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

Valid 12Z Thu Feb 11 2021 - 12Z Sun Feb 14 2021

...The West onto the Central Plains...
Days 1-3...
An upper low currently moving across the top of an upper ridge
centered over the eastern Pacific is forecast to drop southeast
across Oregon and Northern California late Thursday into early
Friday -- bringing heavy snows to portions of the Sierra. 
Meanwhile, energy moving southeast across western Canada will
support an arctic airmass moving farther south across the
Northwest.  Guidance shows a sharpening ridge along the Cascades,
with easterly flow supporting cold air spreading east into the
lowlands of eastern Washington and Oregon -- setting the stage for
a potential significant snow and/or icing event across the region.
 With this initial event, accumulating snow is expected across the
lowlands of southwestern Washington and northwestern Oregon, with
significant icing possible across portions of northeastern Oregon
as well.  This initial system will be quickly followed by a second
shortwave dropping southeast into the region late Friday into
early Saturday.  This will likely be the wetter system of the two
for the Northwest, with widespread heavy snows expected across the
Olympics and Cascades.  With the cold air locked in, significant
snow and/or ice is likely across the lowlands -- with several
inches of snow possible along the Puget Sound, including the
Seattle metro area.  Accumulating snow and/or ice is also likely
for the Portland region, with significant icing likely farther to
the west across northwestern Oregon and southwestern Washington. 
The threat for heavy snow across the Northwest is expected to
continue through Saturday, before diminishing early Sunday.  As
this system continues to dig southeast into the Great Basin, heavy
snows are expected to develop across  portions of the Great Basin
on Saturday before shifting into the central Rockies and the
northern to central Arizona mountains Saturday night into Sunday. 
Meanwhile, moisture associated with both systems is expected to
spread east across the northern Rockies, supporting rounds of
heavy snow across the eastern Idaho and western Wyoming ranges as
it interacts with an arctic boundary in place. 

...Southern Plains to the Mid-Atlantic and Northwest...
Days 1-3...
A wintry pattern is expected to continue through the period as a
series of low-amplitude shortwaves moves through the base and
along the leading edge of the longwave trough anchored over the
central U.S.  The ongoing wintry mix that extends from Texas to
the Mid-Atlantic is expected to continue on Thursday, with light
snow and/or ice accumulations expected from central Texas to Long
Island.  After a brief break, another round of wintry weather is
expected to develop Friday night from the lower Mississippi valley
to the southern Mid Atlantic.  On Saturday, a weak coastal low
moving along the Mid-Atlantic coast is expected to bring
precipitation farther north into the northern Mid Atlantic and
Northeast, with cold air damming expected to support the potential
for a wintry mix and significant icing across portions of the Mid
Atlantic.

Pereira