729
FXUS62 KGSP 251407
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1007 AM EDT Wed Sep 25 2024

.SYNOPSIS...
A moist and cooler airmass will remain in place across the forecast
area through mid-week. Tropical Cyclone Helene is forecast to make
landfall over the Florida Panhandle late Thursday, with the remnant
circulation moving quickly north bringing gusty winds and heavy
rainfall to our area Thursday night into Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
1000 AM Update...An upper trof axis has setup west of the FA and is
instigating an active PRE along a sfc frontal zone. Showers and
thunderstorms have developed and will continue to train along the
front into the sw/rn NC mtns an nrn NE GA. So far, new rainfall amts
have approached an inch across wrn Graham Co with little impact as
far as flooding is concerned. Good confidence is had in this pattern
holding rather steady state into the afternoon and evening, which
will increase the threat of flash flooding over the aforementioned
areas.

The WPC has a Day 2 Moderate Risk area that includes our western
zones and we have a Flood Watch in effect for that same area. In
addition, our area will be under the gun again this aftn/evening for
strong to severe convection with some organization possible due to
the continued high shear environment. These storms will be capable
of producing high rainfall rates and pose a risk of localized
flooding outside/east of the PRE area mentioned above. All this
precip is expected to occur before the rainfall associated with TC
Helene begins moving into our area on Thurs. Thus, more widespread
flooding is likely with this tropical system. Please remain weather
aware over the next few days, the potential for major to
catastrophic flooding over parts of our fcst area is looking more
likely.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As of 245 am Wednesday: ***MAJOR RAINFALL EVENT FOR THE SOUTHERN
APPALACHIANS AND VICINITY WITH POTENTIAL FOR WIDESPREAD FLASH
FLOODING***

Predecessor rainfall event will be ongoing at the start of the short
term, and isolated flash flooding will likely already be developing
in areas along and near the Blue Ridge escarpment. With inverted
surface trough interacting with fire hose of moisture extending
north from T.C. Helene being the primary impetus for the PRE event,
rainfall rates could diminish somewhat during the afternoon as the
surface trough begins shifting west. However, any relief will be
brief as SE upslope flow into the escarpment will be relentless, and
the outer rain bands from Helene will will likely begin invading the
CWA by evening as the cyclone begins its fairly quick trek through
Georgia. The heaviest rainfall rates of the entire event will occur
within tropical bands Thu night into Fri morning on top of already
saturated soils across the southern Appalachians and adjacent
foothills. Flash flooding is expected to become widespread in these
areas during this time frame, with locally severe and life-
threatening flooding likely. By the time the rainfall finally tapers
off Friday morning, 48-hour storm totals are expected to range from
5-9 inches across much of the area west of ~U.S. Highway 321, with
as much as 15 inches expected on southeastern-facing slopes in the
vicinity of the Blue Ridge. At least moderate flooding is also
likely to develop on some main stem rivers. (Amounts of 3-5 inches
are expected for areas to the east). It`s also looking more likely
that we will see numerous landslides in the mountainous counties
encompassing the Blue Ridge escarpment...with a couple of large,
damaging debris flows/slope failures possible.

Helene will likely be of tropical storm strength as far north as
central Georgia, so wind gusts of 50 to 65 mph remain a distinct
possibility Thu night/early Fri across far western portions of the
CWA (i.e., near the GA/SC border & southwest NC)...as well as in the
high elevations. Most of the remainder of the area should see gusts
in the 35 to 45 mph range. No decisions will be made on this shift
regarding wind headlines. It should be stated that even if winds
don`t quite reach those magnitudes, numerous to widespread downed
trees will still be a strong possibility across the western half of
the area due to the saturated soil conditions. In terms of the
tornado threat...can`t rule out an isolated spin-up across southern
portions of the area on Thursday...especially with any initial
rainbands punching into the area late in the day. However, any
sbCAPE is expected to diminish quickly as the sun sets and E/NE flow
developing north of Helene helps to stabilize the atmosphere.
Quickly improving conditions are expected throughout Fri morning
into the afternoon, although ongoing flooding will likely continue
well into the afternoon, while main stem river flooding may continue
for several days.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 325 am EDT Wednesday: Upper low is forecast to be centered
somewhere over the Ohio Valley at the start of the extended, and
should begin wobbling east over the weekend, as a series of strong
short wave brush the northern part of the Conus. We should have an
opportunity to dry out over the weekend, as precip chances are
expected to be limited to spotty, mainly diurnal showers. Precip
chances ramp up a bit (although remaining of the token chance
variety) early in the work week as the upper low opens up and ejects
over the region. Any additional rainfall amounts should (hopefully)
be light during this time frame. Temperatures are forecast to be
close to normal through the period.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: Expect MVFR to IFR cigs to linger over the
area thru the morning with cigs improving to MVFR by noon or so,
and then low-end VFR by mid-afternoon. Another round of convection
is expected again this afternoon/evening which I account for with
PROB30s at all taf sites. Cigs are expected to lower to IFR/LIFR
again later tonight and stay that way through the end of the taf
period. Winds will pick up from the S to SE during the late mor-
ning and will eventually back to a more ELY direction by evening
and remain ELY thru early Thursday.

Outlook: Fog/low stratus will likely develop again the next few
mornings, especially in the mtn valleys. A tropical cyclone will
bring heavy rain and strong winds Thurs into Fri before moving out
of the area late Fri into Sat. Isolated to sct mainly diurnal con-
vection will be possible thru the weekend.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flood Watch through Friday afternoon for GAZ010-017-018-026-
     028-029.
NC...Flood Watch through Friday afternoon for NCZ033-048>053-058-
     059-062>065-501>510.
SC...Flood Watch through Friday afternoon for SCZ010-101>108.

&&

$$

SYNOPSIS...JPT
NEAR TERM...JPT/SBK
SHORT TERM...JDL
LONG TERM...JDL
AVIATION...JPT