-- Discussion containing changed information from previous version are highlighted. --
335
FXUS62 KGSP 270208
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1008 PM EDT Thu Sep 26 2024

.SYNOPSIS...
Tropical Cyclone Helene is forecast to make landfall over the
Florida Panhandle early tonight. The remnant circulation will move
quickly north through Georgia bringing continued heavy rainfall,
significant flooding and strong winds tonight into Friday morning.
Expect a calmer weekend with temperatures near seasonal normals.

&&

.NEAR TERM /THROUGH FRIDAY/...
-- Changed Discussion --
***A MAJOR RAINFALL EVENT FOR THE SOUTHERN APPALACHIANS AND VICINITY
IS OCCURRING WITH THE POTENTIAL FOR WIDESPREAD AND CATASTROPHIC
FLASH FLOODING***

As of 931 pm...A Tornado Watch has been issued for the area
from Greenwood to Union SC to the Charlotte Metro area through
12Z Friday, in anticipation of improving tornado potential in
the overnight hours. In these situations, it`s best to get the
Watch out before it gets too late in the evening. While the
thermodynamic environment is not particularly conducive at this
time...the sfc-based and most-unstable CAPE was generally 100 J/kg
or less southeast of I-85... the latest HRRR suggest that 300-500
J/kg or so may work in from the southeast, particularly into the
southern portions of metro CLT after 05Z or 06Z. Needless to say,
the low level and deep layer shear will remain more than adequate
as Helene makes its approach from the south through the overnight
hours. The tornado threat may continue through late Friday morning
if the RAP and HRRR are correct, as they bring a plume of 500
J/kg CAPE into the area along/E of I-77 as the main circulation
of Helene lifts northward in the morning.

Meanwhile...the rain rain rain came down down down...across the
entire forecast area for the better part of the day. The radar loop
shows a brief lull moving up from the south, so we should get a
break for an hour or two. Even so, the increasing moist upslope
flow will likely force more light to moderate rain as the lull
moves in, so the rain won`t really taper off, or not long enough
to matter. More showers and a few storms were already filling in
over the Coastal Plain. Needless to say, a 100pct will suffice
for the precip prob overnight.

As to the wind...no change to earlier thinking. The leading edge
of the significant wind gust potential will lift north across the
region from roughly 08Z to 11Z, maximizing from 10Z to 14Z south to
north. Indications of strong tropical storm force gusts continue in
the most recent guidance. Outside the mtns, there is some concern
that a very shallow stable boundary layer will keep the strongest
gusts from reaching the surface, but there will still be frequent
50kt gusts in many locations. Officially, the gust forecast will
not change until after the 11 pm Advisory. The mountains will have
no such protection. It seems likely that peaks and ridgetops will
get frequent gusts to hurricane force. These winds will combine
with saturated soils and down widespread swaths of trees, producing
structural damage and knocking out power for many communities.

Otherwise...confidence is high that rainfall intensity will pick
back up after 06Z as the bands set up perpendicular to the NC
mtns. This will likely add an additional 8- 10 inches of rain across
the Blue Ridge and should high-end flooding, which may eventually
require flood warnings with the EMERGENCY designation. All the
ingredients are in place for numerous landslides and several
damaging debris flows over the mountains as the final bursts of
very high rainfall rates move across the region. Reservoirs along
the Catawba River chain are also running high and there could be
breaching problems as enuf water may not be able to be released
beforehand. Please remain vigilant to any warnings issued this
evening and overnight as conditions can worsen rather quickly.

The system will continue to advance north of the forecast area late
Friday morning thru the afternoon. Improving weather conditions are
in store Friday afternoon as the column dries quickly from above,
however, the impacts and effects of this system will last thru
the weekend.
-- End Changed Discussion --

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
As of 200 PM Thu: By Friday evening, Helene`s center will be
northwest of the Appalachians and interacting with the lower OH
Valley upper low. The CWA should be pretty much entirely in the dry
slot of the transitioning system, but the low will be positioned to
allow vort lobes to swing across the mountains, and we accordingly
keep a small PoP generally west of I-26 and northwest of I-85. 850mb
flow will continue to be southerly/upslope, and although the dry
slot will bear PWATs well under an inch initially, those values
may rebound toward daybreak Saturday. That said, the moisture
Fri night will be beneath a deep subsidence inversion and rainfall
rates/amounts are expected to pale in comparison to those earlier in
the day, so it is unlikely there will be any new flood or landslide
impacts, except perhaps on slopes made unstable by the earlier
rainfall. Gusty winds will continue particularly in higher terrain
overnight, where frequent gusts of 25-30 mph will remain possible
into Saturday morning; shallow instability may still tap into 20+
kt winds in some lower elevations over the western half of the area.

