-- Discussion containing changed information from previous version are highlighted. --
879
FXUS62 KGSP 180826
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
426 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/...
-- Changed Discussion --
As of 400 AM Sat: Ill-defined, somewhat unusual pattern in place
over the region, with synoptic warm front seemingly near I-20,
ahead of a very weak low pressure system in the Mississippi
Delta region. Southwesterly low-level flow continues between
the associated upper low and a flat ridge centered off the SE
coast. Another weak shortwave is moving up the Ohio River well to
our north. A few sfc obs from NE GA and the Upstate are reporting
light rain and this is plausible due to isentropic lift north
of the front, but from a shallow enough layer it is difficult to
discern on radar.

Although there is some MUCAPE on short term analyses, prog soundings
depict a deep inversion. 925-850mb flow has veered to SW via the
orientation of the ridge but also following the shortwave to our
north. PoPs have been trimmed back mainly to the Upstate and I-85
corridor mainly as a nod to the light stratiform precip; this may
not expand much farther north with the flow veering. That said,
850-700mb lapse rates are shown to increase with the warm front
after daybreak along with slight increases in sfc dewpoints;
thinning higher cloud decks should allow diurnal warming and
perhaps even a few breaks in the stratocu nearer the sfc. These
developments suggest the inversion will mix out and/or be overcome
by sfc parcels by late morning, so PoPs ramp up from the south from
then into early afternoon. The mountain ridges also will destabilize
via the warming and the westerly flow may kick off a few showers
there as soon as late morning. Abundant cloud cover will persist
and looks to keep temps a little below normal once again.

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 20-21z, 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.
-- End Changed Discussion --

&&

.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 /09Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Abundant low-level moisture beneath inversion
early this morning, but murky forcing regime, makes for tricky
TAFs. Weak warm front south of the terminal area. Some -DZ will
occur prior and accelerate decline to IFR which otherwise is likely
by 09-10z. Approach of shortwave from the west will activate warm
front in mid to late morning and focus light -SHRA development. The
front will also allow parcels to overcome the inversion aloft,
leading to lifting and perhaps partially scattering cigs by late
morning. With already favorable upper level lapse rates this also
will support development of TSRA during the afternoon, handled with
PROB30s. Southerly flow will continue today. Tonight, shortwave
will drift over the area while occluded front advances over the
Eastern Seaboard and weak sfc high builds in from the north. This
will induce a shift to northerly winds across the Piedmont in a
wedgelike scenario, with cigs again deterioriating from north to
south overnight. Elevated convection may be able to re-fire along
the backdoor front and persist in some fashion into Sunday.

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...Wimberley
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...Wimberley