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359
FXUS62 KGSP 241427
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1027 AM EDT Tue Sep 24 2024

.SYNOPSIS...
A moist and cooler airmass will remain in place across the forecast
area through mid-week. What is expected to become Hurricane 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 and seasonably warm conditions are expected over the
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
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1000 AM Update...The forecast is running rather steady state this
morning thanks to a shallow layer of StCu engulfing much of the
non/mtns. These clouds will be slow to dissipate as warming surface
levels and thermal lift take some to get going. Any precip over the
next few hrs will be brief and light, with maybe an isol strike of
lightning across NE GA. Temps and td/s are running well within the
fcst curve and only made locally minor tweaks.

Otherwise, today will be similar to yesterday (Monday) with morning
low clouds slowly eroding and lifting from SW to NE thru the morning
and into the afternoon. Low pressure will develop to our NW over the
Ohio River Valley and help keep a broad area of moist air in place
over the region. Moderate instability along with 30 to 45 kts of deep-
layer shear can be expected across our area during the afternoon and
evening. Expect sct convective coverage during the afternoon with the
best coverage more likely during the evening hrs. The severe potential
looks better for our area today/tonight with SPC`s current day 1 Con-
vective Outlook having most of our CWA under a Marginal Risk for svr
thunderstorms. The main threats will be damaging wind gusts and hail.
Otherwise, high temps will top-out roughly 3 to 6 degrees above normal
with our SW zones approaching 90 degrees again.
-- End Changed Discussion --

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY/...
As of 245 am Tuesday: A strong upper low is forecast to be centered
near the confluence of the Ohio and Mississippi Rivers at the start
of the period, slowly wobbling south through Thursday, as what is
expected to become Hurricane Helene makes landfall in the Florida
panhandle Thu evening. Throughout Wednesday, an inverted surface
trough is forecast to organize across the region...most likely
centered across the TN Valley, while increasing southerly mid/upper
level flow on the east side of the upper low begins to draw a plume
of tropical moisture from Helene into the region. The interaction of
this plume with increasingly deep low level convergence in the
vicinity of the surface trough. This could result in evolution of a
heavy predecessor rainfall event (PRE) from Wed into Wed night,
which may incorporate much of southwest NC, eventually including an
upslope component as southeast flow accelerates above the surface
layer late Wed through early Thu. Whether the axis of heavy rain
sets up within, or just west of our forecast area is uncertain at
this juncture...it just needs to be noted that widespread amounts of
at least a couple of inches could accumulate across a portion of the
area before direct impacts from Helene arrive. Regardless, PoPs will
increase throughout Wed as spokes of vorticity rotating around the
upper low approach the forecast area and interact with a moderately
unstable and increasingly moist environment.

Other than the potential for lingering upslope showers on Thursday,
there may be a bit of a lull in precipitation through much of
Thursday, as inverted surface trough shifts well west of the
Appalachians. Meanwhile, Helene is expected to lift quickly N/NW
through Georgia Thursday night, with the center likely passing near
the intersection of  GA/TN/NC Friday morning. This will place at
least the western part of the forecast area on the favorable right
side of the cyclone track for heavy tropical rain bands. Heavy
rainfall may pile up quickly Thu night, with excessive rainfall and
at least isolated flash flooding becoming likely, especially from
northeast GA and the Upstate into southwest NC. By noon Friday,
total rainfall amounts (including anything falling prior to Helene`s
arrival) are expected to range from 4 to 7 inches across much of the
area west of ~I-77. Locally higher amounts as high as a foot are
expected across southwest NC...especially along the Blue Ridge
escarpment.

Confidence in winds and wind gusts remains modest at best. However,
a consensus of the latest short term guidance suggests Helene will
still be of solid tropical storm intensity as it passes near the CWA
Thu night. Our current best guess is that much of the western half
of the area will see plenty of 35-50 mph gusts, with damaging wind
gusts very much a possibility just to the east of the cyclone`s
center. Being on the right side of the cyclone also poses some
concern for isolated tornadoes, as shear parameters will be more
than adequate for rotating cells within rainbands. However,
tornadoes tend to have a strong diurnal correlation in association
with inland tropical cyclones, and guidance suggests surface-based
instability will be largely absent by the time rainbands arrive Thu
night, so the tornado threat appears to be small (but non-zero) at
this juncture. Otherwise, wind and rain are expected to steadily
diminish throughout Fri morning, with quiet weather returning by
evening.

&&

.LONG TERM /FRIDAY NIGHT THROUGH MONDAY/...
As of 335 am EDT Tuesday: An upper low is still expected to be
wobbling near Memphis at the start of the extended, but should
finally weaken and be nudged into the eastern Conus early next week
as a strong short wave trough grazes the north central part of the
country. After a brief lull in significant precip chances over the
weekend, PoPs begin ramping up again early next week, albeit
remaining in the chance range. Temperatures begin the period close
to normal, but fall below normal early next week, when global model
guidance suggests development of cooling E/NE flow in association
with an inverted surface ridge/potential cold air damming.

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: Patches of sct showers continue to linger
over the NC Piedmont this morning. This was handled with VCSH at
KHKY for the next few hrs. Another round of IFR to LIFR cigs has
settled over the area and will likely linger thru the morning, with
the lower cigs likely lingering into the early afternoon for KCLT
and KHKY, but probably not quite as long for the Upstate terminals
or KAVL. Any lingering fog should dissipate out by 15z or so, with
the lower cigs likely to linger a few hrs longer. Another round of
sct showers and thunderstorms is expected this afternoon/evening,
so I have PROB30s for TSRA at all taf sites from roughly 18 to 24z
with VCSH and/or TEMPOs for lingering showers after 00z. Winds
should remain light to calm across the area thru the morning and
pick up modestly this afternoon from the S to SW. They will become
light again later this evening.

Outlook: A slowly approaching weak cold front will stall west of
the fcst area on Wednesday, keeping the chance for diurnal showers
and thunderstorms around. Fog/low stratus may develop each morning,
especially in the mtn valleys. Confidence is increasing that 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...
GA...None.
NC...None.
SC...None.

&&

$$

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