FXUS62 KGSP 030752
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
352 AM EDT Sat May 3 2025

.SYNOPSIS...
Showers and thunderstorms return today and tonight ahead of a cold
front. Above-normal temperatures today fall back to near normal
Sunday and Monday under partly cloudy skies. Another unsettled
pattern likely to develop by midweek.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 302 AM EDT Saturday: A pocket of convection continues to
produce lightning along and east of I-77 after apparently spinning
up along a remnant outflow that arrived in the eastern Upstate
earlier tonight.  Some elevated instability there allowed a neat
line of showers to gain strength, and though no severe weather
is expected out of it, some gusty winds might fall out over at
most, the next hour, for those east of I-77 in the NC Piedmont.
Otherwise, it should remain quiet through the remainder of the
overnight, as a highly amplified trough and associated frontal
boundary inch eastward out of the Midwest and Ozarks, but remain
much too far west of the forecast area to provide any impetus for
active weather in the immediate near term.  Patchy fog is expected
through daybreak, particularly for spots that received a good
shot of rain yesterday afternoon/evening, but wider-scale issues
requiring any dense fog-related products aren't expected.  Low temps
remain on track to land 4-6 degrees above normal as widespread
cloud cover and very weak WAA hinder better nighttime cooling.

Active weather will pick up by day, however.  As the z500 trough
now centered over western Illinois cuts off into a closed low,
it'll gain momentum through the day and begin tracking eastward,
driving its surface reflection into the Tennessee Valley over the
next 12 hours, and resulting in an uptick in deep synoptic forcing
across the western Carolinas.  As a result, the hi-res guidance
depicts widespread convection breaking out as early as noon,
and multiple waves of showers and thunderstorms possible through
the period.  The parameter space will not be super impressive, but
should also present enough to expect a few strong to severe storms:
CAMs are in general agreement on 1200-1800 J/kg sbCAPE during
peak heating; meanwhile, as the upper flow becomes constrained by
the advancing upper low, and a weak LLJ develops in the southerly
low-level flow over the area, 15-25kts of 0-3km shear and possibly
in excess of 40kts of 0-6km shear could develop (though, notably,
soundings suggest not *quite* all of this 0-6km shear will actually
be realized by updrafts).  The real limiting factor for the first
wave of convection, though, will be a lack of synoptic forcing over
the most unstable and most sheared portions of the CWA farther east.

Instability of the surface-based variety will be maintained into
the evening and first part of the overnight as lobes of DPVA slide
east of I-26 and subtly improve lapse rates there.  Although sbCAPE
itself declines sharply after sunset, the thinking at this time
is that strong enough low-level flow should be present over the
eastern zones to make MLCAPE a better metric for instability...and
as the low- and mid-level kinematics continue to improve ahead of
the front, shear over the eastern third of the forecast area may
even increase, if not remain at their previous levels, during the
late evening and early overnight period.  Thus, severe risk will
continue for the eastern zones into the first part of Saturday
night, and a second wave of convection is expected during this
timeframe with a semi-organized QLCS makes tracks across the area
leading up to midnight.  A separate issue during this timeframe
will be the increasing risk of localized flash flooding along
the I-77 corridor...where soundings depict almost (but not quite)
boundary-parallel shear vectors.  Moisture will have had all day
to pile up here, with PWs as high as 1.4-1.5"...or somewhere in
the 90th to 95th percentile of NAEFS climatology.  Despite this,
the bulk of hi-res guidance doesn't depict especially impressive
QPF response (perhaps because the system's overall motion is
still fast enough to prevent it), with the 00z HREF keeping 6-hour
totals just a hair under 2" across the I-77 corridor...and so no
flood watch appears necessary, as these totals would keep flooding
issues isolated and localized enough not to justify it.  All in all,
the frontal boundary should finally push things east of the CWA by
the wee hours of Sunday morning, resulting in markedly decreasing
PoP and an end to both the severe and heavy rain threat.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 200 AM Sat, key messages:

1. Severe weather threat from prefrontal convection likely will have
ended by 12z Sunday, although isolated to scattered showers or storms
producing gusty winds and small hail could develop during the day.

2. Isolated/scattered showers will be possible Monday near the TN
border.

3. Aside from the above, dry and fairly quiet Sunday thru Tuesday
morning, with temps falling back to around or a little below normal.

