.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
An upper level split-flow wave will cross the Ohio Valley into the
mid-Atlantic Wednesday into Thursday, bringing a swath of moisture
across the Upper Ohio. Associated sfc low pressure will develop
across the upper Mississippi Valley before deepening off the
Carolinas and riding up the Atlantic coastline. Transient left-exit
and coupled jet support are progged to generate broad ascent to
support mainly snow beginning Wednesday morning, then ramping up
during the afternoon and overnight.
Significant uncertainty remains with regards to snow amounts across
the forecast area. Deterministic and ensemble guidance continue to
support a large range in possible snow totals, primarily due to
large differences in QPF amounts and placement of the sfc low to our
east. The 00z ECMWF run in particular places the sfc low much farther
west and north than previous cycles, though without full EC ensemble
and other deterministic agreement, this was not enough reason to
significantly inflate our official snow totals at this time.
Some question also remains with the extent of warm advection across
the lower elevations of WV into SW PA on Wednesday afternoon/ early evening
as this could impact initial snow totals. With further penetration
of warm air, some potential also exists for brief freezing rain
across this area and into the lower ridges. However, the window for
this is small, as more substantial warm advection would likely bump
marginal sfc temperatures above the freezing mark.
Rapid system progression also presents a barrier to high snow totals,
as the system quickly shifts off the coast early Thursday morning and
shuts off efficient snowfall.
Highest snow totals remain over the Allegheny Mountains and ridges,
with lesser snow farther west. Confidence in 8+ inches of snow is
high enough across the higher elevations to support an upgrade from
Watch to Winter Storm Warning in those areas. With increased snow
amounts forecast farther west, the Watch was also expanded to include
areas including Pittsburgh. Lower confidence exists across the lower
elevations of Monongalia and Marion counties where warmer
temperatures will likely inhibit initial snowfall totals. Watches and
Warnings are in effect from 7AM Wed through 7AM Thu. Advisories will
likely be needed for areas farther west, especially if the upward
trend continues.