939
FXUS62 KGSP 162131
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
531 PM EDT Fri May 16 2025

.SYNOPSIS...
Very warm and humid conditions remain in place through tonight as
a cold front sends a line of showers and thunderstorms across the
area late tonight into Saturday morning. The front stalls in the
vicinity of the area Saturday through early next week, keeping a
relatively active pattern in place.

&&

.NEAR TERM /THROUGH SATURDAY/...
Key Messages:

1) Severe Thunderstorm Watch has been cancelled for this
afternoon/evening.

2) A line of strong-to-severe thunderstorms is expected to impact
the mountains late tonight, with the threat decreasing eastward.

3) Temps around 10 degrees above normal.

As of 521 PM EDT Friday: The cap appears to have won the battle,
with not a singel storm having initiated over the Piedmont in the
last few hours, and round 1 of severe potential essentially over
for the afternoon and early evening.  Severe Thunderstorm Watch
#261 has been cancelled.  Expect quiet conditions the next few
hours, as attention turns upstream, to organized convection now
underway across southern Illinois and western Kentucky.  Afternoon
soundings there depict a more robust convective environment, with
a weaker/nonexistent cap and thus a similar but more accessible
parameter space.  The latest hi-res guidance is remains in fairly
good agreement on quasi-linear convection accelerating cross
Tennessee late this evening along a robust coldpool...but likely
weakening upon its arrival in the Appalachians after midnight.

The big question then becomes whether convection can reorganize
along the cold pool as it moves off the high terrain into the
foothills and Piedmont during the pre-dawn hours. The environment
east of the mountains is expected to remain largely capped...arguing
against the potential for reorganization. Additionally, shear
parameters are strong...and studies have shown that strong shear cam
actually be detrimental for regeneration along a terrain-crossing
cold pool. Nevertheless, there`s more than enough uncertainty to
advertise chance PoPs east of the mountains. While the main severe
wx threat overnight will be from damaging wind gusts...low level
shear will be adequate for brief spin-ups, mainly across the TN
border counties.

The cold pool will effectively push a frontal boundary through
the CWA Saturday morning, leaving behind lower theta-E air (mainly
manifest as lower surface dewpoints). As such, forecast soundings
during the daylight hours feature much weaker instability, and
CAMs develop little in the way of diurnal convection. Therefore,
after some lingering token small morning PoPs, chances drop to
less than 20% across most of the area during the afternoon.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 130 PM EDT Friday: Most of the model guidance are in
agreement with the synoptic frontal boundary completing a full
fropa across the CFWA and stalling south of the I-20 corridor by
Saturday evening as the vertically stacked parent low sets up shop
over the northeastern CONUS. The frontal boundary will be draped
from the Southern Plains through the Lower MS Valley and Deep South
Saturday night as the cold front becomes reactivated when convection
initiates over the Southern Plains and Ozarks Saturday evening
before becoming clustered and translating east. The flow aloft
becomes more progressive Sunday into Monday as a digging upper low
over the western CONUS carves out a ridge over the central CONUS,
while orientating the flow from the northwest to southeast across
the CFWA. Early Sunday morning, the flow aloft won`t be fully
established, but a potential decaying MCS could make a run at the
southwest mountains and Savannah River Valley. Model guidance differ
with the overall evolution, with the ECMWF/CMC indicating very
little SBCAPE, while the NAM/GFS show a couple hundred J/kg of
SBCAPE as it slips towards the area. Kept some mentionable PoPs in
this region of the CFWA as a result, while any convective debris
could alter afternoon highs on Sunday, but warm thicknesses and
slightly lower dewpoints (upper 50s-lower 60s) will help afternoon
highs top out ~5 degrees above normal.

Deep layer northwesterly flow becomes better established later
Sunday as the upper ridge axis gradually shifts over the MS Valley,
while an upper low churns over the western CONUS and the other upper
low meanders over New England, creating an Omega-like blocking
pattern. In this case, drier air will filter into the area, but
convective initiation on the leeside of the western upper low and
dryline across the Plains will create an assembly line for the
activity to grow upscale and make a couple of MCS runs towards the
CFWA during the early part of next week with the first MCS possibly
tracking towards the area sometime on Monday. Confidence is low on
the exact timing and placement of the MCS, but the synoptic setup is
favorable and will likely throw a monkey wrench into the temperature
and PoP forecast with the associated QPF response and convective
debris. However, higher heights build into the area by early next
week and should keep temperatures on track to rise a category or so
above normal outside of the monkey wrench.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 230 PM EDT Friday: The upper ridge gradually shifts further
east towards the eastern-third of the CONUS, while breaking down
as the upper low over the western CONUS gets pushed from behind
as a Pacific Northwest jet punches in. This will disorientate
the northwesterly regime, but guidance try to send one more MCS
towards the region late Monday into Tuesday, but will likely hit
the northern tier of the CFWA this time around or miss to the north
altogether. Diffluent flow aloft becomes better establish later
Tuesday into Wednesday as the ridge continues to slide eastward
and the upper low deepens into a closed low, which eventually
becomes established across the Great Lakes region. Expect an
increase in deep layer shear as a result, which will coincide
with daily diurnal instability to help instigate strong to severe
storms ahead of an encroaching frontal boundary. Model guidance
also show a potent shortwave rounding the base of the closed
low Wednesday into early Thursday to help the frontal boundary
advance eastward and complete a fropa sometime during the first
half of Thursday. Drier air is expected behind the front as broad
cyclonic flow aloft remains in place and low-level CAA filters
in with weak high pressure, setting the stage for a post-frontal
regime by the latter half of the forecast period. Northwesterly
flow will commence and could produce a few orographically enhanced
showers along the TN border. Continued northwesterly flow pattern
remains in place through the end of the forecast period and could
make the area vulnerable to another favorable MCS pattern, but
likely to become more set and stone after D7. Temperatures will
be at or slightly above normal Tuesday and Wednesday, before the
post-frontal regime allows temperatures to fall a category or so
below normal Thursday and Friday.

&&

.AVIATION /22Z FRIDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: VFR is forecast through the period. Isolated
thunderstorms are possible late this afternoon near KHKY, but the
probability of a storm in the vicinity is around 20%, and therefore
not worthy of a TAF mention. A line of thunderstorms organizing over
the TN/OH Valleys later this afternoon/evening is expected to begin
weakening as it moves E/SE across east TN and western NC late this
evening into the early part of the overnight. At this time, it is
not clear whether this convection will hold together long enough to
reach any of the TAF sites. Nevertheless, Prob30s for TSRA are
warranted at most sites at some point between 06-12Z Saturday. Gusty
winds are likely if the convection holds together. (Even if the
convection doesn`t hold together, remnant outflow boundary may
still be capable of gusty winds.) Otherwise, S/SW winds are expected
to increase to around 10 kts by this evening...generally persisting
through tonight before increasing to 10-15 kts with higher gusts by
early afternoon Sat.

Outlook: The potential for mainly diurnal convection increases again
Sunday, with chances continuing through the middle of next week.
Early morning fog/low stratus will be possible, especially in the
mountain valleys and in locations where appreciable rain fell the
previous afternoon/evening.

&&

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

&&

$$

SYNOPSIS...CAC
NEAR TERM...JDL/MPR
SHORT TERM...CAC
LONG TERM...CAC
AVIATION...JDL