Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 AM EST Tue Feb 16 2021

Valid 12Z Tue Feb 16 2021 - 12Z Fri Feb 19 2021


...Central and Southern Rockies to the Northeast...
Days 1-3...
Shortwave energy dropping southeast through the Great Basin into
the Southwest will continue to amplify the western extent of the
broad, large-scale trough that remains positioned over the U.S. 
Broad, large-scale ascent supported by left-exit region upper jet
forcing is expected to encourage widespread precipitation across
the Great Basin, Southwest into the central and southern Rockies,
with orographic effects supporting locally heavy amounts on
Tuesday.  Areas impacted are expected to include the Utah, western
Colorado, eastern Arizona, and New Mexico ranges.

By late Tuesday as the upper shortwave begins to move east of the
Rockies, precipitation is forecast to develop out across northern
Texas and Oklahoma before expanding farther north and east during
the overnight hours.  Guidance is showing a period of mid-to-upper
level forcing along with low level theta-e advection sufficient
for light to moderate snows spreading from the eastern Texas
Panhandle to Arkansas and southern Missouri Tuesday night.  Latest
PWPF shows high probabilities for snow accumulations of 4-inches
or more extending across much of central and eastern Oklahoma into
western Arkansas by Wednesday afternoon.

Impacts downstream have become less certain, with the general
consensus of the overnight guidance showing less amplification
with the upper trough as it moves across northern Texas and
Oklahoma Wednesday night.  A notable exception is the NAM, which
remains more amplified, drawing more moisture into the colder air
and developing heavy snows out across Arkansas into western
Tennessee and lower Ohio valley.  Following the trends of the GFS,
UKMET and ECMWF, WPC leaned away from the NAM solution.  While the
potential for heavy snow appears to be decreasing, there remains a
strong signal for significant icing from eastern Texas to northern
Mississippi, with accumulations of 0.25-inch or more likely across
portions of the region.

As shortwave energy continues to translate through the base of the
longwave trough and lift to the northeast, precipitation will
continue to spread farther north and east from the Tennessee and
Ohio valley into the Mid Atlantic Thursday morning and into the
Northeast by late in the day.  A cold air wedge in place will
support a prolonged period of freezing rain, with significant ice
accumulations (0.25-inch or more) expected across portions of the
North Carolina and Virginia Piedmont.  Farther to the north within
the deeper cold air, widespread snow accumulations of 4-inches or
more are expected across the northern Mid-Atlantic, New York and
southern New England.

...Pacific Northwest to the Northern Rockies...
Deep northwesterly flow, with embedded energy aloft will continue
to support periods of mountain snow, with additional heavy
accumulations expected from the Cascades to the northern Rockies
on Tuesday into early Wednesday.  An upper ridge moving east
across the region, will provide a break in precipitation late
Wednesday into early Thursday.  However, precipitation is forecast
to return as the ridge gives way to a shortwave trough and its
associated frontal band during the day on Thursday.  This system
is expected to bring another round of heavy snows to the Olympics
and Cascades, with lighter accumulations spreading east across the
Intermountain West into the northern Rockies late Thursday into
early Friday.

Pereira