By Saturday morning, the sfc/upper low will be stacked, and
Helene should be no longer distinguishable from a regular inland
closed low. Heights will continue to rise early Saturday but then
fall a bit from the west as another vort lobe rotates over the
CWA. Convective layer deepens and appears a little more viable
in our west, so PoPs expand across the mountains again. PWATs may
rebound over 1.5 inches but CAPE does look to remain no more than
a couple hundred joules. Effective shear looks unlikely to support
a severe threat; locally heavy rain could redevelop as well as a
localized flood risk. WPC maintains Marginal Risk excessive rain
outlook for the mountains on Saturday, which seems reasonable in
light of antecedent conditions. All of the above factors will be
in play again Sunday, just shifted slightly further north/east,
so PoPs end up similar but expanded slightly in those directions.

Temps will be mild, slightly above normal for the Piedmont each
afternoon, and slightly below over the cloudier mountains.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 230 PM Thu: Sunday night the upper low looks to begin to
rejoin the westerlies, at least partially, although the process is
slow and somewhat unsettled weather will continue through Monday
if not Tuesday. Some guidance depicts pattern almost getting to
a Rex block, with the remnant upper low beneath the eastern CONUS
ridge, but it looks somewhat transient. We retain small PoPs over
the northwest half of the CWA both days due to the upper low, but
with less potential enhancement from vort lobes. Rainfall rates are
likely to be light and currently appear unlikely to hinder recovery
efforts. Temperatures trend slightly warmer through Wednesday,
but remain within a couple degrees of normal in all locations.

&&

.AVIATION /02Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --
At KCLT and elsewhere: Widespread IFR/LIFR cig/vis will continue
across the western Carolinas this evening, with occasional MVFR
holes, under an expansive rain shield still well ahead of the
circulation around fast-moving and powerful Helene. Will continue
with the strategy of moving a four-hour TEMPO group ahead with
semi-regular amendments to handle the changes. A developing radar
trend looks like a bit of a lull moving up from the south over
the next 2-4 hours, but the strength of the upslope flow and very
high moisture content will probably keep the light precip and at
least MVFR/IFR restrictions in place. Finally, the rain shield with
Helene will arrive in the pre-dawn hours, with intense bursts of
rain that bring vis down to IFR/LIFR. As this happens, basically
after the end of whatever operations are conducted this evening,
the wind will increase dramatically, with the potential for frequent
gusts of 50kt or higher toward daybreak as the center of circulation
lifts quickly northward over GA. Winds aloft will be even stronger,
with intense LLWS possible. The center moves past and winds veer
around quickly between daybreak and mid-morning. Guidance suggests
a quick end to the precip restrictions after 14Z or so, with the
lowest ceiling restrictions scattering out around midday. Lots
more details were glossed over on the back end.

Outlook: The remnants of Helene will move north of the area Friday
evening. Isolated to sct mainly diurnal convection will be possible
thru the weekend.
-- End Changed Discussion --

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flood Watch through Friday afternoon for GAZ010-017-018-026-
     028-029.
     Tropical Storm Warning for GAZ010-017-018-026-028-029.
NC...Flood Watch through Friday afternoon for NCZ033-035>037-
     048>053-056>059-062>065-068>072-082-501>510.
     Tropical Storm Warning for NCZ033-048>053-056-058-059-062>065-
     068>071-082-501>510.
     Wind Advisory until 2 PM EDT Friday for NCZ035>037-057-072.
SC...Flood Watch through Friday afternoon for SCZ008>014-019-
     101>109.
     Tropical Storm Warning for SCZ008>014-019-101>109.
&&

$$

SYNOPSIS...DEO
NEAR TERM...PM/SBK
SHORT TERM...Wimberley
LONG TERM...Wimberley
AVIATION...PM