554
FXUS62 KGSP 161600
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1200 PM EDT Mon Sep 16 2024

.SYNOPSIS...
A semi tropical low will move inland over the Carolinas
tonight and Tuesday bringing heavy rain and gusty winds. The low
will weaken through mid week with less wind and rain. 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 1200 PM EDT Monday: Main change this afternoon was the
issuance of a Flood Watch for Flash Flooding for the Charlotte Metro
area as rainfall amounts from 2-4 inches are expected, with locally
higher amounts from 4.5-5.0 inches possible. Otherwise, the broad
deck of lower clouds associated with Potential Tropical Cyclone 8
continues over eastern two-thirds of the CWA. Still hard to pinpoint
a defined circulation with the system and it has not become any
better organized since this morning. The area of light rain
continues pushing west across the eastern zones of the CWA so
increased PoPs slightly to account for this.

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. Potential Tropical Cyclone 8 is
currently situated about 100 miles east of Charleston, SC and will
begin its gradual track inland before making landfall sometime this
evening. Model guidance still differs on the overall timing and
location of the potential tropical cyclone as the operational GFS
still fast tracks the system into the heart of SC by this evening,
while the ECMWF/Canadian/NAM are slower and do not show the
potential tropical cyclone making landfall until this evening. 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 this 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. The other main
hazard in association with Potential Tropical Cyclone 8 will be wind
gusts as model guidance sources indicate a 40-50+ kt LLJ punching in
on the north and northwest periphery of the system. Did issue a
mostly impact based Wind Advisory for most locations along I-77 as
the last few runs of the HRRR continue to show 30-40 kt gusts in
these locations. These winds may allow for some scattered trees to
come down and may lead to isolated power outages. Also, 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
very 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 CWA 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 as
extensive cloud cover and rainfall continue and remain over at least
the northeastern half of the CWA.

&&

.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 MONDAY/...
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 /16Z 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. 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 today. 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/KAND/KAVL by tonight. Showers are
possible at KAND, but confidence has increased a little more to
support a PROB30 mention for SHRA during the overnight period as
guidance continue to trend further south with the rain shield. Still
expect for MVFR cigs to filter in before the end of the TAF period.
Eventually, MVFR/IFR should overspread all terminals by tonight,
with vsby restrictions being included, especially with any ongoing
rainfall. Winds will take on a northeasterly component, but may back
more northerly by late this 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...Wind Advisory until 2 AM EDT Tuesday for NCZ057-070>072-082.
     Flood Watch from 8 PM EDT this evening through Tuesday morning
     for NCZ070>072-082.
SC...Wind Advisory until 2 AM EDT Tuesday for SCZ009-014.
     Flood Watch from 8 PM EDT this evening through Tuesday morning
     for SCZ009.

&&

$$

SYNOPSIS...DEO
NEAR TERM...AR/CAC
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...CAC