Additionally, significant ice accretion is likely in NW OR were
WPC probabilities for 0.25" are 20-40%.

A more robust low pressure system will follow quickly on the heels
of the first one, moving onshore the PacNW coast Saturday evening.
This low will be accompanied by better moisture, and strong deep
layer ascent through mid-level height falls and jet level
diffluence. Additionally, WAA streaming ahead of this low on
locally backed mid-level winds will drive intense ascent and
moisture eastward, with some low-level fgen contributing to
intense omega. The guidance has trended a bit north today with
this low, which makes a big difference in the thermal structure
near Portland, OR. While it is likely Portland will receive both
snow and freezing rain from this event, the risk for snow is
somewhat lower while the risk for freezing rain has increased as
easterly flow through the Gorge maintains cold surface air but
better WAA drives a warm nose aloft. Heavy freezing rain is likely
in NW OR, and WPC probabilities are again 20-40% for 0.25", with
more than 0.1" possible in the Portland metro. Further north, more
moisture should produce heavy snow into Seattle and the WA
lowlands, where WPC probabilities are high for 4", and some areas
including Seattle may receive near 8". Moderate accumulations are
possible in Portland, but it would be highly dependent on how the
warm nose evolves.

Elsewhere across the West, heavy snow is likely to spread across
all the terrain north of 35N latitude on D2 including the Sierra,
Wasatch, CO Rockies, and ranges to the north, and then sinking as
far south as the Mogollon Rim and Sacramento Mountains on D3. Two
days of snowfall could accumulate to more than 3 ft in the highest
terrain.


 ...Central and Southern Plains...
Days 1-3...

A potent upper jet streak approaching 150kts will race across the
Northern Plains Friday leaving favorable RRQ diffluence for ascent
across the Central Plains. Some additional ascent is likely
through the ageostrophic fgen response to this jet streak, and a
band of moderate to heavy snow is likely to spread from the High
Plains of Nebraska D1 towards Chicago on D2. WPC probabilities
within this band are as high as 50% for 4 inches despite limited
QPF due to intense ascent and SLRs that could reach 25:1.

A more significant storm system will develop D3 as the shortwave
dropping out of the Pacific Northwest advects towards the Four
Corners and develops a neutral tilt. As this occurs, the longwave
trough moving across the West drives downstream jet development,
with a coupled jet structure likely forming over the Central
Plains Sunday morning. The combination of these features will
drive surface low development over West Texas or even as far south
as Mexico. WAA ahead of this feature will become robust, with 290K
isentropic upglide intensifying atop the cold surge of arctic high
pressure. An exceptionally cold column will lead to SLRs that are
nearly 20:1, and the WAA combined with the deep layer omega should
produce a swath of moderate to heavy snow expanding southeast from
Nebraska trough the Texas Panhandle Sunday. WPC probabilities for
this event are already above 40% for 6 inches in parts of
KS/OK/TX, with significantly more possible. South of the heaviest
snow, some light freezing rain is possible in SE TX and northern
LA, locations that do not typically receive wintry precipitation.


...Northeast...
Day 3...

A low pressure system moving along the coast will push northeast
Sunday into Sunday night while a wave of mid-level vorticity
swings across the Great Lakes. The guidance is vastly different in
both the intensity and track of this surface low, but the trend
today has been for it to be a bit further northwest with slightly
more moisture. As the low lifts up the coast and interacts with
the shortwave, precipitation will spread northward. Along the
coast there is likely to be an area of freezing rain due to warm
mid-level temperatures overrunning the cold retreating surface
high, but further inland across Upstate NY and central New
England, some light snow is likely. WPC probabilities are 10-20%
for 4 inches on Sunday.


...Mid-Atlantic...
Days 1-3...

Two distinct low pressure systems will bring a wintry mix to the
Mid-Atlantic through the weekend.

The first is a weak wave of low pressure expected to move west to
east across the Deep South and then offshore the Carolinas. This
system is embedded within broad SW mid-level flow but strong upper
divergence within the RRQ of a departing 170kt jet streak over New
England. WAA ahead of the wave will spread precipitation
northward, while a wedge of high pressure asserts itself down
against the Appalachians. This overrunning setup will create a
period of sleet and freezing rain near the NC/VA border, with snow
further north as far as Washington, D.C. The heaviest snow is
likely in a weak fgen band which may align west to east over
central VA, but the overall system is weak and WPC probabilities
for 2 inches are less than 30%. South of the snow, a narrow
corridor of freezing rain is likely with 30-40% probabilities for
0.1" of accretion.

This first system ejects quickly Friday aftn, but only a brief
lull is expected as a more significant wave approaches Saturday
aftn. This secondary low will blossom from the Gulf Coast in
response to a weak shortwave and renewed RRQ jet level diffluence.
However, the Gulf low should wane as a secondary low develops
offshore the Carolinas and then lifts northeast through Sunday.
The guidance continues to suffer from quite a bit of spread with
the track and intensity of this system, but the trends today have
been for subtly more amplification with a further NW track. This
implies that more moisture will spread inland across the region,
and with arctic high remaining wedged in place, a more significant
sleet/freezing rain scenario is likely. For D2-3, WPC
probabilities for 0.25" of accretion are higher than 40% from far
northern NC through much of central VA and into the I-95 corridor
near Washington, D.C. There is potential for more than 0.5" in a
few locations, especially across central VA where overrunning
precip will be more significant. Further adjustments of the
heaviest freezing rain axis are likely in the upcoming days, and
if the system continues to trend NW more accretion could impact
the major urban areas of I-95.


Weiss