Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 090837

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
337 AM EST Sat Dec 9 2023

Rain will develop ahead of an approaching cold front today through
tonight, with thunderstorms becoming more likely on Sunday. As the
rain ends Sunday night, colder air behind the front may produce some
light snow in the North Carolina Mountains. The forecast remains
quiet through the rest of next week.


As of 300 am EST Saturday: With showers starting a bit early in the
southwest upslope flow into the southwestern NC mountains, the onset
of better PoPs has been hastened in that area.

Otherwise, the forecast remains on track. A 500 mb cutoff low center
continues to deepen over the upper Midwest early this morning, with
the associated longwave trough axis digging southward through the
Plains. This trough will become the primary player in a forecast
featuring deteriorating conditions across the western Carolinas and
northeast Georgia over the next 24 hours. Light shower coverage will
continue to improve from the west today in the deepening moisture
and weak forcing ahead of the approaching trough and frontal system.
Anticipate thickening and lowering cloud coverage keeping a lid on
temperatures, but maxes will still likely reach the 50s in the
mountains and 60s elsewhere due to the warm start this morning. PoPs
will ramp up steadily from the west today, with better light
rain/shower coverage across the western half of the forecast area
for the bulk of the day.

Forcing and deeper moisture will increase tonight as the digging
upstream trough migrates eastward over the Mississippi River Valley.
An upper jetlet east of the trough axis will improve divergence
aloft sharply over the southern Appalachians from 06Z to 12Z Sunday,
and the convection-allowing models exhibit a robust response to this
with an uptick in coverage through the early morning hours. With no
strong signal suggesting that Gulf coastal convection will rob
moisture influx into the region, fairly widespread heavy rainfall
should develop in the pre-dawn hours, especially in the southerly
upslope preferred regions of the extreme southern Appalachians. With
some wetting rainfall today through this evening, overnight QPF
could be sufficient for isolated flooding to develop with any
training showers in and near the mountains late tonight. In
addition, the 850 mb southwesterly flow jet will see an uptick to 45
to 50 kt and a steady increase in associated bulk shear by daybreak.
Cannot rule out some surface based instability of a few hundred J/kg
sneaking into the Savannah River area and the southwest NC mountains
through daybreak, and a marginal severe thunderstorm risk is
indicated there as well late tonight. Very warm minimum temperatures
will run around 20 degrees above climo tonight.


As of 330 AM EST Saturday: A pronounced shortwave is progged to
ride thru the base of a full-latitude longwave trough across the
Southeast CONUS, sharpening the overall trough and pivoting its axis
from positive to negatively tilted as it crosses the forecast area
(FA). This trend in the guidance is resulting in a slight slowing
down of the attendant cold front, expected to be pushing into the
western part of the FA at 12z Sunday. The right entrance region of
a 300 mb jet should be coupled with the frontal forcing supporting
a fairly broad frontal precip band. The latest CAMs show a leading
edge of the convective line in the FA, but disagree on the exact
placement at 12z Sunday. The NAM seems to be a slow outlier,
while the HRRR is right in the middle. Based on the HRRR timing,
have a chc thunder mention pushing into the center of the FA
(just east of the Blue Ridge escarpment). MUCAPE of up to 500
J/kg along/ahead of this line may support some strong storms, as
some of this instability may be sfc-based. Strong low-level shear
should be in place to support a damaging wind and isolated tornado
threat. This will continue to be highlighted in the HWO. The new Day
2 Convective Outlook has pushed back the marginal risk to include
most of our Piedmont zones. Heavy rain will also be a concern, as
the frontal speed has trended a little slower in the guidance. Highs
will be above normal, mainly in the 50 mountains and 60s Piedmont.

A sharp gradient in thicknesses along the front should result in
snow levels falling across the NC mountains fairly quickly Sunday
aftn-eve. PoPs should start to cut off east of the mountains as
downslope NWLY flow dries out the low-levels by late evening. The
NAM is the most aggressive on turning profiles from rain to
snow across the NC Foothills and NW Piedmont, with Hickory
possibly seeing a few snowflakes before precip completely tapers
off. The GFS shows something similar, but with less overlap with
moisture. For now, have just a slight chc to low-end chc of snow
mixing in with rain showers east of the mountains Sunday night. But
no accums are expected. Across the mountains, elevations above
3500 ft may see 1-3" of snow, while the bigger concern in the
valleys will be wet roads freezing, as temps plummet into the 20s
overnight. Temps approach freezing across much of the Piedmont
by daybreak Monday, but roads will have more time to dry and will
start out warmer. Another concern in the mountains will be gusty NW
winds with strong CAA. May see advisory-level gusts above 3500 ft,
especially in the Northern Mountains.

NWLY upslope flow snow showers should taper off along the TN border
by midday Monday, as dry air filters in behind the departing cold
front and upper trough. Winds should gradually weaken thru the day,
but it will be breezy across the mountains. Cold high pressure
building in from the west will keep temps well below normal in
the mountains and a couple categories below normal east.


As of 130 AM EST Saturday: The medium range looks pretty quiet,
starting out with slightly below normal temps, but overall,
seasonable conditions for mid-December. Dry high pressure will
settle over the region under a confluent NWLY flow aloft, which
should moderate gradually thru the end of the week. The 00z GFS
develops a low pressure system in the Gulf of Mexico and tracks it
northeastward, spreading moisture across the FA by early Friday. But
this seems to be an outlier, with none of the other deterministic
models showing this, and very few of any of the various ensemble
members. Precip chances won`t likely return until next weekend at
the earliest.


At KCLT and elsewhere: Conditions are expected to steadily
deteriorate across the terminal forecast area through the period,
and beyond. Lowering VFR clouds will give way to occasional MVFR
cigs from the west through the day, with IFR cigs developing late
today through tonight as better coverage of the showers arrives
along with deeper moisture. Some thunderstorms will be possible
tonight, but mainly just beyond the current TAF period. Winds will
be light southerly on average, perhaps a bit more southwesterly
early this morning and then more southeasterly later in the day. Any
low end gusts should be confined to KAVL given the limited mixing
across the foothills and Piedmont.

Outlook: Widespread showers, and embedded thunderstorms, with IFR or
lower restrictions are expected for much of Sunday ahead of an
advancing cold front. The front will move east of the area Sunday
night, with northwest flow snow showers briefly possible at or near
KAVL. Dry and VFR conditions will then return by Monday and persist
through the week.




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