275
FXUS62 KGSP 230000
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
800 PM EDT TUE SEP 22 2026
WHAT HAS CHANGED
REVISED POPS THIS EVENING THROUGH THE OVERNIGHT WITH LATEST HI-
RES MODEL GUIDANCE.
UPDATED AVIATION DISCUSSION FOR THE 00Z TAF ISSUANCE.
KEY MESSAGES
1. SLOW-MOVING THUNDERSTORMS MAY PRODUCE LOCALLY EXCESSIVE RAINFALL
THIS EVENING, POSING A RISK OF ISOLATED FLASH FLOODING. CANNOT
RULE OUT AN ISOLATED SEVERE STORM PRODUCING DAMAGING WIND GUSTS.
2. MUCH COOLER TEMPERATURES AND CLOUDY SKIES WEDNESDAY AND
THURSDAY, WITH OCCASIONAL LIGHT RAINFALL OR DRIZZLE ESPECIALLY IN
THE PIEDMONT. A FEW POCKETS OF HEAVY RAIN OR THUNDER COULD OCCUR
WEDNESDAY IN THE FAR SOUTHWEST NORTH CAROLINA MOUNTAINS. DRIER
AND WARMER BUT STILL MILD CONDITIONS RETURN FRIDAY INTO EARLY
NEXT WEEK.
DISCUSSION
KEY MESSAGE 1: SLOW-MOVING THUNDERSTORMS MAY PRODUCE LOCALLY
EXCESSIVE RAINFALL THIS EVENING, POSING A RISK OF ISOLATED FLASH
FLOODING. CANNOT RULE OUT AN ISOLATED SEVERE STORM PRODUCING
DAMAGING WIND GUSTS.
CONVECTION HAS BEEN EXPANDING ACROSS THE EASTERN SC PIEDMONT
AND SOUTHERN NC PIEDMONT. THIS ACTIVITY MAY CONTINUE TO EXPAND
WESTWARD ON OUTFLOWS AND EVENTUALLY MAXIMIZE OVER THE EASTERN
ESCARPMENT, WHERE INCREASING EASTERLY LOW-LEVEL FLOW PRODUCES
SOME UPSLOPE FORCING. THE MAIN THREAT APPEARS TO BE EXCESSIVE
RAINFALL GOING FORWARD, AS LOW-LEVEL LAPSE RATES AND MUCAPE ARE
DECREASING, RESULTING LOW-TOPPED, MORE EFFICIENT RAIN-PRODUCING
CONVECTION. STORM MOTIONS REMAIN SLOW, AND STORMS MAY TRAIN ALONG
AN APPROACHING BACKDOOR FRONT AS IT PUSHES INTO THE NC PIEDMONT
THIS EVENING.
1034+ MB HIGH PRESSURE NOW CENTERED OVER ONTARIO/QUEBEC. WITH THE
AID OF CONFLUENT FLOW OVER THE NE CONUS DOWNSTREAM OF A REX BLOCK,
THE HIGH WILL BUILD SOUTH ALONG THE EASTERN SEABOARD, AND SUPPORT
DEVELOPMENT OF COLD-AIR DAMMING WHICH WILL PERSIST THROUGH AT LEAST
THURSDAY. PER SATELLITE IMAGERY, A BOUNDARY IS EVIDENT NEAR THE
SOUTHERN AND EASTERN BORDERS OF OUR CWA, PROBABLY A CONVERGENCE ZONE
BETWEEN MARITIME AIR AND THAT ASSOCIATED WITH THE HIGH. HOWEVER,
MODELS LARGELY DEPICT A BACKDOOR COLD FRONT SETTLING SOUTHWARD
THROUGH VA INTO NC DURING THE EVENING, NORTH OF WHICH ISENTROPIC
UPGLIDE WILL DEVELOP ALONG WITH CLOUDS AND LIGHT STRATIFORM PRECIP.
