-- Discussion containing changed information from previous version are highlighted. --
565
FXUS62 KGSP 231423
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1023 AM EDT Thu May 23 2024

.SYNOPSIS...
Temperatures will remain several degrees above normal each
day through Monday as humid subtropical air remains over the
Southeast. Chances for showers and thunderstorms will remain higher
than normal, with some chance lingering each night. A cold front
passing late Monday will bring cooler and drier conditions in the
middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
As of 1014 AM EDT Thursday: No substantial changes to the forecast
this morning.  Aligned dewpoints a little closer the raw guidance,
and blended the in-house ensemble guidance to tweak PoPs through
the afternoon.  Unfortunately, none of the CAMs are initializing
well, with scattered non-severe showers over the Tennessee Valley
conspicuously absent in every run of the HRRR, RAP, and ARW.
Not taking any comfort in the fact that the NSSL-WRF is doing
better than any of the other CAMs with this activity, and happens
to depict one of the more dire solutions for this afternoon.
Will need to monitor this closely to see how it evolves.

Today still looks to be the start of a more convectively active
pattern, as the Southeastern upper ridge gets flattened and allows
a series of embedded shortwaves/MCVs to cross the region. The
most pronounced impulse appears to be migrating into the upper
Mississippi Valley attm, and this feature will reach central to
eastern TN by early aftn. Convection is expected to fire with
this impulse just west of the forecast area. Meanwhile, there may
be a lot of cirrus over the area that may help delay convective
initiation until mid aftn. From there, mid-level cooling is expected
atop the forecast area, as heights continue to fall thanks to the
flattening ridge. This should erode the cap and allow convection to
fire across the high terrain, with steering flow taking activity
into the Piedmont late aftn thru the evening. The CAMs are not in
good agreement on the exact timing or the storm mode. The 00z HRRR
was very active and showed some strong multicell clusters tracking
thru the eastern half of the FA. But the last few runs have been
more subdued. The two HiResWindow CAMs are quite active, while the
NAM is in between. So all that to say, confidence is below average
on convection/PoPs. Will stick with the hourly NBM, which is a
little lower than previous fcst. As for severe potential, the new
Day 1 SPC outlook keeps the entire area in a marginal risk. This
seems reasonable, although convective coverage across our NE GA
Piedmont and southern Upstate is expected to be low. Moderate
instability of 1500-2000 J/kg of sbCAPE with 0-6 km bulk shear
of 35-45 kt should support some organization of multicells and
possibly some rotating storms. So damaging winds and large hail
will be a threat. The tornado threat should be low, however, as
the low-level shear will remain weak. Temps are expected to be
similar to yesterday, about 5-7 degrees above normal.

Tonight, convection should drift east of the area, with some clear
expected west to east overnight. If precip coverage is decent today,
we could see some areas of fog form as winds should be light. This
will mainly be in the mountain valleys. Lows will be mainly in
the 60s, a few degrees above normal.
-- End Changed Discussion --

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 230 AM Thu: Confidence has not really increased in
the forecast for Friday and Saturday.  The Southeast states
will remain in the warm sector of a low pressure system in the
northern Plains as it matures and begins to occlude, and very large
diurnal instability is expected to develop in the lower Mississippi
Valley. For our CWA, temperatures will remain several degrees above
normal and dewpoints generally in the lower to mid 60s and we too
should be able to generate respectable CAPE each afternoon. One
of the factors limiting confidence is the quasi-zonal, slightly
anticyclonic upper pattern, which will allow any weak shortwaves
generated upstream to drift over our area with relative ease. Models
remain in some disagreement as to the nature and timing of these
waves. The weak waves, upstream instability and westerly steering
flow together suggest MCSs could develop in the Cumberland or TN
Valleys and make a run at our area either Friday night or Saturday
night, which might suppress convection the following days. PoPs
remain slight-chance to chance overnight both nights this period
as a nod to that possibility, particularly in the western CWA.

There remains decent agreement for a fairly well-defined wave to
cross the area Friday afternoon, which still suggests PoPs above
climo (40-50% Piedmont and 60-70% mountains). Shear has trended
a bit downward but still enough to suggest loosely organized
clusters of storms producing marginally severe hail and/or
wind. Upper ridging may build slightly over the area Saturday, and
midlevel drying looks to occur if the shortwave does pass Friday as
anticipated. That permits stronger sfc-midlevel delta-theta-e and
an increased risk of damaging wind. However, without the enhancing
effect of the shortwave and with post-MCS subsidence or cold pool
somewhat more likely, PoPs overall are 15-20% less on Saturday
afternoon compared to the previous day.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 310 AM Thu: A cold front will linger in the mid-Mississippi
and Ohio Valleys into Sunday, being left behind by the dying low
then having moved into central Canada. The next low spinning up
in the lee of the Rockies will reactivate this front and push it
toward the Mid-Atlantic states, reinforcing the already hot and
humid airmass in place over our CWA. Heights fall slightly as
a shortwave rides over Sunday invof the warm front, and upper
trough in the Plains will take on a negative tilt. PoPs trend
upward again on Sunday although the best DPVA should occur to our
north. Shear presently looks likely to increase over 40 kt and risk
of organized severe storms thus increases as well. The developing
cyclone looks likely to bring a cold front to the area perhaps as
soon as Sunday night, but more likely Monday (Memorial Day). We
reintroduce likely PoPs in the mountains Sunday night and across
the area Monday. Severe risk would appear to continue until the
front passes. Upper trough over the region Tuesday and Wednesday
suggests small chances for showers beneath it. Temperatures trend
back to about normal Tuesday and slightly below normal Wednesday.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
At KCLT and elsewhere: Uncertainty continues to be above average
for the 12z TAFs.  Plenty of mid and high clouds are expected
to stream in from the west thru the day which will likely delay
convective initiation until later this aftn. From there, guidance
is still not in good agreement on timing and overall evolution
of convection thru tonight. Some guidance has a fairly typical
diurnal late aftn/early evening peak, while others have the
greatest coverage late evening into the overnight. And depending
on how much rain falls and how much debris cloudiness thins out,
some guidance is hinting at areas of fog and LIFR stratus forming
before daybreak Friday. Given all that uncertainy, the 12z TAFs are
not too different from the previous fcst. Still handle convection
with PROB30s and hint at some fog and stratus toward the end of
the 12z TAF period. Winds will be generally light out of the SW,
but become vrb with thunderstorm outflows and may favor a N or NW
direction this evening at KCLT, KAVL and KHKY.

Outlook: Scattered to numerous showers and thunderstorms, along
with their associated flight restrictions, are expected again on
Friday. A more active pattern may persist thru early next week.

&&

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

&&

$$

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