Upper low will remain centered over KY/TN Sunday. Cold front will be
near our eastern border at daybreak and will continue to progress
east, with dry slot wrapping into the low and over the area during
the morning hours. Perhaps dependent on how well the atmosphere is
worked over Saturday night, a small amount of SBCAPE may develop in
the vicinity of I-77 during the morning, with moist enough profiles
above the PBL that a shower or even t-storm could develop. The dry
slot will erode that moisture by midday and likely will make profiles
too dry for a viable updraft in that area, so PoPs diminish below
slight-chance with the arrival of the drier air. However, closer to
the center of the low, with cooler temps and more moisture aloft,
unstable profiles persist through the day over much of the mountains,
and accordingly PoPs. The best chances will be in the far north and
along the TN border. Although SBCAPE probably will be no more than a
few hundred joules, effective shear could be 30-40 kt part of the
afternoon, and freezing/WBZ levels below 10000 ft, so small hail or
graupel along with brisk outflow winds may result from any of the
mountain cells. Cloud cover generally will decrease east of the
mountains, aside from that resulting from diurnal instability. Max
temps will fall back to about climo, with mins Sunday night a few
degrees below climo.

The low will wobble north into Indiana/Ohio over the course of
Monday. Lapse rates won't be quite as good, but somewhat higher RH
will be present aloft and isolated to scattered mountain convection
again will be possible, although with less effective shear. Showers
probably will be unremarkable.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 300 AM Sat, key messages:

1. Dry, mild, mostly sunny weather Tuesday, which looks to be the
only day meeting that description until next weekend.

2. A warm front will lift into the area Wednesday and effectively
stall in our vicinity, bringing back abundant cloud cover and chances
for precipitation from then through Friday.

3. Model consistency appears to have improved in the Wednesday-Friday
period, although with the slowly evolving blocking pattern resulting
in nebulous forcing, confidence is lower than usual at that forecast
range.

The aforementioned upper low is part of an omega block which will
begin to break down Tuesday-Wednesday as the low is absorbed into a
trough over the eastern Great Lakes region. Heights will rise over
the CWA Tuesday in response. We will be downstream of the ridge
component of the block and mostly clear skies, allowing temps to rise
slightly above climo that day. Under the ridge, the western closed
low (then in the Plains) and associated sfc low will bring a warm
front north into the lower Mississippi Valley, and possibly farther
east into the Carolinas, by Wednesday. Chance to likely PoPs are
forecast along with abundant cloud cover. The GFS, for one, suggests
this will ride over the dry sfc airmass to produce in-situ CAD or a
lookalike event.  NBM max temps are the coolest of the period that
day; there may be bust potential in either direction depending on
whether diabatic cooling does come into play.

The pattern by Wednesday night starts to look more like a Rex block,
as the Plains low remains nearly stationary while ridging develops
north of it near the Canadian border. One pretty certain forecast
element for the second half of the week is that blocking will
continue across the CONUS, and our weather will change only slowly.
The initially warm front thus looks to remain quasi-stationary over
our area through around Friday. A protracted period of mid-range PoPs
(mostly 40-60%) remains in the forecast throughout Wed-Fri, with
thunder coming and going diurnally. After the cool Wednesday, temps
will return just on the warm side of climo Thu-Fri. Model consistency
was worse on the 02/12 and 02/18z cycles than on the 03/00z cycle,
but the general consensus is now for the trough to cut off again over
the Northeast Coast and push the front south again.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: What a forecast.  Over the next 24 hours,
multiple rounds of potential convection and flight restrictions
are expected, and TAFs are messy to say the least.  Most sites
are currently VFR (circa 140 AM LT) and should stay that way
for a few hours...while a lingering cluster of cells west of
KCLT is still producing occasional lightning even at this hour.
Don't expect too much out of it, but have included a TEMPO for
SHRA at KCLT, which may need to be amended to TSRA if the thunder
holds together as it drifts east.  Otherwise, the other TAF sites
should mainly just expect some MVFR or even brief IFR restrictions
develop through daybreak...with most guidance depicting some
low ceilings developing across the Piedmont and western Upstate.
These'll scatter out once the sun comes up.

Then, it should be an active day convection-wise, as widespread
SHRA and embedded TSRA develop across the terminal forecast area.
Multiple waves can be expected...but timing remains in question.
It looks like sometime in the 18z-to-00z timeframe is the best
guess for the first round.  For now, this has been handled with
PROB30s, but the hope is that we can narrow this down and upgrade
to TEMPOs with a future amendment based on the 06z guidance, or
more likely with the 12z TAF issuance in the morning.  In short
order, a second round of convection is progged to arrive during
the first part of Saturday night, in the form of a robust QLCS
pushing out of the west...and a second round of SHRA/TS may be
ongoing or barely over at the end of the 06z TAF period.  For now,
have left a mix of -SHRA or VCSH in the prevailing lines, depending
on confidence, but as existing PROB30s are upgraded to TEMPOs,
and confidence on tomorrow night's timing improves, expect PROB30s
to be added to the tail end of newer TAFs.  Associated with this
activity will come another round of flight restrictions...with
potentially more-aggressive IFR ceilings and visibility issues.

Outlook: Unsettled weather and associated restrictions may
continue into early Sunday across parts of the area. In the wake
of a cold front after dawn Sunday, drier weather and predominately
VFR conditions are expected to return.

&&

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

&&

$$

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