406
FXUS62 KGSP 231751
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
151 PM 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/...
As of 1236 PM EDT Thursday: Unexpected convection over eastern
TN has wrought havoc on the forecast for today, as although it
shouldn`t incite any activity for us, it appears to be somewhat
stabilizing parts of east TN.  The result is that some of the
rapid-refresh guidance is slowing down the progression of convection
later this afternoon and evening.  Weak cells are just firing up
over the NC-TN border, though, so PoPs didn`t require too much
tweaking.  The trend is toward the severe risk coming later in
the day, however.  Evidence in water vapor imagery of a shortwave
making tracks across the upper Mississippi Valley, which when it
arrives here should still uncap profiles as originally expected
and set us up for thunder from there on out.

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.

&&

.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 THURSDAY/...
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 /18Z THURSDAY THROUGH TUESDAY/...
At KCLT and elsewhere: The TAF forecast is difficult, with every
model basically choosing its own version of reality and presenting
a different picture of how the afternoon and evening will evolve.
Currently (c. 1745z) cells are just starting to fire up over
southeast TN and northeast GA.  The expectation is that these
cells will translate eastward, becoming greater in coverage, as
the afternoon wears on.  In general, the timing of best coverage
looks to be later - post-4pm - for most terminals than advertised in
previous forecasts.  Guidance has also doubled down on the idea of
widespread fog tonight outside the mountains, in response to rain
from this afternoon/evening`s thunderstorms.  For now, have added
mention of low-VFR/MVFR vis at KCLT and the Upstate terminals,
and IFR at KHKY and KAVL.  For KHKY and KAVL, it`s unclear if
things will clear out enough to permit stratus development...but
IFR mountain valley stratus is certainly not out of the question.
Winds will be out of the SW for most of the period (variable,
of course, during thunderstorms) but may turn slightly N of W
during the evening or overnight hours after convection slides east.
Expect overall less coverage of storms on Friday, but at least some
convection seems likely.  The evolution of such activity remains in
question, and confidence on either timing or location is very low.

Outlook: A low-confidence pattern emerges for the weekend, with
some rain likely on Saturday.  Sunday and beyond, an unsettled
pattern will set in with diurnal rain/thunder chances and mountain
valley fog possible each night.

&&

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

&&

$$

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