Severe Thunderstorm Warning 229, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
737 PM EDT Sat Jul 29 2023

.SYNOPSIS...
A cold front approaches out of the north tonight with showers and
thunderstorms continuing into the evening. A few storms may become
strong to severe. The cold front then tracks across the forecast
area on Sunday before pushing south of the region Sunday night. High
pressure returns behind the cold front early to middle of next week,
bringing cooler temperatures.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 551 PM EDT Saturday: Thunder has finally broken containment up
and down the Blue Ridge, with a more coherent line of thundershowers
upstream...and just beginning to encroach on the NC/TN border.
With the rapid loss of upper-level cloud cover, temps have shot
upward, and some 3000-3500 J/kg of sbCAPE has managed to develop
east of the mountains...which should support robust updrafts
through the coming hours.  So, high temps needed to be bumped up
a tad...but will still come in among the lower third of guidance
for this afternoon.  All this ahead of an advancing cold front,
now apparently eeking its way into the Tennessee Valley.

Otherwise...the cold front will track into the Carolinas into
tonight while the area of low pressure gradually pushes off
the Carolina Coast tonight.  It remains uncertain how thunder
will evolve over the next six hours; CAMs continue to depict the
advancing line of convection making tracks across the CWA through
late this evening and into the first part of tonight...but with
some loss of veracity as instability wanes after dark.  It seems
likely, though, that thunder will cross the mountains over the
next 1-2 hours, then make a run at the Piedmont and Upstate zones
through midnight or later.  SPC's Day 1 Severe Weather Outlook
places a Slight Risk across the northern NC mountains, with a
Marginal Risk across the rest of western NC, extreme NE GA, and the
northern SC Upstate. Despite only 15-20 kts of 0-6 km bulk shear,
strong to severe storms can be expected given a very unstable
environment with ample DCAPE.  Damaging wind gusts are the main
potential threats.  Lows tonight will be around 3-6 degrees above
climo thanks to lingering cloud cover.  Mountain valley fog and
low stratus should return again overnight into daybreak Sunday.

The cold front will push southward across the CWA on Sunday leading
to another round of at least scattered showers and thunderstorms
just ahead of the FROPA. Capped PoPs to chance for now as CAMs do
not agree on the coverage of storms Sunday afternoon. The Day 2 SPC
Severe Weather Outlook has most of NE GA, the SC Upstate, and the
southern NC Piedmont in a Marginal risk for severe storms. Thus,
a few strong to severe storms cannot be ruled out. Damaging wind
gusts look to be the main potential threat. Temps will be much
warmer Sunday due to lower cloud cover and downslope flow. Highs
will end up around 3-6 degrees above climo.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
As of 155 PM EDT Saturday: A series of short waves moving through
the northwest flow over the area will keep the East Coast trough in
place through the period. A weak frontal boundary will move to our
south Sunday night but stall there Monday. Diurnal convection ends
Sunday evening. With drier air in place on Monday, the forecast is
looking dry for now, but that could change if the boundary stalls
over the Upstate. Convection associated with an MCS boundary or
short wave/MCV could move in from the NW or redevelop over the area
on Tuesday, but details remain uncertain. Temps cool on Monday but
remain above normal. The good news is drier dew points will keep
Heat Index values below 100. Highs cool to near or slightly below
normal on Tuesday.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 110 PM EDT Saturday: NW flow over the area Wednesday weakens
Thursday along with some height rises as the east coast trough moves
east and an anti-cyclone attempts to move east out of Texas. The
advance of the anti-cyclone is halted Friday, and even retrogrades,
as a series of short wave move across the area from the NW
reinstating a flat trough. At the surface, mainly dry high pressure
over the area Wednesday moves off shore with southerly flow
returning Thursday. A weak cold front approaches from the NW Friday,
moving into the area on Saturday. The result is an active period
with diurnal convection developing Thursday through Saturday after a
dry Wednesday. Temps a little below normal Wednesday and Thursday,
rise back a little above normal Friday, then back to near normal
Saturday.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
At KCLT and elsewhere: A line of severe thunderstorms is tracking
across the forecast area and located across the western Piedmont
as of 2330z.  Retained TEMPOs for TSRA at all terminals as
the line is expected to push east by about 03z...with trailing
showers handled with a VCSH extending later into the evening.
Some overnight drying will permit gradual clearing of low-level
cloud cover, which should permit another round of low-end VIS
restrictions in some of the mountain valleys.  In particular,
with KAVL having already received a good amount of rainfall this
evening, it seems like a better case for some fog than last night.
Whatever mountain valley restrictions develop should lift quickly
after sun-up, giving way to FEW/SCT cloud cover through the morning
and early afternoon.  Prevailing VFR will persist for the remainder
of the period...with any restrictions limited to another round of
afternoon convection...handled with PROB30s for now.  It's unclear
how coverage will evolve at this point: with a frontal boundary
expected to settle across at least part of the forecast area by
afternoon, the southeast terminals most likely to be under the gun
going into the evening.  As of yet, CAMs are inconsistent in their
handling of tomorrow's thunder.  Winds will become increasingly
NNW through the overnight...with periods of ENE wind even possible
going into Sunday as the front pushes south.

Outlook: Drier conditions will return early next week as high
pressure pushes in behind the cold front.  Fog and low stratus
will be possible in the mountain valleys each morning.

&&

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

&&

SYNOPSIS...AR/RWH
NEAR TERM...AR/MPR
SHORT TERM...RWH
LONG TERM...RWH
AVIATION...MPR

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