311
FXUS62 KGSP 250416
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1216 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 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 TODAY/...
As of 12:05 AM Wednesday: We`ve still got 2 fairly large clusters
of robust convection moving NE just north of the I-85 corridor.
A severe warning was issued for a few of these cells over the past
couple of hrs, but hopefully they will remain sub-severe going
forward into the overnight. Regardless, it`s looking like some
amount of convective activity will continue well into the over-
night. Otherwise, an upper trof will continue to advance toward
our area with increasing diffluent flow aloft. This will help
produce decent lift over our CWA and allow for more shower acti-
vity across the mtns and adjacent foothills. Not expecting too
much rainfall in any one given location as the cells will push
NE thru the morning hours, however if they train over the same
location(s), then flooding could be an issue. Temperatures will
remain well above normal overnight with lows in the mid to upper
60s across the lower terrain.

On Wednesday, the synoptic pattern comes together to produce a Pre-
decessor Rain Event (PRE). This pattern is looking more defined on
the current model runs and has shifted east from previous solutions
and into our fcst area. This is not a good signature as far as accu-
mulating rainfall goes as rounds of energy traverse a nearly statio-
nary front while moisture flux combines off both the ATL and gulf
thru the period. Accordingly, model QPF response has increased over
the wrn/srn escarpment and NE GA zones where large-scale forcing will
combine with enhanced upslope lift. Right now, it`s looking like 2
to 3 inches of rain, with locally higher amounts, will fall thru 00z
Thursday which would likely create flash flooding issues. WPC has a
well placed MDT risk which 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 Wednesday 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 will occur before
the tropical rainfall associated with TC Helene begins moving into
the area during the short-term period. Thus, more widespread flooding
is likely in store. Highs on Wed will be near normal, if not a few
degrees above.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...
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.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
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.

&&

.AVIATION /04Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: Scattered convection will slowly lift north
this evening with TSRA threat ending from S to N. Have TEMPOs in for
this timing. Expect VFR outside of convection this evening, then MVFR
to IFR cigs to spread back in from NE to SW overnight. There will be
some mainly MVFR fog, but IFR is possible. Light S wind becomes
light and variable overnight. Any vsby restrictions dissipate by mid
morning with cigs slowly lifting to MVFR then low VFR by early
afternoon. Convection expected again during the afternoon and have
PROB30s in place for that. S to SE wind develops during the morning
and continues into early afternoon.

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. Isolated to scattered mainly
diurnal convection will be possible through the weekend.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flood Watch from 11 AM EDT this morning through Friday
     afternoon for GAZ010-017-018-026.
NC...Flood Watch from 11 AM EDT this morning through Friday
     afternoon for NCZ033-048>053-058-059-062>065-501-503-505-
     507-509.
SC...Flood Watch from 11 AM EDT this morning through Friday
     afternoon for SCZ101>105.

&&

$$

SYNOPSIS...DEO
NEAR TERM...JPT/RWH
SHORT TERM...PM
LONG TERM...PM
AVIATION...JPT/RWH