937
FXUS62 KGSP 181422
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1022 AM EDT Sat May 18 2024

.SYNOPSIS...
A weak low pressure system will track east across North Carlina
today, bringing another rounds of showers and thunderstorms to the
area. The low exits off the Carolina Coast on Sunday, with weak
high presure building in from the north. Somewhat drier conditions
return early next week resulting in only isolated afternoon and
evening showers and storms. A cold front will bring better shower
and thunderstorm chances back into the forecast late in the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1012 AM Saturday: All is quiet for the most part for the time
being. Still not entirely clear how the afternoon will play out,
with a Marginal Risk for severe thunderstorms retained by the SPC
on the Day 1 Convective Outlook update. The latest vis satellite
imagery suggests a plausible scenario would be for convection to
begin along/near the Blue Ridge Escarpment later this afternoon,
because those locations have already seen partial clearing and
have started to destabilize. The 12Z HRRR seems to have that
idea in mind. Fortunately, none of the CAM solutions imply much
organization to the threat, so the most likely outcome looks
like mostly garden-variety thunderstorms, with a few having some
potential to become severe with wind damage and marginal hail. Temp
trends might be a mess through early afternoon as various locations
break out to sunshine quicker than other places.

Otherwise...height falls will occur later in the afternoon as
shortwave arrives from the west. CAMs are in rather good agreement
on more vigorous convection developing over the Escarpment around
19Z-21Z, expanding in coverage through early evening and propagating
SE. 700-500mb lapse rates will already be fairly good but should
be maintained by the wave, and altogether 1000-1500 J/kg SBCAPE is
likely to be in play at that time. Near-sfc winds will be light, and
midlevel flow is initially modest but weakens under the shortwave,
so the aftn convection could be fairly pulsey. Models vary as
to how dry the midlevels remain, and that is likely key to the
level of severe risk. Sfc-midlevel delta-theta-e values look lower
than we usually see with severe pulse storms. SPC Marginal Risk
is certainly sufficient for this low-end threat. Storms appear
to be very efficient rainfall producers and given consensus in
initiation time/area, PMM QPF is quite healthy with areas of 1-2"
storm totals likely. Some storms could produce tree damage simply
from weak organization along cold pools if they develop, and trees
may topple more easily with many areas having saturated soils.

Pattern remains murky for tonight. A tilted ridge develops to
our north in the wake of the shortwave, and a weak sfc high
will build down the Eastern Seaboard. This looks to bring a
wedgelike backdoor front into the area, likely encountering
enough sfc moisture to create a low stratus deck and possibly
also initiate elevated convection, with lapse rates still good
above the associated inversion. PoPs thus diminish from north to
south across the Piedmont overnight, although see fit to keep a
slight chance all night there; chance values for the GA/SC zones
and Escarpment. Lows will be several degrees cooler than this
morning despite continuing clouds.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 230 AM EDT Saturday: The positively tilted upper trough axis
will reach the Carolina Coasts by late Sunday, ushering in strong
height rises atop the forecast area. The associated sfc low pres
system be off the coast by 12z Sunday, with wrap-around moisture on
the north side of the circulation from the Mid-Atlantic SW into the
western Carolinas. Fcst soundings across the NC Piedmont and eastern
Upstate show potential for a stratus deck to gradually scatter out
thru the day, with a decent inversion atop the cloud layer. However
over the mountains and across the Upper Savannah Valley, sbCAPE
may still be realized with little to no CIN. This looks almost like
wedge setup, albeit not from CAD. So have trimmed PoPs back within
the wedge-like air mass, with solid chc PoPs in the mountains, but
only slight chc across most of the I-77 corridor. Any tstms that
do develop will have fairly weak shear and a somewhat suppressive
synoptic environment, so severe threat will be low. With steering
flow roughly parallel to the Blue Ridge out of the NE, training
convection could lead to isolated heavy rainfall. Highs will be
below normal, especially across the I-77 corridor.

Dry air continues to filter in from the N/NE Sunday night thru
Monday, but there may be some redevelopment/expansion of stratocu
overnight, before dissipating during the day Monday. Subsidence
from an upper ridge should cap most deep convection, except for the
higher terrain. There, enough CAPE and mountain-top convergence
could lead to scattered showers an perhaps a few garden-variety
tstms. Temps return to near normal under partly to mostly sunny
skies.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 300 AM EDT Saturday: A warming trend with generally dry
wx is expected thru Wednesday, as an upper ridge builds over
the Southeast. Guidance continues to back off PoPs, and it
looks like even the high terrain may be largely capped from deep
convection. Will keep some slight chc PoPs in the NC mountains for
now, both Tue and Wed aftns. With instability remaining weak and
a lack of upper support, severe threat should remain low. Highs
will be a category or two above normal, while lows will be 2-3
categories above normal.

A cold front will approach the forecast area from the NW Thursday,
as shortwave energy rides over the eastern CONUS ridge. Thanks
to the influence of the ridge, the front will struggle to reach
the area, and guidance seems to be trending drier for the latter
half of next week. There still should be some uptick in PoPs,
as shortwave energy ejects out of the Rockies and works to break
down the eastern ridge, but the National Blend of Models may be
overdone, given the model trends. Temps will continue to be above
normal with modest humidity.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Murky forcing regime results in lower
confidence than usual for TAFs. Patchwork cigs at various levels at
issuance time with -DZ or sprinkles across the SC sites, north of
a weak warm front. Warm front will ooze northward and promote low
level warming, promoting mixing and allowing destabilization. Light
SW`ly winds will continue today. Cigs should lift to MVFR or low
VFR. Convection could develop south of the warm front or over
the mountains as soon as midday, but more likely will develop in
the mid to late afternoon with peak heating as well as DPVA from
shortwave to the west. Any storm will be capable of +TSRA and IFR
vsby, but mention of IFR is limited to where confidence greater on
TS coverage and timing. Storms that develop along cold pools will
be capable of strong to damaging wind gusts as well. Convection
should work W to E from the mountains to the Piedmont, likely
ending by around 02z. Backdoor cold front settling into the area
tonight may however spawn an isolated elevated SHRA or TS. More
likely impact is IFR cigs and flip to NE winds overnight.

Outlook: SHRA/TSRA remain possible Sunday and perhaps even Sunday
night until stronger high pressure settles over the region
Monday. Late night/early morning restrictions in low stratus
and/or fog will remain possible Sunday night; isolated mountaintop
convection still appears possible Monday.

&&

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

&&

$$

SYNOPSIS...ARK
NEAR TERM...PM/Wimberley
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...Wimberley