308
FXUS62 KGSP 171044
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
644 AM EDT Tue Jun 17 2025

.SYNOPSIS...
Hot and humid with daily chances for showers and thunderstorms
through Thursday. A cold front pushes across the area early
Friday, bringing a reduction in rain chances lasting through the
weekend. Although humidity looks lower this weekend, temperatures
trend hotter through the beginning of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 645 AM EDT Tuesday...a smattering of patchy dense fog and
low stratus at daybreak...more or less as expected...but that
shouldn`t last too long into the morning hours as sky is
essentially clear above it. Temps are going to get off to
another warm start with a very humid air mass.

A weak mid/upper trof over the mid-MS Valley region this morning
will fill and lift over the OH Valley region today and this
evening, keeping a warm and humid air mass across our region,
supporting the development of mainly diurnal thunderstorms
again. Probs look better than climo, meaning likely over the
mtns/foothills and chance east of there. There are some subtle
changes to the environment, though. The Precipitable Water should
drop just a bit this afternoon as some slightly drier air works
in at mid-levels. Shear is just a bit stronger and the flow in
the cloud layer will be southwesterly at 25-35 kt, so storms will
be moving along at a steady pace. While we can`t ever completely
rule out enough rain to cause isolated flash flooding in a warm
and moist summertime air mass, the risk appears to be lower than
yesterday, and the Excessive Rainfall Outlook has the fcst area
removed completely. Meanwhile, the environment is slightly more
favorable for an isolated severe storm, but far from ideal. Wind
damage would be the main concern there. The CAMs suggest that
storms will weaken and move out during the late evening. Thereafter,
another quiet but muggy night is expected. Temps will continue to
run around normal, maybe a degree or two above.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As of 147 AM EDT Tuesday...

Key Messages

1) Limited convection on Wednesday, with very limited risk of
severe weather and heavy rainfall.

2) A cold front crosses the area Thursday, with an associated risk
of severe thunderstorms.

3) Hot and humid conditions continue through the period, with only
brief and limited relief behind the cold front.

By Wednesday morning, shortwave energy aloft will have more or less
merged into the southwesterly mean flow, and a deep slot of modestly
drier air will be digging into the region from the south...with
an overall negative effect on Wednesday`s convective prospects.
Behind the wave, mid-level lapse rates appear unimpressive (with
some guidance, most notably the 00z GFS, even depicting a weak
inversion just above the LFC, which would act to inhibit parcels
already struggling to overcome weak instability.  Across the NC
mountains, operational guidance depicts weak low-level convergence
evidently left behind from the shortwave...which ought to help
overcome the lack of better thermodynamics.  What storms develop
will do so in an environment of ~20kts deep shear, permitting loose
organization...and dry air aloft should result in marginal dCAPE
values.  All that to say, while the severe risk isn`t especially
impressive...a strong to severe storm or two can`t be ruled out.
Hydro issues, meanwhile, will be limited as well...with PWs trending
back toward climatological values and storms expected to be less
stationary owing to aforementioned marginal shear values.

Meanwhile, ensembles are quite consistent on the development of
a surface low over the central Great Plains as early as Tuesday
night beneath a broad, low-amplitude z500 trough...and this feature
will track across the Ozarks into the Midwest through Wednesday,
while its attendant cold front only slowly translates into the
Tennessee Valley.  Though prior model cycles struggled to come to
a consensus on the timing of this front`s arrival in the western
Carolinas, the latest 00z LREF cycle seems fairly confident on a
mid-afternoon arrival time of the prefrontal trough...with over 75%
of members developing enough surface convergence to take advantage
of some 1400-1800 J/kg surface-based instability during and after
peak heating.  With the latest round of operational guidance
depicting a z500 speed max located significantly farther south
than prior runs...forecast profiles also boast some 25-30kts deep
shear...enough to support organized multicellular convection and an
associated severe wind risk, at least over the northern NC Piedmont,
where bulk shear will be greatest.

While all this is happening, temperatures are also expected to
pose a separate issue through the short term...with the influx of
moisture in SW flow between the advancing low and the Bermuda high
still churning over the western Atlantic pushing dewpoints into the
mid-70s.  The combo of this humidity with high temps in the 90s will
elevate heat risk...as heat index values surge into the upper 90s
and even some lower 100s south of I-85 each afternoon.  The passage
of the front will serve to modestly lower dewpoints...but will do
little to dislodge the Bermuda high and the onslaught of moisture.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 303 AM EDT Tuesday...

Key Messages:

1) Some relief from hot and humid conditions is expected on Friday
and Saturday.

2) By Sunday, a growing heat dome will drive heat index values
back into the upper 90s and lower 100s.

3) Showers and storms will be limited from Friday onward by
various mechanisms.

Robust subtropical ridging will remain in place going into
the weekend.  For Friday, however, dry air behind a departing
cold front to our southeast will provide at least some reprieve
from hot and muggy conditions experienced earlier in the week.
Marginally-lower dewpoints on the backside of the front paired with
a decent dry slot atop the boundary layer should allow for some
daytime drops in dewpoint that probably also aren`t being captured
well in the NBM or the other ensemble blends...but it`s reflected
well in some of the MOS products, which are forecasting afternoon
dewpoints to the tune of 4-6 degrees lower than the NBM on Friday,
and to a limited extent on Saturday as well.  Based on this...still
think that we`ll get a reprieve from dangerous heat index values the
first half of the weekend.  Unfortunately, by Sunday, continued Gulf
moisture flux driven by the ever-present Bermuda high will overcome
any lingering dry air, and the subtropical ridge will gradually
evolve into a ~600dm upper high centered over the Mid-Atlantic.
This will drive temperatures into the mid- to upper-90s by the
end of the weekend, so by Sunday and beyond, we can expect to be
right back into dangerous heat index territory.

As an aside, some diurnal convection certainly can`t be ruled out
any afternoon through this period.  Coverage looks...not great...on
Friday and Saturday, because there`s just no good forcing mechanism
to initiate updrafts, except maybe some ridgetop convection over
the mountains.  By Sunday and Monday, boundary layer moisture will
be better...but the growing upper anticyclone still looks like
it`ll create an increasingly capped environment that will inhibit
convective growth.  So...undercut NBM PoPs through basically the
entire extended...and may still not have undercut them enough.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: As expected, we are starting out
with patchy fog and stratus around mostly the NC part of the
fcst area, affecting KAVL and KHKY with variable ceiling and
vis. The fog and low stratus should break up an hour or two after
sunrise. Thereafter, looks like a typical hot and humid summer day,
with a brief MVFR ceiling possible in mid/late morning, then a
chance of thunderstorms in the afternoon and early evening. Will
opt for PROB30s as coverage looks scattered in the model guidance
and confidence is too low for TEMPOs. Once the convection ends
in the mid/late evening, more convective debris clouds can be
expected. We can expect more low stratus and fog restrictions around
daybreak Wednesday. Wind will generally remain SW thru the period,
but variable and gusty near storms.

Outlook: Isolated to scattered showers and thunderstorms are
expected each afternoon/evening through Thursday. Drier conditions
return behind a cold front Friday into Saturday. Fog and/or low
stratus will be possible each morning.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$

SYNOPSIS...MPR
NEAR TERM...PM
SHORT TERM...MPR
LONG TERM...MPR
AVIATION...PM