596
FXUS62 KGSP 211852
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
252 PM EDT MON SEP 21 2026
WHAT HAS CHANGED
NO MAJOR CHANGES TO FORECAST THINKING.
KEY MESSAGES
1. A FEW SEVERE THUNDERSTORMS ARE POSSIBLE TODAY AND TUESDAY,
MAINLY IN THE LATE AFTERNOON AND EARLY EVENING. THE PRIMARY
CONCERNS FROM ANY STRONGER STORMS WILL BE DAMAGING WIND GUSTS
AND EXCESSIVE RAINFALL.
2. TEMPERATURES FALL BELOW NORMAL WEDNESDAY AND THURSDAY WITH LIGHT
PRECIPITATION LIKELY FOR MUCH OF THE AREA, PERHAPS WITH EMBEDDED
HEAVIER SHOWERS. DRIER CONDITIONS ARE EXPECTED TO DEVELOP FRIDAY
ALONG WITH NEAR-NORMAL TEMPERATURES THROUGH THE WEEKEND.
DISCUSSION
KEY MESSAGE 1: A FEW SEVERE THUNDERSTORMS ARE POSSIBLE TODAY AND
TUESDAY, MAINLY IN THE LATE AFTERNOON AND EARLY EVENING. THE
PRIMARY CONCERNS FROM ANY STRONGER STORMS WILL BE DAMAGING WIND
GUSTS AND EXCESSIVE RAINFALL.
SOMEWHAT UNSEASONABLY WARM AND HUMID CONDITIONS PERSIST THIS
AFTERNOON. WE LACK MUCH IN THE WAY OF A DISTINCT FORCING MECHANISM,
THOUGH A WEAK CONVERGENCE AXIS (LEE TROUGH?) IS EVIDENT IN THE
LOWER PIEDMONT, SOUTH AND EAST OF CHARLOTTE. UPWARD OF 2000
J/KG OF SBCAPE APPEARS TO BE IN PLAY, ALONG WITH 600-900 J/KG
OF DCAPE; WITH WINDS VERY LIGHT UP THROUGH 700 MB OR SO, SHEAR
IS ACCORDINGLY ALSO WEAK. RIDGETOP CONVERGENCE SHOULD MAINTAIN
SCATTERED SHOWERS/STORMS OVER THE MOUNTAINS, WITH A SECONDARY
MAXIMUM IN COVERAGE IN THE AFOREMENTIONED CONVERGENCE ZONE OVER OUR
LOWER PIEDMONT. CAN'T RULE OUT LATER DEVELOPMENT IN BETWEEN. SPC
HAS MOST OF THE PIEDMONT IN A DAY 1 MARGINAL RISK CONTOUR; WITH
A PULSE STORM ENVIRONMENT THAT ISN'T UNREASONABLE THOUGH SEVERE
THREAT IS EXPECTED TO REMAIN ISOLATED. PWATS ARE HIGH FOR THIS TIME
OF YEAR AND WITH SLOW STORM MOTION AN ISOLATED EXCESSIVE RAINFALL
OR FLASH-FLOOD THREAT MAY MATERIALIZE.