THE UPPER LOW COMPONENT OF THE MIDWEST REX BLOCK IS SURROUNDED
BY A BROAD TROUGH, AND PER WATER VAPOR IMAGERY A SHORTWAVE WILL
EJECT OUT OF THE LOWER OH/TN VALLEYS AND OVER OUR AREA TONIGHT. NAM
AND SPC MESOANALYSIS DEPICTIONS OF CAPE CURRENTLY APPEAR OVERDONE
COMPARED TO GOES SOUNDINGS, BUT AT LEAST A FEW HUNDRED JOULES OF
SBCAPE ARE PRESENT ACROSS THE AREA THIS AFTERNOON, AND SOME MUCAPE
IS EXPECTED TO REMAIN FOR PARCELS LIFTED ABOVE THE WEDGE FRONT
TONIGHT. BETWEEN THAT AND THE SHORTWAVE, SHOWERS/STORMS SHOULD
INCREASE IN COVERAGE FROM LATE AFTERNOON THROUGH THE EVENING. CAMS
HAVE TRENDED TOWARD MORE ACTIVITY FIRING NEAR THE WEDGE BOUNDARY AND
LESS OF A SIGNAL FOR ACTIVITY PROGRESSING WEST TO EAST AS IF FORCED
BY THE SHORTWAVE. ASSUMING SKINNY CAPE PROFILES AS SEEN ON MOST
GUIDANCE THIS EVENING, PWATS BEING OVER THE 90TH PERCENTILE, AND
PARTICULARLY THE SLOW STEERING FLOW, A LOCALLY HEAVY TO EXCESSIVE
RAINFALL THREAT WILL PERSIST THROUGH LATE EVENING. CAM CONSENSUS
SUGGESTS THE MAIN THREAT IS IN OUR SOUTHEASTERN CWA, PARTICULARLY
THE CHARLOTTE METRO AND LOCATIONS SOUTH AND EAST. HREF AND REFS
PMM OUTPUT FEATURES THE GREATEST LIKELIHOOD OF EXTREME VALUES
(3"/3HR) IN THAT PORTION OF THE CWA.
KEY MESSAGE 2: MUCH COOLER TEMPERATURES AND CLOUDY SKIES WEDNESDAY
AND THURSDAY, WITH OCCASIONAL LIGHT RAINFALL OR DRIZZLE ESPECIALLY
IN THE PIEDMONT. A FEW POCKETS OF HEAVY RAIN OR THUNDER COULD OCCUR
WEDNESDAY IN THE FAR SOUTHWEST NORTH CAROLINA MOUNTAINS. DRIER AND
WARMER BUT STILL MILD CONDITIONS RETURN FRIDAY INTO EARLY NEXT WEEK.
THE REX BLOCK CONFIGURATION PERSISTS OVER THE GREAT LAKES AND
LOWER OH VALLEY WEDNESDAY THOUGH IT DOES PROGRESS SLIGHTLY SOUTH
AND EAST. BY WED MORNING CAD SHOULD BASICALLY BE ENTRENCHED
ACROSS OUR CWA WITH LOW CLOUDS AND FOG, PLUS OCCASIONAL DRIZZLE,
CONTINUING THROUGH THE DAY. MAX TEMPS WILL BE SHARPLY COOLER,
MAINLY IN THE 60S EXCEPT LOW TO MID 70S ON THE SOUTHWEST PERIPHERY
OF THE CWA. A SMALL AMOUNT OF MUCAPE IS SHOWN TO LINGER OVER THE
WEDGE THROUGHOUT THE AREA, INCREASING TOWARD THE SOUTH AND WEST. A
PORTION OF THE NC MOUNTAINS WEST OF THE FRENCH BROAD, AS USUAL,
MAY REMAIN OUTSIDE THE WEDGE WITH WARMER TEMPS AND POTENTIAL FOR
SURFACE BASED INSTABILITY TO DEVELOP. A VORT LOBE SURROUNDING THE
BLOCKING UPPER LOW IS SHOWN TO ROTATE OVER EAST TN AND FAR WESTERN
NC IN THE LATE AFTERNOON. IN THE WESTERN MOUNTAINS, THE RESULTING
DYNAMIC LIFT MAY REALIZE SBCAPE OF 500-1200 J/KG. EFFECTIVE SHEAR
WILL BE NEAR 30 KT, SO SOME CLUSTERS OF STORMS MAY ORGANIZE INTO
LINE SEGMENTS AND STRONG OR EVEN DAMAGING WIND GUSTS COULD RESULT,
PERHAPS WITH SOME EMBEDDED DIME TO NICKEL SIZE HAIL. SPC DAY 2
MENTIONS THIS POSSIBILITY IN TEXT BUT THE CHANCE WAS NOT SUFFICIENT
TO INCLUDE A RISK AREA, ONLY GENERAL THUNDER. THAT SHOULD PROVIDE
SOME DEGREE OF LIFT TO EXPECT SOME OF THE MUCAPE TO BE REALIZED
OVER THE WEDGE, SO A FEW EMBEDDED HEAVIER SHOWERS MAY RESULT IN THE
PIEDMONT, THOUGH PRECIP IS EXPECTED TO BE MAINLY STRATIFORM. FOR THE
PIEDMONT UNDER THE WEDGE, A FEW EMBEDDED HEAVIER SHOWERS MAY RESULT.
