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252
FXUS62 KGSP 231845
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
245 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 FRIDAY/...
-- Changed Discussion --
As of 242 PM EDT Thursday: Convection is just starting to fire
across the NC mountains this afternoon as ripples of synoptic
forcing make tracks across a deteriorating upper ridge to our east.
Widespread convection to the east earlier this afternoon somewhat
stabilized parts of northeastern TN, but the environment has
rebounded quickly, and now boasts some 500-1000 J/kg with only
modest CIN.  Our own forecast area is itself socked into a warm
sector-like environment, with a plume of some 1500 J/kg sbCAPE...and
on the rise...already creeping into the Upstate.

The general trend today is for advancing shortwave energy to steepen
lapse rates and erode an already-weak cap over what are otherwise
excellent profiles for deep convection.  The best upper forcing will
not arrive in the western zones until c. 4pm, at which point most of
the CAM guidance depicts widespread convective initiation.  This`ll
take place in an environment of only modest (<15kt) low-level shear,
but increasingly potent (>30kt) deep-layer shear.  Consequently,
many of the CAMs depict some level of convective organization.
DCAPE values will also soar to 900-1200 J/kg in many sites outside
the mountains.  All that to say, developing cells will encounter
an environment ripe for severe weather.  What remains in question
is the exact evolution.  Some guidance sources - especially the
later runs of the HRRR and NAMnest - depict an initial wave of
weaker convection during the next 3-4 hours, followed by a second,
late evening round that won`t fully exit the area until midnight
or later.  The 12z cycle of the ARW, FV3, and NSSLWRF all depict
a more robust, semi-organized line crossing the area after 4pm,
and stripping the atmosphere of enough instability in the process
to effectively shut convection down thereafter.  Which of these
scenarios will actually play out is hard to say.

Once convection exits the area tonight, gradually clearing skies
should permit fog development.  There are early indications in
guidance that this won`t be restricted to the mountain valleys,
but may also occur in parts of the NC Piedmont.  Right now, it
seems like it`ll be patchy at best...but as always, we`ll need
to evaluate trends once convection is underway.  Fog will quickly
scatter out tomorrow morning, giving way to another partly cloudy
day.  Profiles should once again support some afternoon thunder,
with CAMs advertising ~1200-1500 J/kg sbCAPE and , but somewhat
more nebulous synoptic forcing and more borderline deep layer shear
owing to a less prominent upper wave than the one inciting today`s
convection.  Consequently, while some severe weather is certainly
possible tomorrow, it looks overall less likely than today.
-- End Changed Discussion --

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
-- Changed Discussion --
As of 150 PM Thu: A short wave ridge builds over the area Saturday
then slides east on Sunday as a short wave trough crosses the area.
Any surface frontal features will be weak and likely remain north
and west of the area. A weak lee trough seems to be the dominant
surface feature if there is any. Convection should diminish Friday
evening with isolated showers possibly lingering overnight. Diurnal
convection returns Saturday, but the coverage while favoring the
mountains, remains somewhat questionable, especially over the
Upstate and NE GA given the ridge building in. That said, where
storms do form, isolated severe storms with damaging downbursts
would be possible given moderate instability, low shear, and mid
level dry air. Isolated heavy rainfall would also be possible.
Convection should taper off quickly Saturday evening.

Coverage on Sunday should be greater, still favoring the mountains.
The atmos could become very unstable with shear increasing to around
40 kts. This could lead to a more organized severe storm threat
adding large hail to the damaging wind potential. There is still
some uncertainty, so this will need to be monitored. Lows around 5
degrees above normal and highs 5 to 10 degrees above normal both
days.
-- End Changed Discussion --

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
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As of 220 PM Thu: A phasing short wave digs a trough over the
eastern CONUS and crosses the area Monday night. The trough remains
in place as another phasing short wave moves through on Wednesday.
NW flow aloft will be over the area Thursday. A cold front will
slowly move toward the area Monday and across the area Monday night.
The front may stall near the area Tuesday before a weaker cold front
crosses the area Wednesday. Likely to good chance PoP expected
Monday with the front. Lower chance PoP expected Tuesday with
moisture along the stalled front, with only isolated mountain
showers with the Wednesday front. Some severe storms and heavy
rainfall will be possible with the Monday front. Much lower chance
of either Tuesday. Thursday looks to be dry. Temps start out above
normal on Monday falling to near normal by Thursday.
-- End Changed Discussion --

&&

.AVIATION /19Z 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...DEO
NEAR TERM...MPR
SHORT TERM...RWH
LONG TERM...RWH
AVIATION...MPR