733
FXUS62 KGSP 120743
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
343 AM EDT Mon May 12 2025

.SYNOPSIS...
Low pressure over the lower Mississippi Valley region today will
move slowly northeast through mid week, spreading showers and
thunderstorms across our region. Periods of heavy rain may result
in some flooding today and tonight, mainly over the eastern side of
the mountains.  Drier high pressure moves over our area Thursday
then a cold front stalls to our north next weekend with scattered
to numerous showers and thunderstorms and above normal temperatures
across our region.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 330 AM EDT Monday: Little change in thinking for this
morning`s forecast package. A stacked low pres system will slowly
drift NE across the Lower MS Valley region today, bringing an
850 mb southerly jet into the CWFA. This will be the period of
peak moisture transport help erode the cold air damming (CAD)
wedge that has been in place since yesterday. With some diurnal
destabilization and the eroding wedge, should set up a period of
greatest rain rates and even a non-zero tornado threat this aftn.
The one thing keeping the flash flood threat in check is the
residence time of the moisture plume over the FA, with a dry slot
lurking just to our west on WV imagery, this will start to enter
the forecast area this evening. There has been a decreasing trend
of QPF along the Escarpment in the CAMs, but hints that bands
of training cells outside the mountains may occur. Of course,
the CAMs disagree on where these bands will set up, but the
HRRR has been doing the best with this event so far, and shows
one axis of heaviest rainfall east of I-77 and into central NC,
and the other over NE GA into the western Upstate. These amounts
still look to be in the 1-3" range, which should be under flash
flood guidance, unless locally higher amounts occurs. Meanwhile,
the 3-5" across the southeasterly upslope areas near the Blue
Ridge Escarpment still looks similar to the previous forecasts. So
with all that said, no changes will be made to the Flash Flood
Watch. The other concern today will be the threat for isolated
tornadoes this aftn during peak instability. The 00z HREF members
generally don`t show strong updraft helicity streaks. But with 0-1
km shear possibly 25-30 kt and helicity 150-200 m2/s2, stronger
cells may show low-level rotation and could spun short-lived weak
tornadoes. The threat looks to be greatest roughly along and west
of the I-26 corridor in the Piedmont today, based on where the
expected stronger convection will be. Otherwise, it will be another
rainy, cloudy day, but with temps slightly warmer than yesterday,
thanks to the eroding wedge. Still several degrees below normal.

Tonight, the upper low will pivot toward the NE, providing decent
upper divergence atop the forecast. The forcing should keep
scattered to numerous showers across the CWFA, but with a dry
slot working in from the west, PWATs should come down some. Also,
the low-level jet will weaken and begin to veer to more due south
over the CWFA, helping weaken the upslope component. However,
with additional rain, could see a lingering flash flood threat
thru the overnight in the Watch area, as rainfall totals continue
to accumulate. Lows will be mainly in the upper 50s to mid 60s.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 338 AM Monday: We will continue to deal with the passage of
the vertically-stacked low pressure system through the middle
of the week. However, by daybreak Tuesday, the bulk of the
forcing will have moved off to the northeast, leaving us with
little more than lingering weak low level convergence and some
decent thermodynamics. Essentially this portends a transition to a
diurnally-enhanced convective precip regime with rather high precip
probs in the afternoon because of the colder air aloft associated
with the upper system allowing for steeper lapse rates and decent
buoyancy with some daytime heating. But, as the convection consumes
the bouyancy and we lose the heating, probs drop back down after
sunset both Tuesday and Wednesday. The threat for severe storms
appears to be low both days. Temps will undergo a gradual warmup
toward normal thru Wednesday. Guidance ultimately pivots a trof
axis across the region on Wednesday as the old upper low opens
up over the upper Ohio Valley region late in the day. With that
trof axis to our east Wednesday night, dry air finally returns
from the NW overnight, giving us a short break.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 236 AM Monday: With the old system finally out of the way,
a quick transition is expected on Thursday to something more
summerlike for the bulk of the medium range. Upper ridging builds
in from the west, raising temps above normal, while helping to
suppress deep convection, so only a token slight chance will be
included over the mtns. The ridge axis should cross the region
Thursday night while dampening, meanwhile an upper anticyclone
becomes established across the Gulf. Thereafter, we`re under a
belt of fast-moving westerlies through the weekend, featuring an
old frontal boundary laid out by a passing northern stream low
which should help to focus diurnally-enhanced convection through
the period. We essentially end up in what would be a potential MCS
track, with plenty of instability to fuel convective storms and
enough shear to organize them. There`s a good chance that we will
see some kind of organized severe weather threat at least once in
the period from Friday thru Sunday, most likely Friday/Friday night
according to some of the machine learning/AI guidance. We`ll still
be under the MCS track into Monday, but by that time the pattern
will be undergoing amplification with a strong ridge building to
our west. Thus, each day will get a decent chance of convective
storms, perhaps more likely on the west side of the mtns, and
temps on the order of 5-10 degrees above normal.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Widespread showers moving into the area
to start the 06z TAFs, with all TAF sites now IFR or lower. A
lingering wedge should generally keep the cigs stable thru this
morning, but vsby may fluctuate due to the on-and-off showers. The
wedge will begin to erode after 12z, with winds shifting to SE
and perhaps some improvement of the cigs to low MVFR. There is
below average confidence on how the cigs will behave, as deeper
convection develops across the area, and may mix out some of the
clouds, only for them to drop again during lulls. The peak daytime
heating ours of mid aftn thru early evening looks to be the time
most likely to see some TS embedded in the widespread SHRA. So
will continue PROB30 for all sites, mainly in the 18z-00z time
frame. Precip coverage should decrease somewhat late evening into
tonight, but scattered SHRA will likely continue thru the 06z TAF
period. Despite the wedge eroding, IFR conditions look likely at
all sites tonight, as low-levels remain very moist.

Outlook: Showers and TS are expected to gradually taper off Tue
night. Drier conditions return by mid-week, but sufficient moisture
will likely remain to support at least scattered diurnal showers
and thunderstorms.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...Flood Watch from 10 AM EDT this morning through Tuesday
     morning for NCZ033-035-049-050-063>065-501>510.
SC...Flood Watch from 10 AM EDT this morning through Tuesday
     morning for SCZ102-103.

&&

$$

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