THE UPPER LOW SHOULD INCREASE WINDS ATOP THE WEDGE SO IT SHOULD
CONTINUE TO BE REINFORCED WED NIGHT. WENT A LITTLE WARMER THAN
NBM THAT NIGHT AND A LITTLE COOLER THAN NBM ON THU GIVEN WEDGE
SHOULD STILL BE STRENGTHENING. MAX TEMPS WILL BE A CATEGORY OR
SO COOLER--LOWER 60S FOR THE NW NC PIEDMONT AND ESCARPMENT AREAS,
AND MID TO UPPER 60S IN MOST OTHER AREAS. THE ALREADY BREEZY WINDS
SHOULD TURN A BIT GUSTIER ON THURSDAY WITH THE BETTER FLOW ALOFT. AS
THE LOW DRIFTS STILL FURTHER EAST TOWARD THE COAST THU NIGHT,
NORTHWESTERLY CAA IS SHOWN TO OCCUR ALOFT, WHICH WOULD PROMOTE
WEDGE EROSION WITH CLOUDS THINNING AND SCATTERING FROM TOP DOWN,
THOUGH OVERNIGHT WEDGE EROSION IS GENERALLY HARD TO DO. THIS
CAA REGIME CONTINUES INTO FRIDAY WHEN UPPER LEVELS ARE SHOWN TO
CONTINUE TO DRY, SO ANY REMAINING CLOUDS SHOULD ERODE. SUNSHINE
SHOULD OFFSET CAA, BUT WE ADVERTISE ONLY A MODEST REBOUND IN TEMPS
FRI SINCE THE WEDGE COULD HANG ON LONGER THAN MODELS SUGGEST, AS
USUAL. THE WARMING TREND SHOULD CONTINUE SATURDAY THROUGH MONDAY,
AT WHICH TIME AFTERNOON TEMPS RISE BACK TO AROUND NORMAL.
AVIATION /00Z WEDNESDAY THROUGH SUNDAY/
AT KCLT AND ELSEWHERE: CONVECTION IS INCREASING IN COVERAGE ACROSS
THE PIEDMONT, AND GENERALLY DRIFTING TOWARD THE WEST. THIS PUTS
CONFIDENCE ON DIRECT IMPACTS AT THE UPSTATE SITES THIS EVENING,
SO HAVE CONVERTED PROB30S TO TEMPOS. ENOUGH INSTABILITY AROUND TO
WARRANT KEEPING PROB30 AT KCLT THRU 03Z. A BACKDOOR COLD FRONT
WILL SLIDE THRU THE AREA OVERNIGHT, BRINGING ONSET OF COLD-AIR
DAMMING. HIGH CONFIDENCE ON WIDESPREAD IFR CIGS FORMING, ALONG
WITH PATCHY LIGHT SHRA AND DZ. WITH THE CAD, TYPICAL BREEZY NE
WINDS EXPECTED ACROSS THE PIEDMONT, WHILE KAVL WINDS TURN OUT OF
THE SE. LITTLE, IF ANY IMPROVEMENT IN CIGS IS EXPECTED THRU THE
DAY WEDNESDAY, WITH PERIODS OF -DZ AND SPOTTY -SHRA POSSIBLE.
OUTLOOK: CIGS MAY START TO IMPROVE THURSDAY, AND VFR RETURNING BY
FRIDAY, AS DRY AIR WORKS IN FROM THE NORTH. DRIER CONDITIONS RETURN
FRIDAY INTO THE WEEKEND, THOUGH MOUNTAIN/RIVER VALLEY FOG/STRATUS
COULD STILL DEVELOP NEAR DAYBREAK EACH DAY.
GSP WATCHES/WARNINGS/ADVISORIES
GA...NONE.
NC...NONE.
SC...NONE.
ARK/JCW
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