Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
444 AM EST Sun Feb 14 2021

Valid 12Z Sun Feb 14 2021 - 12Z Wed Feb 17 2021


...Southern Rockies to the Northeast...
Days 1-3...
A major winter storm will continue to impact a large portion of
the U.S. into early next week, with significant impacts reaching
as far south as the western and central Gulf Coast. 

A well-defined shortwave trough digging into the southwestern U.S.
overnight is forecast to swing east across the southern Rockies
later today.  As it does, models show an area of strong ascent
supported by left-exit region upper jet dynamics, along with low
level easterly flow, producing locally heavy precipitation across
southern New Mexico, with heavy snow accumulations of up to a foot
or more likely along the upslope regions of the Sacramento
Mountains Sunday morning. 

Meanwhile, favorable mid-to-upper level forcing and modest low
level frontogenesis/convergence will begin to support some
organized areas of light to moderate snow moving east across
northern Texas and Oklahoma into southeastern Kansas and
southwestern Missouri on Sunday.  WPC PWPF shows high
probabilities for snow accumulations of 4-inches or more extending
from the western portion of North Texas across central into
northeastern Oklahoma.  By late Sunday and continuing into early
Monday, models continue to show a strong signal for organized
snows developing farther to the southeast across East Texas into
the ArkLaTex region.  Low-to-mid level frontogenesis along with
favorable upper jet forcing is expected to help support the
potential for moderate to heavy snowfall bands developing across
the region, with WPC PWPF indicating high probabilities for
snowfall accumulations of 4-inches or more.  As this area
develops, lighter snows are forecast to spread farther south into
South Texas, while a wintry mix with accumulating ice develops
along the Texas coast. 

Heavy snows are expected to move northeast across northern
Mississippi into western Tennessee and the lower Ohio valley on
Monday.  Meanwhile, freezing rain with significant icing is
forecast to develop farther to the southeast from the central Gulf
Coast to Middle Tennessee.  The consensus of the forecast
guidance, which was already cold, has trended colder overnight --
shifting the axis of heavier snow and ice farther to the southeast
than previous runs.  This is reflected in the latest WPC PWPF,
which shows high probabilities for ice accumulations of 0.25-inch
or more extending now covering a large portion of central
Mississippi and southern Louisiana.

Moderate to heavy snows will continue to move northeast through
the Ohio valley into the Lower Great Lakes late Monday into
Tuesday, while a wintry mix, including accumulating ice extends
from the Tennessee valley to southern New England.  Light to
moderate snows are expected to quickly move from Update New York
across northern New England on Tuesday, diminishing as the system
moves into the Canadian Maritimes late in the day.

...Pacific Northwest to the Southern Plains...
Unsettled weather is expected to continue across the Northwest as
a series of shortwaves drop southeast across the region.  Snow
levels are expected to remain low intialy -- supporting additional
lowland snow early before snow levels begin to gradually increase
on Sunday.  Additional heavy accumulations are likely across the
Olympics and Cascades, with lighter accumulations spilling east
across the Intermountain West through early Monday.  On Monday and
continuing into Tuesday, heavier amounts are expected to develop
farther to the east across the southeastern Washington and
northeastern Oregon mountains into the northern Rockies.  By early
Tuesday, energy dropping southeast will begin to re-amplify the
trough over the Great Basin and Southwest once again, with areas
of heavy snow developing across the higher elevations of the Great
Basin into the central Rockies.  Similar to the previous
shortwave, this system is expected to dig across the Southwest
into the southern Rockies.  By early Wednesday, the threat
organized snow is forecast to the return to portions of the
southern Plains, with WPC PWPF indicating increasing probabilities
for significant accumulations from the eastern Texas Panhandle
into western Oklahoma by Wednesday morning.

Pereira