821
FXUS62 KGSP 290731
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
331 AM EDT Sat Jun 29 2024

.SYNOPSIS...
Seasonable temperatures are expected today as shower and
thunderstorm chances increase ahead of a robust cold front arriving
Sunday.  Drier and cooler conditions will filter in on Monday and
Tuesday behind the front, before summer heat returns on Wednesday
and beyond.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 314 AM Saturday...The KGSP radar continues to show shower
activity developing and moving north in the weak moist southerly
flow across the Piedmont and foothills of the Carolinas. Suspect
this will continue for the remainder of the pre-dawn hours in some
form or fashion, mainly concentrating toward the foothills and Blue
Ridge Escarpment through daybreak. Still won`t rule out a brief
thunderstorm. Temps will remain warm with high RH through daybreak.

We remain under a subtle weakness in the broad upper ridge
stretched out across the srn Plains, across the Southeast, and
offshore. At low levels, a light southerly flow of moisture is
expected to continue through late tonight. In this environment,
a shower could pop up at any time at any location, though the
lack of mid/upper forcing means that coverage will be isolated
to scattered at best. Eventually, as we warm up and destabilize
in the late morning and into the afternoon, we should see an
increase in convective coverage concentrated over the mtns where
differential heating and weak mechanical lift will help develop
the precip. Guidance remains in line with this scenario, so precip
probs are allowed to climb into the likely range this afternoon
over the mtns, but kept at chance east of the mtns. The convection
should persist well into the evening as the overall flow changes
little. Eventually, the chances will move to the TN side of the
mtns as a cold front over the OH Valley region approaches. Within
this regime, the risk of severe thunderstorms looks low because
of poor lapse rates, a lack of mid-level drying, a moist boundary
layer, and no shear. High temps today should be similar to Friday,
and lows will be mild once again tonight.

&&

.SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/...
As of 212 AM EDT Saturday: Sunday`ll be a dynamically-active day
as a trough axis sharpens up over the Mid-Atlantic on the leading
periphery of a deeper upper wave dipping down out of the Great
Lakes.  Operational guidance still depicts the little "kink" in
low-level flow early Sunday in response to that initial shortwave,
which should lead to an initial suppressive effect on convective
initiation while helping temperatures get a jump start earlier in
the day.  A cold front will be making tracks across the central
Tennessee Valley at this point, but it`ll take until late morning
for the bulk of the forecast area to get into the prefrontal
trough...so initially the atmosphere won`t be super conducive
to convection.  Indeed, even at 18z many forecast profiles depict
shallow lapse rates or even modest low-level subsidence at sites
east of the NC mountains.

At best, though, this can be expected to merely delay CI.  By Sunday
afternoon, as the frontal circulation draws near and low-level
dynamics become enhanced, lapse rates will somewhat steepen and
any low-level inversion should mix out entirely...giving way to
impressive sbCAPE values on the order of 2000 J/kg across much of
the forecast area.  Especially toward late afternoon and evening,
the upper height gradient will tighten up a bit, enhancing deep
layer shear across these areas.  And with some lingering dry air
aloft, the most recent HREF cycle is hinting at DCAPE values of
800-1000 J/kg...enough to support an outside risk of severe weather
(the primary threat being damaging wind, and large hail a close
secondary).  So, the SPC`s recent inclusion of most of the area in a
Marginal Risk of Severe Weather for Sunday looks reasonable enough.

Separately, Sunday may present a heat risk...with dewpoints
forecast to climb into the mid 70s alongside temps in the mid 90s.
Heat index values could exceed 100 degrees across much of the NC
Piedmont and SC Upstate, and may well climb to values near 105.
There`s high potential for the forecast to fail, though...it`s
going to be a delicate balance between increasing heat / moisture
versus convection.  Cloud cover due to developing convection,
as well as the rainfall itself, could easily skewer a higher-end
temperature forecast.  And indeed, the latest HREF cycle is hinting
at lower probabilities of hitting advisory-criteria heat index
values than before.  This`ll need to be monitored closely over
the next 36 hours.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
As of 241 AM EDT Saturday: High pressure will build into the area
Monday and beyond as an active frontal boundary pushes east of the
forecast area.  Upper heights will rise sharply through mid-week
as an upper anticyclone deepens over the Southeast...while at
the surface, high pressure will migrate across the Ohio Valley
and eventually, off the Mid-Atlantic coast.  This`ll actually set
up a brief period of cold-air damming on Tuesday, such that high
temperatures, which will already have fallen significantly on Monday
following the FROPA, will take another hit.  Expect highs to stay in
the low- to mid-80s, with relatively dry dewpoints, both afternoons.

Thereafter, temperatures will rise steadily as the heat dome
intensifies and increasingly S flow develops over the forecast area.
Forecast profiles each afternoon look increasingly conducive to
diurnal convection, though sans any discernible synoptic forcing,
we`ll be relying on parcels to auto-convection, which could put
coverage into question.  More than likely, any meaningful shower
activity will be focused across the mountains, where the terrain
will serve as a trigger.

By Friday, northern stream troughing looks like it may put a
dent in the upper pattern and carry a cold front into the Ohio
Valley...although this forecaster finds such a scenario a little
dubious.  Whatever the case may be...temperatures by this point
in time will be back in the 90s, and we may well be once again
looking at heat index problems on and after Independence Day.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: A light moist southerly flow continues
across the region early this morning, which should continue to push
a broad and loosely organized band of shower activity northward
across the NC foothills and western Piedmont. This stuff should
either drift northward out of the forecast area or weaken before
daybreak. Other showers could pop up at any time, but the chances
are too low to include. Otherwise, the guidance still suggests
that low clouds will develop steadily across the region through
the pre-dawn hours, with an MVFR-level cloud ceiling developing
shortly and then dropping down possibly into the IFR category,
especially near the Blue Ridge Escarpment. The TAFs will continue
to feature this restriction through mid/late morning. Wind will
stay SSE to SSW and light. On Saturday, the ceiling will eventually
lift to VFR and scatter out by afternoon, thereafter we will look
for better coverage of showers and thunderstorms in the mid/late
afternoon. This will be handled with a PROB30 until confidence in
timing improves. Expect a similar evolution late Saturday night.

Outlook: A cold front approaches the area on Sunday, resulting in
higher aftn/eve convective coverage. The front pushes trough and
stalls out just south of the area, resulting in quieter weather
for Monday and Tuesday.

&&

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

&&

$$

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