A STRONG 1030MB SURFACE HIGH IS ALREADY CENTERED OVER UPPER
ONTARIO. CONFLUENT UPPER FLOW DOWNSTREAM OF THE BLOCK OVER
THE NORTHERN PLAINS WILL ALLOW THAT HIGH TO STRENGTHEN FURTHER
TONIGHT INTO EARLY TUESDAY. AN AXIS OF VORTICITY ASSOCIATED WITH A
WEAK SHORTWAVE MOVING THRU THE OH VALLEY COULD PROVIDE A BOOST TO
FORCING LATE TONIGHT OR TUESDAY MORNING. IN ADVANCE OF THAT FEATURE,
WE SHOULD SEE SOME DEGREE OF WEAK UPGLIDE TAKE SHAPE OVER OUR CWA
TONIGHT, THOUGH THE SIGNAL APPEARS MUCH STRONGER TO OUR NORTH, NEAR
AN EXISTING STATIONARY FRONT. MOST GUIDANCE DEPICTS A FEW HUNDRED
JOULES OF MUCAPE LINGERING OVERNIGHT, SUCH THAT IN LIGHT OF THE VORT
AXIS AND UPGLIDE, OUR FORECAST WILL MAINTAIN A CONTINUING CHANCE FOR
ISOLATED TO SCATTERED SHOWERS/STORMS OVERNIGHT. THE UPGLIDE COULD
ALSO LEAD TO DEVELOPMENT OF A LOW STRATUS DECK OVER THE MOUNTAINS
AND NW NC PIEDMONT BY MORNING, AND MORE DIURNALLY DRIVEN CLOUD COVER
IS EXPECTED ACROSS THE REMAINDER OF THE AREA. TEMPERATURES SHOULD
TREND A BIT COOLER THOUGH THEY WILL REMAIN ABOVE NORMAL. WHILE
MORNING CLOUD COVER COULD HINDER DESTABILIZATION, WE WILL REMAIN
UNDER BROADLY CYCLONIC FLOW ALOFT, AND SHEAR WILL INCREASE A
BIT. SPC DAY 2 OUTLOOK FEATURES A MARGINAL RISK OVER OUR SOUTHWEST
CWA FOR MUCH THE SAME REASON AS TODAY; THE SMALLER AREA REFLECTS
THE POTENTIAL STABILIZING EFFECT OF THE CLOUD COVER.
KEY MESSAGE 2: TEMPERATURES FALL BELOW NORMAL WEDNESDAY AND THURSDAY
WITH LIGHT PRECIPITATION LIKELY FOR MUCH OF THE AREA, PERHAPS WITH
EMBEDDED HEAVIER SHOWERS. DRIER CONDITIONS ARE EXPECTED TO DEVELOP
FRIDAY ALONG WITH NEAR-NORMAL TEMPERATURES THROUGH THE WEEKEND.
AS THE CANADIAN HIGH BUILDS FURTHER INTO THE ST LAWRENCE
VALLEY TUESDAY NIGHT, IT COMES INTO POSITION TO FOSTER COLD-AIR
DAMMING. CAD WILL BE FURTHER STRENGTHENED AS THE MOIST BOUNDARY IS
FORCED SOUTHWARD, AND DIABATIC COOLING OCCURS. WITH THE WEDGE AND
WIDESPREAD CLOUD COVER WEDNESDAY, TEMPERATURES WILL PEAK AROUND
10 DEGREES BELOW NORMAL. WE WILL REMAIN UNDER CYCLONIC FLOW AT
500 MB, WITH AN UPPER LOW ROTATING OVER THE LOWER GREAT LAKES
AND OH VALLEY. MODELS DEPICT A TONGUE OF SBCAPE AND HIGHER MUCAPE
EXTENDING INTO EAST TN AND AROUND THE PERIPHERY OF THE WEDGE INTO
SW NC. HENCE THOSE AREAS STILL HAVE A CHANCE AT A FEW THUNDERSTORMS
WEDNESDAY, THOUGH THINKING DEPICTIONS OF ELEVATED T-STORMS OVER THE
WEDGE ITSELF ARE OVERDONE, SO WE ONLY ADVERTISE SHOWERS THERE. A
FEW MEMBERS OF THE REFS DEVELOP ORGANIZED CLUSTERS OF STORMS NEAR
THE SMOKY MTNS WHICH IS A SMALL POSSIBILITY, GIVEN 30+ KT OF 0-6KM
SHEAR THERE; WILL HAVE TO MONITOR THAT TREND IN GUIDANCE AS IT
