Discussion from NWS CAE (Columbia)
Key Message 2: The trend continues towards a potentially significant
winter storm this weekend.
Overview: The forecast thinking has not changed much for the weekend
system with potential for a highly impactful winter storm
continuing. The overall synoptic pattern is quite consistent across
all guidance with a deep digging
trough off the NE
CONUS,
strong
confluence and an associated strong surface high in the
central- eastern
CONUS, and an ejecting
cutoff shortwave in the
SW
CONUS. This patterns sets up a broad
overrunning scenario as
the arctic high digs southeastward presenting an all-hazards
impactful winter storm potential for much of the southern and
eastern US, including GA and SC as strong cold air damming sets
up.
Trends and Forecast Challenges: While some disagreements continue,
guidance remains in above-typical agreement over the potential
impacts from this system given the 72-96 hour window, with historic
analogs and
climatology concurring; this setup distinctly favors
mixed precip with snow-sleet-freezing rain-rain potential. A subtle
shift in some of the 18z guidance and more evident in much of the
00z guidance is a northerly shift in the axis of heaviest
qpf,
thanks to a slightly more amplified
flow and associated low pressure
development; this also pushes the surface edge of the
CAD dome
north. This trend in guidance is seemingly driven by a very
subtle slowing of Pacific
cutoff ejection and positioning of the
broad surface high; aircraft recon early Wednesday morning
should help improve this sampling, but not until the 12z
guidance. Thanks to this trend, the GEFS has steadily become
more and more of a cold- southerly outlier compared now with the
ECE, Canadian
Ensemble, and AI suite being warmer- northerly.
As noted previously, shifts in the handling of both of the low
level cold air and the Pacific
cutoff are expected as these are
two of the most notoriously difficult meteorological features
for guidance to handle. The current trends, and anticipating
typical model biases, reinforce the thinking that the I20
corridor will potentially lie along a steep
gradient zone for
qpf and therefore potential winter impacts.
Potential Impacts: A range of impacts continues to be possible from
this system, ranging from a notable
ice storm to a inconvenience-
nuisance event with a
gradient of impacts across SC and eastern GA
looking more
likely. A mix of precip-types is expected, but trends
over the last 24 hours suggest snow is less
likely for central SC
and eastern GA. Confidence remains high that much of, if not the
entire, area will see at least some wintry weather from late
Saturday-Sunday, however specific geographic impacts remains unclear.
Based on the guidance trends, confidence is increasing that the I20
corridor will lie along the
gradient of impacts; the probabilistic
WSSI for moderate impacts summarizes this well, with ~60-80%
chance north of I20 and then ~20-40% chance south of I20. While
details in the spatial impacts are still unclear, the high end
ceiling for this system remains, with notable
ice storm potential
and lingering cold weather behind the system extending impacts
in time.
Summary: Guidance remains in fairly good agreement over much of this
forecast but subtle differences result in a range of impact
potential. Confidence is already fairly high that many areas of SC
and eastern GA will see at least some winter weather this weekend
but a
gradient of impacts is
likely geographically. Unlike many
southern winter events, the
ceiling for this event is very high with
significant impact potential.