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

Valid 00Z Wed Feb 17 2021 - 00Z Sat Feb 20 2021


...Southern Rockies and Southern Plains to the Mississippi Valley,
Central Appalachians, Mid Atlantic, and the Northeast...

Days 1-3...

Shortwave energy emerging from the Southern Rockies causes
warm/moisture advection to develop across the Southern Plains this
evening and spread into the MS Valley tomorrow as the trough/low
level front move east. A strong 850-700 mb jet of 50-60 kt leads
to well defined low level convergence maxima crossing LA and AR
into MS and then TN. Widespread precip is expected with locally
heavy snow in eastern OK across AR on Day 1.  On Day 2, the east
northeast movement of the 700 mb wave leads to precipitation
moving downstream across the Oh Valley to the central
Appalachians, with a continuing north-south spread of how far
northwest the QPF and resultant snow spreads.
On Day 3, the models show the system departing NY/New England,
with the mid level frontogenesis leading to a period of snow in
upstate New York before the low moves off the coast and mid level
front relocates offshore with it.  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.

Lake effect snows develop off Lake Superior and Lake Erie, with
upslope flow bringing additional snow showers into the western
Adirondacks of NY.

Latest PWPF shows high probabilities for snow accumulations of 4
inches or more extending across much of Oklahoma across Arkansas
on Day 1, with low to moderate probabilities in the Ohio Valley to
central Appalachians due to the north-south spread in QPF that
persists.

Further south, the snow changes to sleet and freezing rain across
southeast Texas to central/northern LA, southern AR, western MS,
much of TN, and across the central Appalachians. The highest
probability for 0.25 inches of icing is focused in southeast TX
across northern LA and western MS. on Day 2, icing extends
northeast from TN and eastern KY across the Appalachians of WV to
western NC, with excellent overlap on an area of quarter to half
an inch of icing across the Piedmont of northwest NC to southwest
and south central VA.
The day 2 probability of a quarter inch of ice is high in
southwest VA to the NC border.

As the system moves northeast on Day 3 and redevelop off the
coast, the axis of icing moves northeast across portions of
eastern VA and then across eastern MD to southeast PA/northern NJ
before ending.

...Pacific Northwest to the Northern Rockies and Great Basin...

Deep northwesterly flow, with embedded energy aloft will continue
to support periods of mountain snow, with additional accumulations
expected to focus from the WA/OR Cascades to the Blue Mountains of
OR/ranges of Idaho.  As the confluent mid level flow funnels
moisture downstream into the UT Wasatch, along with sporadic 700
mb  vertical velocity maxima, several inches of snow area expected
where lifted is aided by orographics.

An upper ridge moving east onshore from the northeast Pacific Wed
evening will provide a break in precipitation late Wednesday into
early Thursday.  However, precipitation is forecast to return as a
700 mb trough approaches with moisture advection and warm
advection combining to produce lift during the day Thu, so snow
will redevelop in the WA/OR Cascades. With the continued low-mid
level confluent flow and periodic 700 mb ascent maxima, the snows
should continue on Thu in the WA/OR Cascades, streaming across the
OR Blue Mountains and ranges of Idaho, then reaching the WY
Tetons.  This system is expected to bring another round of heavy
snows to the Olympics and Cascades, with potential for a
widespread 12 inches and isolated maxima to 2 feet on day 3.

Petersen