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

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


...Pacific Northwest...
Days 1-3...
The upper low associated with the ongoing initial round of
precipitation across the region is already dropping southeast of
the region, and is expected to continue to dig progressively
southeast across the western U.S. on Friday.  However, continued
onshore flow interacting with arctic high pressure will continue
to support precipitation, including lowland snow and freezing rain
across portions of the Pacific Northwest early Friday.  A strong
surface ridge centered east of the Cascades will support easterly
flow and cold air spreading east across western Washington and
Oregon.  Amounts west of the Cascades are expected to be
relatively light before the next system begins to impact the
region Friday night.  This second system is expected to bring
heavier accumulations into the region, with significant snowfall
accumulations expected across both the Portland and Seattle metro
regions.  Ongoing freezing rain across portions of northwestern
Oregon into southwestern Washington is expected to continue, with
locally heavy ice accumulations likely.  Meanwhile, heavy snow is
likely across the Cascades, with several feet expected along the
southern Washington and Oregon Cascades.  Similar to the first
system, this second shortwave is expected to move quickly
southeast of the region on Saturday.  However, unsettled weather
is forecast to continue through the remainder of the weekend, with
another system expected to impact the region by early Monday. 
Snow levels however will begin to increase, with the potential for
lowland snow decreasing late Sunday into early Monday.

...Great Basin/Southwest/Central and Southern Rockies...
Day 2...
The shortwave impacting the Northwest Friday night 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.

...Central to Southern Plains/Lower Mississippi to the Ohio
Valley...
Days 2-3...
Favorable upper jet forcing, along with low level easterly flow
will support snow developing east of the central Rockies into the
Plains Saturday night into Sunday, with at least a few inches
likely across portions of eastern Colorado, western Nebraska and
western Kansas.  As the upper trough continues to dig across the
Southwest and southern Rockies.  Snows are expected to develop
farther to south across the southern Plains, with left-exit region
upper jet forcing along with low-to-mid level frontogenesis
supporting the potential for heavy snowfall rates and
accumulations across the Texas Panhandle region into central
Oklahoma and south-central Kansas.  As cold air continues to
extend farther south, accumulating snow may reach as far south as
interior portions of South Texas Monday morning, with perhaps a
wintry mix and accumulating ice reaching the coast.  Periods of
freezing rain, with at least minor ice accumulations are expected
to extend from eastern Texas across the lower Mississippi and into
the Ohio valleys late Sunday into early Monday.

...Mid Atlantic...
Day 2...
Low amplitude energy and a weak coastal wave interacting with a
cold air wedge are expected to produce the next round of wintry
weather for the Mid Atlantic on Saturday.  Overall trend in the
models showed lighter QPF across  areas with the greater potential
for freezing rain -- lowering probabilities for significant ice
accumulations across North Carolina and Virginia.  However
impactful ice accumulations are still likely, especially across
portions of southern and central Virginia, extending northeast
into the Northern Virginia and Maryland I-95 corridor.

Pereira