COULD RESULT IN A SMALL SEVERE RISK OUTSIDE THE WEDGE. WHILE THE
BLOCKED FLOW CONTINUES, IT PROGRESSES ENOUGH FOR THE UPPER LOW TO
DRIFT EAST OF THE APPALACHIANS WED NIGHT INTO THURSDAY, SO HEAVIER
SHOWERS COULD RESULT IN ADDITION TO CONTINUING STRATIFORM PRECIP FOR
THE AREA. THE ADDITIONAL CLOUD COVER AND PRECIP SUGGESTS THURSDAY
WILL BE A FEW DEGREES COOLER THAN WEDNESDAY IN MOST LOCATIONS;
MOST OF OUR CWA IS FORECAST TO REMAIN IN THE MID TO UPPER 60S
THAT DAY. THE MODELS DISAGREE ON THE FINE DETAILS, BUT THERE IS
GENERAL CONSENSUS FOR THE UPPER LOW TO ENLARGE AND CENTER OVER THE
MID-ATLANTIC COAST BY LATE THURSDAY. A NOR'EASTER ALSO APPEARS
TO DEVELOP BY THEN. A DEEP DRY NORTHWEST F LOW REGIME SETS UP;
THOUGH THE HIGH REMAINS TO OUR NORTH THERE WOULD APPEAR TO BE COLD
ADVECTION LEADING THE WEDGE TO TECHNICALLY ERODE BY FRIDAY. PRECIP
CHANCES DECREASE AND SKIES TREND CLEARER; UNDER DOWNSLOPE FLOW
AND SUNSHINE TEMPS RETURN TO NEAR NORMAL, UPPER 70S TO NEAR 80
FOR MOST OF THE PIEDMONT. HUMIDITY WILL BE COMFORTABLY LOW. THIS
PATTERN SHOULD LAST THROUGH THE END OF THE PERIOD. OVERNIGHT LOWS
FRI AND SAT NIGHTS WILL REACH THE 40S OVER MUCH OF THE MOUNTAINS.
AVIATION /18Z MONDAY THROUGH SATURDAY/
AT KCLT AND ELSEWHERE: SLOW-MOVING SHRA/TSRA COULD AFFECT ANY
OF THE TAF TERMINALS THIS AFTERNOON; THE CHANCE WARRANTS PROB30
EVERYWHERE EXCEPT KAND, WHERE ONLY VCTS WAS MENTIONED DUE TO CHANCE
BEING SLIGHTLY LOWER. CANNOT RULE OUT ISOLATED REDEVELOPMENT OF
SHRA/TSRA TONIGHT, WHICH WARRANTS A PERIOD OF VCSH AT ALL SITES LATE
EVENING TO EARLY MORNING. WINDS ARE EXPECTED TO REMAIN LARGELY VRB
UNTIL TONIGHT WHEN HIGH PRESSURE BEGINS TO BUILD SOUTHWARD ALONG
THE EAST COAST, LEADING TO MAINLY NE WINDS, EXCEPT SE AT KAVL
DUE TO VALLEY EFFECTS. VALLEY/RIVER FOG WOULD APPEAR LIKELY AT
KAVL AND KHKY PRIOR TO DAYBREAK TUE, SO LIFR CIGS ARE INCLUDED AT
THOSE SITES. THE PATTERN SUGGESTS ISENTROPIC UPGLIDE COULD LEAD TO
DEVELOPMENT OF MVFR TO IFR LEVEL STRATUS IN THE PREDAWN HOURS TUE,
BUT THE SIGNAL FOR SUCH DEVELOPMENT IS WEAK ENOUGH THAT ONLY FEW-SCT
DECKS ARE MENTIONED AT THE OTHER SITES. RESTRICTIONS SHOULD BE SLOW
TO LIFT. CONTINUING NE WINDS ON TUESDAY, WITH A RETURN OF AFTERNOON
SHRA/TSRA CHANCES. AT KCLT A PROB30 IS ALREADY INCLUDED FOR SUCH.
OUTLOOK: DEVELOPMENT OF COLD-AIR DAMMING TUESDAY NIGHT SUGGESTS
PERIODS OF VSBY/CIG RESTRICTIONS WILL BE LIKELY THROUGH THURSDAY,
WITH BREEZY WINDS. DRIER CONDITIONS ARE EXPECTED TO 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.
JCW
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