614
FXUS62 KGSP 160744
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
344 AM EDT Mon Sep 16 2024

.SYNOPSIS...
A low developing off the South Carolina coast is expected to track
northwestward into the Carolinas later today, possibly become
a Tropical Storm, bringing heavy rain and gusty winds. The low
will slowly drift inland tonight through Tuesday, and weaken. By
Wednesday, the remnant low will stall out over the Mid-Atlantic,
while high pressure begins to build into the region from the north
and west late in the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 330 AM EDT Monday: Broad deck of lower clouds associated
with Potential Tropical Cyclone 8 is covering at least the
eastern two-thirds of the CFWA. Still hard to pinpoint a
defined circulation, but indications on satellite show the storm
intensifying. Once a circulation center becomes established,
model guidance should have a better handle on the overall track
(more on this below). The area of precip remains to the southeast
and will have to overcome a dry boundary layer for rain droplets
to reach the surface, which won`t be an issue after a while as
moist 40-50 kt easterly LLJ begins to filter in after daybreak
Monday. Can`t rule out a few spotty showers in the vicinity of
the eastern Blue Ridge Escarpment overnight, but not much fanfare
outside of that for the rest of the overnight period.

Weak blocking pattern will remain in place across the eastern CONUS
through a good portion of the forecast period, as an anticyclone
weakens over the Great Lakes and a east-to-west-oriented trough
lingers atop the Southeast. As mentioned above, Potential Tropical
Cyclone 8 is currently situated about 115 miles east-southeast of
Charleston, SC and will begin its gradual track inland, eventually
making landfall late Monday. Model guidance still differ a good bit
on overall timing and location of the tropical disturbance as the
operational GFS still fast tracks the system into the heart of SC by
Monday evening, while the ECMWF/Canadian/NAM are just making
landfall at this point. Even ensemble members on the track guidance
diverge after the 12-24 hour period, but the general consensus is
for the storm to move onshore late Monday evening, while quickly
enduring an extratropical transition as the main axis of precip
remains on the northern and western peripheral of the disturbance as
the deformation zone shifts across the area in response. With the
center of the storm shifting slightly further south across the
Midlands with each run, the heaviest rainfall in the current
forecast shows the main QPF axis stretching from the CLT metro area,
along the NC/SC state line, and eventually the favorable upslope
locations along the eastern Blue Ridge Escarpment as a stout
easterly component will keep rainfall ongoing in this area. CAMs,
hurricane guidance, and operational models show the system somewhat
stalling in the area, which will carry mentionable PoPs longer,
especially in the favorable upslope zones along the Blue Ridge
Escarpment as the easterly component remains established in this
area. Hydro issues may become a concern for the upslope regions
along the Escarpment and the CLT metro, but dry antecedent
conditions and rainfall amounts being more spread out over time are
the reasons for not issuing a Flood Watch at this time. Current QPF
response includes 2-4+" in the NC Piedmont and 3-5+" along the
eastern Blue Ridge Escarpment and 1-2" of rainfall in the northern
Upstate. A sharp gradient will cutoff any potential of significant
rainfall in northeast Georgia and the western Upstate as the system
stalls for a short time period and very slowly tracks northward
during the short term forecast period.

Other hazards in association with Potential Tropical Cyclone 8
are wind gusts as aforementioned model guidance indicate a 40-50+
kt LLJ punching in on the north and northwest peripheral of the
system. Gusts of 30 kts, with an occasional gust of 35+ kts can`t
be ruled out, mainly in the CLT metro and higher elevations that
will tap into the higher gusts. The NAMNest and HRRR display 30-40+
kts of surface wind gusts in these same areas. The current forecast
still doesn`t support enough speed for a Wind Advisory and with dry
antecedent conditions, didn`t think an impact based Wind Advisory
was necessary. One thing to note, with the southern trend that the
center of the tropical cyclone continues on, this may place the
northwest Piedmont and portions of the I-40 corridor into the front
right quadrant of the storm, which could lead to an environment
favorable for an isolated tornado or two in this region late Monday
night into Tuesday morning. Confidence is low on this development,
but the chance is nonzero and is supported by the 00Z HREF, which
places a few updraft helicity streaks in this area. Otherwise,
temperatures will induce a tight gradient for afternoon highs as
the southwest zones will end up close to normal, while the rest of
the CFWA will be 5-10 degrees below normal due to extensive cloud
cover and much better precip chances. Overnight lows will run at
or slightly above normal Monday night as extensive cloud cover
and rainfall continue and remain over at least the northeastern
half of the CFWA.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 315 AM Monday: Overall, the models are trending a little
slower and south with the track of Potential Tropical Cyclone #8
as it tracks into SC, which may be named Hermine at some point
before making landfall along the Carolina coast. This results in
lingering heavy rain potential across the NC Foothills and Piedmont
Tuesday, as the Escarpment keeps a solid upslope ELY flow thru the
day. The low will be weakening and right over the forecast area,
so winds should become fairly light, although may remain breezy
in the far northern zones. PoPs and sky cover were bumped up, and
max temps lowered based on these trends. Additional QPF after 12z
Tuesday may be 1-3" along the Escarpment per the latest HREF PMM.

