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464
FXUS62 KGSP 250806
AFDGSP

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

.SYNOPSIS...
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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.
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.NEAR TERM /THROUGH TONIGHT/...
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As of 3:25 AM Wednesday: Sct showers continue to make their way
NE over the NC mtns and foothills early this morning. Most of
this activity should dwindle over the next few hrs, but we`ll
likely have at least a few showers linger thru the morning hrs.
Temperatures will remain well above normal this morning with lows
in the mid to upper 60s across the lower terrain.

Otherwise, an upper trof will continue to advance toward our area
with increasing diffluent flow aloft. This will help produce decent
lift over our area and allow for more shower activity across the
mtns and adjacent foothills thru the period. Overall, the synoptic
pattern comes together today to produce a Predecessor Rain Event
(PRE). This pattern is looking more defined on the current model
runs and has shifted a bit more east overall. This will allow for
multiple rounds of upper-level energy to translate over a nearly
stationary front while moisture flux combines from both the Atlantic
and Gulf of Mexico. Not surprisingly, model QPF amounts continue to
trend higher over the southern and western Escarpment and our NE GA
and western Upstate zones. We`re still expecting 1 to 3 inches of rain,
with locally higher amounts, thru roughly 00z Thurs. This will likely
create localized flash flooding issues. As such, WPC has a Day 2 Moder-
ate 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 men-
tioned above. All this precip is expected to occur before the rain-
fall 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.
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&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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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.
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.LONG TERM /SATURDAY THROUGH TUESDAY/...
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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.
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.AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: Sct convection should eventually dissipate/
lift north of our area by early morning. Expect MVFR to IFR cigs to
spread back in from NE to SW overnight. There`s a decent chance of
mainly MVFR visby, but IFR will be possible as well. Light SLY wind
becomes light and variable overnight. Any visby restrictions should
dissipate by mid-morning with cigs eventually lifting to MVFR then
low VFR by the early afternoon. Another round of convection is expec-
ted again Wednesday aftn/evening which I account for with PROB30s at
all taf sites. Cigs are expected to lower to IFR/LIFR again later Wed
night as the period is ending. Winds will pick up from the S to SE
during the late morning and will eventually back to a more ELY direc-
tion by evening.

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...
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GA...Flood Watch from 11 AM EDT this morning through Friday
     afternoon for GAZ010-017-018-026-028-029.
NC...Flood Watch from 11 AM EDT this morning through Friday
     afternoon for NCZ033-048>053-058-059-062>065-501>510.
SC...Flood Watch from 11 AM EDT this morning through Friday
     afternoon for SCZ010-101>108.
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&&

$$

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