-- Discussion containing changed information from previous version are highlighted. --
657
FXUS62 KGSP 241933
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
333 PM EDT Tue Sep 24 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 in the Florida panhandle late Thursday, with the remnant
circulation moving quickly north and bringing wind and heavy
rainfall impacts to our area Thursday night into Friday. Calmer
weather over the weekend with high temperatures a little cooler than
normal.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
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Convection is becoming more widespread across the NC mtns this
afternoon with a few storms reaching stg/svr levels. Overall,
instability is increasing and east of the mtns the overcast conds
are giving way to more sct cloudiness, thus increasing the threat of
storm development in this area as well in a few hours. Deep layered
shear is running arnd 30-40 kts and this could help storms maintain
themselves longer and possibly train over the same areas leading to
minor nuisance flooding.

The overnight period will see an upper trof continue to advance
toward the FA and with increasing difl flow aloft decent lift will
be had for increasing showery activity across the mtns and adjacent
fthills. Not expecting too much rainfall in any one given locale as
the cells will continue to push northeast thru the morning hrs.

On Wed, the synoptic pattern comes together to produce a Predecessor
Rain Event (PRE). This pattern is looking more defined on the
current model runs and has shifted east from previous solns into our
wrn FA areas. This is not a good signature as far as accumulating
rainfall goes as rounds of energy traverse a nearly stationary front
while moisture flux combines off both the ATL and GOM through the
period. Accordingly, model QPF response has become more maximized
over of wrn srn escarpment and NE GA zones where large-scale forcing
will combine with enhanced upslope lift. Right now, it`s looking
like 2-3 inches of rain, locally higher, will fall thru 00z Thu
which would likely create flash flooding issues. The WPC has a well
placed MDT risk including our area and a Flood Watch will be issued
this afternoon.

The area will once again be under the gun Wed afternoon for stg/svr
convection with some organization due to continued high shear
levels. These storms will be capable of producing high rainfall
rates and pose a risk of isol flooding outside/east of the PRE area.
All this precip will occur before high amts of tropical rainfall
associated with TC Helene begin moving in during short term period,
thus a sigfnt and more widespread flooding issue is in store after
this near term period. Temps will remain well abv normal tonight
with lows in the m-u60s, while highs Wed reach near normal.
-- End Changed Discussion --

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/...
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As of 245 pm Tuesday: Still a lot of moving parts, but confidence
continues to creep upward with regard to the potential impacts
of Helene. Before we get there, however, we will continue to deal
with an increasing threat of significant rainfall to the east of
a stalled frontal boundary draped across the Cumberland Plateau/TN
Valley region Wednesday night into Thursday. Signals point toward
a Predecessor Rainfall Event (PRE) that may already be underway at
00Z Thursday across parts of northeast GA, southwest NC, and the
western Upstate. Although Helene will still be well to the south
at that time, a plume of deep tropical moisture lifting northward
to the east of a large upper low over the mid-MS Valley region,
combined with deep layer forcing, is expected to result in a high
probability of showers. Rainfall efficiency should be high. Although
parts of southwest NC and northeast GA have been dry recently, the
antecedent conditions may not matter much. The ingredients will
be in place for a good chance of significant rainfall more than
24 hours before Helene arrives. Because of the high likelihood
of the PRE, a Flood Watch will continue through Wednesday night
and Thursday. The new convection-allowing model guidance looks
compelling in the Watch area.  Any amount of significant rain in
that location will only increase the flood threat down the road.
Which brings us to Helene. The fast-moving storm will approach
Thursday evening with precip increasing, followed by the wind
ramping up after midnight into early Friday morning. The most recent
track guidance has raised the threat of seeing tropical storm force
winds in a swath across northeast Georgia and perhaps southwest
NC on Friday morning. Wind gusts of 40 mph are likely across most
of the forecast area, especially Friday morning. The combination
of frequent strong gusts and saturated ground would result in
numerous power outages. The storm might move fast enough that it
would be departing Friday afternoon. However, there remains enough
uncertainty with the timing and speed of the system that effects
could linger well into the day. The wind impacts may diminish by
evening. The flooding rain threat may peak Friday morning. After
all the rain with the PRE, the QPF is high enough with the direct
impacts of Helene to raise the threat for landslides across the
srn Appalachians. Rainfall amounts of greater than 10 inches are
possible close to the Blue Ridge Escarpment, which in the past has
resulted in numerous landslides and some debris flows. We should
be on the back end of it by sunset Friday. Again, the details are
a moving target, but are coming into more focus.
-- End Changed Discussion --

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
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As of 155 pm EDT Tuesday: Still quite a bit of uncertainty regarding
the post-Helene part of the forecast. It seems likely that we will
enjoy at least a brief period of quietness beginning Friday night,
with some dry air wrapping around the large upper low centered
over the mid-MS Valley region and the remnant of Helene moving up
into the OH Valley. The lull may continue through much of Saturday
and Saturday night as the precip stays wrapped closer to the upper
low, which wobbles to the lower OH Valley region. A small chance
of precip will do, mainly for the area north of I-85. From that
point onward, uncertainty only goes up, mostly because of the fate
of the upper low. Most of the guidance keeps at least a remnant
mid/upper trof to our W/NW through the rest of the period, which
on a grand scale, should keep us in a favorable spot for a chance
of rain each period through Tuesday. Details are the sketchy part,
because if any short waves rotating around the weakening upper
low move overhead with the remnant deep moisture, our precip probs
and amounts would go up. The GFS suggests that to be the case for
Monday, and there is some agreement with the old ECMWF. For now,
we will keep precip in the chance range. Meanwhile, the same models
also suggest a hybrid cold air damming wedge, which would keep
temps on the order of five degrees below normal for Sunday into
Monday, perhaps even lingering into Tuesday. Suffice to say, the
pattern looks somewhat damp and cool, but until we have a better
handle on the fate of Helene and the upper low, all bets are off.
-- End Changed Discussion --

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: Low CIGs will persist this afternoon and
begin to lift to MVFR/VFR as a line of -shra/tstms move thru the
area. Have TEMPO thunder all sites this afternoon and a few storms
may be capable of reaching severe levels with high deep shear levels
and and increasing instability. The overnight remains active with
precip continuing mainly across the mtns and fhills while flight
conds reduce to IFR or lower due mostly to CIGs thru the morning hrs
with a better chance of LIFR VSBY developing at KAVL. The atmos
remains unsettled Wed with precip chances returning more widespread
across the mtns as CIGs take a while to improve out of IFR conds.
Winds will remain weak and align generally se/ly to sw/ly.

Outlook: Fog/low stratus will likely develop over the next few
mornings, especially in the mtn valleys. A tropical cyclone will
bring heavy rain and strong winds Thursday into Friday before moving
out of the area Friday night into Sat.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
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GA...Flood Watch from Wednesday morning through Friday afternoon
     for GAZ010-017-018-026.
NC...Flood Watch from Wednesday morning through Friday afternoon
     for NCZ033-048>053-058-059-062>065-501-503-505-507-509.
SC...Flood Watch from Wednesday morning through Friday afternoon
     for SCZ101>105.
-- End Changed Discussion --

&&

$$

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