The low is expected to continue to fill and drift north of the
forecast area Tuesday night into Wednesday. The 850 mb flow will
turn out of the W/SW during the day, allowing for some clearing
of clouds east of the mountains. Sfc dewpts still in the mid 60s
combined with a rebound in temps into the 80s may result in a few
hundred J/kg of sbCAPE by mid-aftn. So isolated to scattered showers
with a few tstms possible mainly in the mountains is expected
Wednesday, and should largely dissipate in the evening. Highs will
be near normal, while lows remain slightly above normal.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 245 AM Monday: An upper low associated with the remnants of
PTC #8 is forecast by the medium range guidance to drift east and
open up along the East Coast late in the week. To our west. a long
ridge axis extending from the Southern Plains to the Great Lakes
will begin to build into the region. At the sfc, a weak low may try
to organize off the mid-Atlantic coast, while an inverted ridge
noses south along the Appalachians. Fairly weak flow across the
region will likely keep some lingering moisture around, and with
more sunshine Thursday, could see some instability for diurnal
convection. Similar setup continues for Friday, with slight chc
to low-end chc diurnal PoPs for mainly shower, but isolated tstms
possible. Temps will be slightly above normal.

By Saturday, the sfc high over New England strengthens
significantly, as a Rex Block pattern sets up across the eastern
Conus. The operational models disagree on how organized a sfc low
will get off the coast, but agree that some moisture may be advanced
in ELY/NELY flow into the area Saturday thru Sunday. This may result
in a CAD-like setup over the forecast area, with increasing clouds
and slightly below-normal highs and above-normal lows. PoPs still
look low given the shallow moisture.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Moist easterly flow just above the surface
layer in response to Potential Tropical Cyclone Eight, located
offshore the Grand Strand will support BKN/OVC MVFR low clouds
across most of the area through daybreak Monday. Conditions may
improve at KAVL and the Upstate terminals during the afternoon,
but should continue to slowly deteriorate at KCLT and KHKY as the
tropical disturbance inches closer to the coast and eventually
making landfall late Monday. As a result, rain chances will increase
by the afternoon and remain in place through the evening, which is
reflected in the prevailing lines at these TAF sites. Low-level
wind shear may become an issue at KCLT and potentially KHKY,
but confidence is higher at KCLT for a TAF mention. Confidence
lowers at the upstate sites and KAVL as model guidance differ on
the timing and extent of the rainfall, but rain and associated
restrictions should eventually reach KGSP/KGMU/KAVL by the end of
the forecast period. Showers are possible at KAND, but confidence
is too low through the 06Z forecast period to include nothing more
than VCSH in the prevailing line due to the sharp cutoff of precip,
but still expect for MVFR cigs to filter in before the end of the
TAF period. Eventually, MVFR/IFR should overspread all terminals
by Monday night, with vsby restrictions being included, especially
with any ongoing rainfall. Winds will take on a northeasterly
component, but may back more northerly by late Monday evening,
depending on the location of the center of Potential Tropical
Cyclone Eight. Gusts in the 25-30 kt range are possible, mainly at
KCLT and KHKY, with some gusts potentially exceeding 30 kts. 20-25
kt gusts are included at the upstate terminals and KAVL as the
storm system pushes towards the region late in the period.

Outlook: Gusty winds and some instances of low-level wind shear
will impact KCLT and KHKY through Monday night and even into
parts of Tuesday. The best chance for rainfall and associated
restrictions will reside at KCLT and KHKY, based on the current
forecast of Potential Tropical Cyclone Eight. MVFR/IFR restrictions
are likely at all TAF sites by late Monday evening through at least
daybreak Tuesday. Still some uncertainty on the extent of rainfall
coverage and associated restrictions beyond daybreak Tuesday,
but winds and precip will begin to taper off at some point during
the day Tuesday. Spotty showers and occasional restrictions will
be possible through the remainder of the week.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$

SYNOPSIS...ARK
NEAR TERM...CAC
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...CAC