Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
403 AM EST Sat Feb 06 2021

Valid 12Z Sat Feb 06 2021 - 12Z Tue Feb 09 2021

...Great Lakes...
Day 1...
The center of trough sprawling across the entirety of the CONUS
(and nearly all of North America) is a gyre over northern/western
Ontario where several lobes of vorticity are apparent in water
vapor imagery early this morning. The next reinforcing shortwave
trough is over the northern MT/ND border and will shift southeast,
crossing the Upper Midwest tonight. The ongoing lake effect event
off Lakes Superior and Michigan will wane this morning on approach
of this trough. Westerly flow and strong cold air advection will
continue to support single band LES development across Lakes Erie
and Ontario, with locally heavy accumulations continuing through
this afternoon across the Buffalo and Tug Hill regions of New York
where there are moderate Day 1 probabilities for 6 or more
additional inches. Flow backs south-southwesterly by this evening,
cutting off the lake fetch.

...Pacific Northwest to the Northern and Central Rockies and High
Plains...
Days 1/2...
Shortwave impulses in a strong NWly get extending from the
northern BC coast to the northern Rockies will shift over the
PacNW and northern Rockies through Sunday, supporting additional
heavy mountain snows from the WA Cascades across the northern
Rockies, with lighter snows spreading east into the northern High
Plains to the central Plains/Neb into Sunday night. Day 1 has the
greatest heavy snow threat with moderate probabilities for 18 or
more inches for the WA Cascades/Bitterroots of ID/MT to the Big
Belt Mtns just east of the Bitterroots as well as the Tetons. The
Day 1 probability for 4 or more inches is moderate to high for the
central MT High Plains and moderate for 4 additional inches in
central Neb on Day 1.5.

...Central Plains to the Mid Mississippi Valley...
Days 1-3...
The confluence of low level flow spreading up the Plains and the
reinforcing trough over the northern Plains is providing favorable
forcing aloft along with strong low-to-mid level frontogenesis and
locally moderate snow moving progressively southeast across Neb
and northern KS this morning. This shifts across northern MO and
southern IA today. Strong forcing along with high snow-to-liquid
ratios in this cold sector activity have helped to bolster the
potential for locally heavy totals, with Day 1 probabilities for 4
or more inches moderate in central to southeast Neb, and low
across northeast KS east over central IL. As the previously noted
upstream energy in the Northwest makes its way into the region, a
second round is expected to develop tonight and continue into
Sunday with low Day 2 snow probabilities for 4 or more inches over
southeast Neb.

...Southern Appalachians to the Mid-Atlantic and Northeast...
Days 1-2...
Model preference is for the 00Z ECMWF/NAM which is slower than the
00Z GFS. This focuses the progressive wave to be a little slower
than the previous forecast and turning closer to the Northeastern
Seaboard due to influence with the northern stream trough crossing
the Great Lakes tonight/Sunday. This makes for more QPF farther
inland/north in NY and New England through Sunday, continuing into
Sunday night for eastern Maine. The shortwave trough is currently
moving into the northern TX Panhandle and will swing east to the
Mid-South states today and the southern Appalachians tonight.
Surface low development along the Carolinas Coast begins tonight
with the low continuing to develop as it tracks northeast from
Cape Hatteras past the Mid-Atlantic and New England coasts through
Sunday evening. The key will be low level frontogenesis allowing
dynamics to overcome marginal antecedent conditions in the central
Mid-Atlantic with cold air located farther inland (where less
moisture is available). Day 1 snow probabilities for 4 or more
inches for the southern Appalachians then across the interior
central Mid-Atlantic (almost entirely northwest of I-95 - but that
can be deceiving as mesoscale forcing is greater south of I-95
where QPF is higher) into eastern PA for Day 1.5.

Farther north, colder antecedent conditions and the offshore track
allows all snow along the Long Island and southern New England
shores, so it's a QPF concern with continued rapid deepening
allowing a intense low level forced bands to lift across southern
New England. Day 2 snow probabilities are moderate for 6 or more
inches over RI, southeast MA, and far eastern ME with low
probabilities extending from southeast PA (back in Day 1.5 probs),
across the NYC metro and southern/central NY up interior southern
New England and the northern New England coast.

Day 1 freezing rain probabilities are about 20 percent for a tenth
inch of ice in the southern Blue Ridge of northern GA and western
NC, but for days 1-3, the probability of significant icing
(0.25-inch or greater) is less than 10 percent.

Jackson