Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
227 PM EST Thu Jan 7 2021
.SYNOPSIS...
A moist storm system arrives from the west this evening and passes
to the east on Friday. Widespread wintry weather will be possible
with this system, with the best chances and most significant
accumulations over the mountains and foothills. Canadian high
pressure returns to the region over the weekend. Another low
pressure system is likely to affect the area early next week.
&&
.NEAR TERM /THROUGH FRIDAY/...
As of 200 pm: ***The Winter Storm Watch has been upgraded to a
Warning for Catawba-NC, Iredell-NC, and Davie-NC. A Winter Storm
Warning will also be issued for Rowan-NC. A Winter Weather Advisory
will be issued for a tier of forecast zones south of the Warning.***
Latest WV imagery reveals upper cyclone centered over northern
Miss., with strongly diffluent flow downstream over much of the Deep
South and the TN Valley. As the short wave ridge currently centered
over the forecast area shifts east, and the upper low continues to
wobble east, forcing and moisture will increase rapidly across our
forecast area this evening, with precip chances ramping up quickly
across western areas through midnight. Surface ridge extending from
surface anticyclone over Ontario will spill into the area through
the period, resulting in increasing low level NE flow and gradually
advecting lower wet bulb temp air into the forecast area. Having
said that, wet bulb temps are already around freezing across much of
the NC mtns, and those areas are poised to see primarily snow as
precip develops this evening. (The Little TN Valley and adjacent
valleys are warmer as usual, and will likely see almost as much RA
as wintry precip).
Locations east of the mountains will, as usual be more problematic,
and will (also as usual) depend upon temperature trends in the
lowest 1000` or so. While all locations east of the mtns will more
than likely start out with rain, advection of the lower wet bulb air
should support a transition to snow along and north of I-40, and in
fact, precip should be more frozen than not in those areas. Closer
to the I-85 corridor, things get a lot trickier. Guidance generally
agrees that mostly rain will be seen closer to the NC/SC border.
However, periods of stronger synoptic forcing will support high
precip rates at times tonight (classic "cross-hair signature"
indicated in model cross sections), and this may force periods of a
mix with or transition to SN, although how much that would be able
to accumulate is debatable. Another source of uncertainty is the
potential for a narrow band of mixed precip, most likely sleet, but
a brief period of FZRA can`t be ruled out, as the NAM has been
consistent in developing a warm nose, albeit not a particularly
strong one near the NC/SC border tonight. Since the NAM usually does
a good job in capturing these features, even when other guidance
does not, we continue to forecast a region of sleet from the GA/SC/
southern NC mtns into the Piedmont. Forecast accumulation is
minimal, but this does create another pitfall for the snow accum
forecast.
Of greater concern for areas east of the mtns, and especially the
Piedmont and I-77 corridor, will be the potential for mesoscale
banding during the daylight hours Friday, as deformation zone
matures in response to cyclone deepening off the coast.
Convection-allowing models, especially those that are NAM-based and
even the operational NAM itself depict potential for locally high
precip rates, esp across the Piedmont from late morning into the
afternoon, possibly resulting in quickly accumulating snowfall, even
in areas outside the warning. Of course, predicting where exactly
this will (or if it will) set up is akin to predicting where a
cluster of thunderstorms will develop 24 hours in advance.
Nevertheless, the potential exists for locally intense snowfall
rates tomorrow, especially across the Piedmont.
In terms of amounts: storm totals are forecast to range from 5 to 9
inches across the NC mtns, except more in the 1-3 inch range across
the valleys of far southwest NC and over much of Rabun County GA.
Four to six inches are expected across the northern NC foothills,
where a developing stout easterly flow is expected to enhance precip
rates tonight, while 2-4 inches are generally forecast for the
northwest NC Piedmont. General 1-2 inch amounts are forecast in the
Advisory areas detailed above, although the southern part of those
zones may see little to no accum.
