Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
615 PM EDT Wed Aug 25 2021

.SYNOPSIS...
An upper ridge will become dominant over the region Thursday through
the weekend, with periods of Atlantic moisture brought into our
region.  A cold front will approach from the north early next week.
Above normal temperatures are expected through the first half
of next week.

&&

.NEAR TERM /THROUGH THURSDAY/...
As of 600 PM EDT Wednesday: As expected, storms scattered around
the CWA are being quite effective at producing microbursts. Although
we have not received any reports of wind damage, several obs sites
have measured 30-40 kt gusts from cells with cores near warning
criteria. SPC maintained Marginal Risk for almost the whole
fcst area on latest Day 1 update. Localized rain accums of 1-2"
have occurred given slow storm motion and elevated PWATs. These
conditions will gradually wane in intensity and storm coverage
should diminish heading thru the evening; MLCAPE and MUCAPE are
depicted by SPC mesoanalysis as being below modeled values at
this hour, but are progged to decline slowly overnight. Lingering
outflows may lead to new development as there remain broad areas
of the CWA that have not been worked over.

Fog will be of some concern in many areas tonight, assuming
convective debris dissipates as expected. Crossover temps are on
the high end, near 70 in many spots. Fog/low stratus development is
likely around areas where rain fell (or will fall) today, probably
locally dense in the mtn valleys.  Convergence axis (weak boundary)
working further inland from the coast later tonight looks unlikely
to be strong enough to kick off anything but highly isolated
convection, if at all, but may bring some cloud cover that could
limit the extent of any fog. Low temps will remain seasonally warm.

The weakness in the pattern aloft will linger into Thursday
before the Atlantic subtropical ridge starts building westward,
so expect Thursday to play out very similar to Wednesday. PW might
be slightly less and mid level temps slightly warmer, but there
should be a good chance of convection at least over the higher
terrain in the afternoon. The trend east of the mtns will be more
toward climo, however, as drier air filters in from the east over
the I-77 corridor at least. Temps will be a bit above normal over
the east, but close to normal elsewhere.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
As of 100 PM EDT Wednesday: A 595dm subtropical ridge will be
situated over the Atlantic coast Thursday night into Friday morning.
Only meager synoptic forcing will be available, perhaps over the
lower SC Upstate.  In response to the building subtropical ridge,
the Bermuda High will retrograde westward through Saturday,
and low-level flow will turn from almost due southerly, to more
southeasterly, allowing surface moisture to tick slightly upward.

Friday, isolated to widely scattered convection is possible given
favorable SBCAPE (~1000-1500 J/kg) and PWATs in the 1.5-1.7" range.
Better coverage should be limited by a growing subsidence inversion
owing to deep high pressure at the surface and aloft.  Saturday,
model soundings indicate a strengthening subsidence inversion,
less favorable instability, and increasingly dry air aloft, which
should once again hinder widespread convection.  Nonetheless,
the presence of dry air aloft means any storms that do break
out could be capable of producing downbursts.  Both afternoons,
convection will be most active over the western part of the CWA,
where terrain effects will enhance lifting.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
As of 130 PM EDT Wednesday: Ridging will persist throughout much
of the medium range forecast.  For the most part, this means a
continuance of diurnally-driven showers and thunderstorms, and
above-normal temperatures each afternoon.  The mid-levels look
to remain dry, as the subtropical ridge funnels moisture around,
rather than over, the Carolinas.  A trough axis should pass well
north of the area on Monday and Tuesday, with the attendant weak
cold front pushing across the area Tuesday afternoon or evening.

For now, the track of an approaching tropical system in the Gulf
of Mexico is unclear.  Global models currently suggest that the
blocking ridge set up over the Carolinas should steer such a
system to the west by Wednesday or beyond; however, the details
are unclear, and even if this situation plays out, some tropical
moisture might be able to filter into the area midweek.

&&

.AVIATION /22Z WEDNESDAY THROUGH MONDAY/...
At KCLT and elsewhere: Looks like a good setup for wet microbursts
across the region this afternoon, with ample buoyancy, mid level
dry air, and a deep well mixed boundary layer. Guidance shows
the development of scattered thunderstorms during peak heating,
so all TAFs get a TEMPO with variable gusty winds. Flight category
should be VFR outside of storms with a light SE wind. Overnight,
expect fog to develop where storms move this afternoon. Most places
get at least a TEMPO for some MVFR vis in the pre-dawn hours, but
the mtns get LIFR for valley fog/stratus. This should mix out by
13Z to 14Z. Thursday should be very similar.

Outlook: Conditions for storms become less favorable for the late
part of the week. Overnight restrictions can be expected in the
mountain river valleys and in any areas that receive heavy rainfall
the previous day.

Confidence Table...

            22-04Z        04-10Z        10-16Z        16-18Z
KCLT       High 100%     High 100%     High  80%     High 100%
KGSP       High 100%     High  97%     Med   77%     High  94%
KAVL       High 100%     High  82%     Low   35%     High 100%
KHKY       High 100%     High 100%     Med   70%     High 100%
KGMU       High 100%     High  95%     Med   77%     High  97%
KAND       High 100%     High 100%     High 100%     High  97%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables are available at
the following link:

www.weather.gov/gsp/aviation

&&

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

&&

SYNOPSIS...DEO
NEAR TERM...PM/Wimberley
SHORT TERM...MPR
LONG TERM...MPR
AVIATION...PM