568
FXUS62 KGSP 262148
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
548 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/...
***A MAJOR RAINFALL EVENT FOR THE SOUTHERN APPALACHIANS AND VICINITY
IS OCCURRING WITH THE POTENTIAL FOR WIDESPREAD TO CATASTROPHIC
FLASH FLOODING***

As of 531 pm...No changes were made to the Tropical Storm
Warning or Flood Watch. All systems appear to be `go` as the rain
continues to fall across the entire forecast area, filling up the
creeks and streams and saturating the soil. From here on out, the
flood situation will only get worse. Confidence remains high that
considerable and significant flooding will occur. The trend in the
short term model guidance continues to support a high likelihood
of frequent gusts into the upper part of tropical storm force,
and hurricane force across the peaks and ridges.

Otherwise...confidence is high with persistent rain bands associated
with TC Helene crossing into the area as the entire system
transitions to extra-tropical and is enhanced while interacting with
a sfc front. Models show rainfall intensity picking up later this
evening and moreso aft 06z as the bands set up perpendicular to the
NC mtns. This will likely add an additional 8- 10 inches of rain
across many mtn locales and produce high-end flooding, which may
require flood warnings with the EMERGENCY designation. Landslides
will become more of realistic threat overnight as slopes will become
more prone to give way due to the constant saturation. 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. We are in coordination with dam operators and
county emergency managers for quick issuance of flood warnings in
case of emergency conditions evolving. Please remain vigilant to
any warnings issued this evening and overnight as conditions can
worsen rather quickly.

Winds will also be picking up very strong after midnight as TC
Helene does not weaken much and is quick to near our area. Right
now, expect wind gusts arnd 50 mph east of the mtns, while gusts
across the mtns increase with elevations sigfnt/ly. Cross sections
show 70+ mph winds at elevations abv 3.5 Kt and even higher values
across the mtn peaks. These winds will combine with saturated
soils and down widespread swaths of trees, producing structural
damage and knocking out power for many communities.

The system will continue to advance north of the FA late Fri morning
thru the afternoon. Lingering rainfall across the NC mtns before
noon Fri has prompted the WPC to issue a HIGH Excessive Rainfall
category for this area, which makes sense as even 2-3 extra inches
of rain will be considered excessive by that time. Improving weather
conditions are in store Fri afternoon as the column dries quickly
from above, however, the impacts and effects of this system will
last thru the weekend.

&&

.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 /22Z THURSDAY THROUGH TUESDAY/...
At KCLT and elsewhere: Low CIGS and VSBY, into IFR or lower, will
persist this evening through the overnight across all TAF sites. A
sfc trof has become a focusing zone for continued widespread precip
over the area, while thunder chances remain low due to limited
instability. Expect heavy rain overnight as bands associated with
Hurricane Helene traverse the area. Winds will also become more
gusty into the high range while very strong winds off the surface
create sigfnt LLWS aft 09z or thru mid/late morning. Wind directions
will generally remain e/ly to se/ly with KCLT veering sw/ly by the
late period.

Outlook: The remnants of TC Helene and lingering low flight cats
will move north of the area Friday afternoon into Saturday. Isolated
to sct mainly diurnal convection will be possible thru the weekend.

&&

.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...SBK