Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 PM EST Fri Feb 12 2021

Valid 00Z Sat Feb 13 2021 - 00Z Tue Feb 16 2021

...Pacific Northwest...
Days 1-3...
As a large cyclonic gyre persists across the Northern Plains
driving broad cyclonic flow across most of the CONUS, a shortwave
will drop out of the Gulf of Alaska through the mid-level NW flow
to move ashore the Pacific Northwest early Saturday. This feature
will be accompanied by a surface low and frontal structure which
will weaken as it approaches the coast, mid-level height falls and
divergence, and LFQ diffluence within an approaching jet streak.
This deep layer ascent will be enhanced by broad WAA as mid-level
flow backs to the SW ahead of the trough axis, and a prolonged
robust precipitation event is likely tonight through Saturday.
Heavy snow is likely in the favored terrain of the Cascades and
other ranges east into Idaho as moisture spillover takes place.
Snow amounts in excess of 12" are almost certain in the Cascades,
with lesser probabilities extending eastward. Potentially more
impactful is the snow and ice that should impact the lowlands,
including the metro areas of Seattle and Portland. For Seattle,
WPC probabilities are high for 4", with more than 6" possible as
snow levels crash to the ground. For Portland it is trickier as
east winds out of the Gorge will try to keep the low-levels cold,
but WAA aloft, especially with a more northern low track shown in
guidance, will try to push a warm nose above 0C. A mixture of snow
and ice is likely in Portland, and WPC probabilities are moderate
for 4" of snow and 0.1" of ice. If the column is just a little
colder (more snow) or a little warmer (more ice) a significant
winter event of either type is still possible. However, SW/W of
Portland, a major freezing rain event is likely where WPC
probabilities for 0.5" are as high as 20% through Sunday morning.

Continued confluent mid-level flow and yet another elongated
shortwave will approach the coast and move onshore Sunday,
bringing renewed precipitation after just a brief respite. Snow
levels remain low in this arctic air mass, and WPC probabilities
are high on D3 for 8 inches in the Cascades, with light snow
accums possible into the lowlands once again.


...Great Basin/Southwest/Central and Southern Rockies...
Days 1-2...

The shortwave impacting the Northwest tonight into Saturday is
forecast to dig southeast into the Great Basin on Saturday and
then into the Southwest by Sunday morning.  Large scale ascent
supported by left-exit region upper jet forcing will help to
produce widespread precipitation across the region, including
areas of locally heavy snow across the mountains.  Areas impacted
are forecast to include the northern Nevada mountains, the Utah
ranges, as well as the Arizona Rim Country.  Southwesterly flow
ahead of the digging trough will draw increasing moisture into the
southern Colorado and northern New Mexico ranges, where orographic
effects along with a low-to-mid level front sliding southwest are
expected to help support heavy snows, and the heaviest snow of the
period is likely D2 in the San Juans where WPC probabilities are
high for 12 inches.


...Central Plains to the Gulf Coast into the Ohio Valley...
Days 2-3...

...Large winter storm to bring heavy wintry mix of precipitation
to the South...

A potent arctic front digging down through the middle of the
country will sink into Texas coincident with an impressive
shortwave digging into the Southwest. This shortwave will
gradually weaken as it moves into the Plains Sunday, but deep
layer forcing will remain intense as a coupled jet structure
develops downstream of the main longwave trough axis to drive
ascent atop mid-level divergence. These features will work
concurrently atop the low-level baroclinic zone to produce a
surface low over the Gulf of Mexico by early Monday which will
then race northeast towards the TN VLY by the end of the forecast
period. This evolution is likely to produce two distinct areas of
heavy precipitation.

For snow, intensifying WAA/isentropic ascent ahead of the
shortwave will overrun the surface boundary and interact with the
arctic high to produce heavy snowfall beginning Sunday morning
over Kansas, dropping south into TX Sunday night, and then lifting
northeast towards TN on Monday. An extremely cold column will
support SLRs that may approach or exceed 20:1, and this "fluff
factor" will lead to rapid snowfall accumulations despite modest
total QPF. WPC probabilities for 6" on D2 are above 50% for the TX
Panhandle into western OK, and then shift into north TX and
central OK D3. The heaviest snow accumulations may approach 12" in
Oklahoma. As the surface low starts to develop the main isentropic
ascent will get shunted east, but increasing frontogenesis and the
synoptic ascent will drive a continuation of heavy snowfall from
east TX through MO and IL on D3, where WPC probabilities for 4
inches are as high as 40%.

To the south of the heavy snow, and primarily on D3 as the surface
low develops, an area of freezing rain is likely. As WAA spreads
moisture northward it will lift atop the arctic front, and
although surface temps will remain well below freezing, the warm
nose should rise above 0C supporting a mix of sleet and freezing
rain. The guidance has trended a little further SE with this axis
today, and at this time the heaviest freezing rain is expected
from near Houston, TX northeast through central LA, MS, and into
TN/KY, with more than 0.25" possible in some locations. This could
become a damaging event for areas that do not frequently receive
significant freezing rain.

Between the snow and sleet a narrow corridor of mixed precip,
including heavy sleet, is likely. At this time it is difficult to
discern exactly where this will occur, however, any sleet could
either cut down on snowfall accumulations or freezing rain from
eastern TX into Missouri and points southeast.


...Mid Atlantic...
Days 1-2...

A wave of low pressure moving up the coast will spread northward
into the Mid-Atlantic as mid-level SW flow spreads moisture atop a
residual cold surface high pressure wedging down east of the
Appalachians. The warm ascent will drive a warm nose >0C quickly
northward, but surface temperatures are likely to remain below
freezing as noted by forecast wet bulb temperatures. This suggests
a period of moderate freezing rain and sleet beginning early
Saturday and persisting into Sunday morning. The guidance today
has backed off a bit on total moisture due to a relatively low
amplitude pattern, but there is still likely to be significant
freezing rain from northern NC through VA and into the I-95
corridor from Washington, D.C. to Philadelphia. WPC probabilities
for 0.25" are as high as 40% in central VA, with generally less
than 0.25" expected elsewhere. Travel along the I-95 corridor late
Saturday into Sunday could become hazardous.


Weiss