National Weather Service Weather Forecast Office

Area Forecast Discussion

588
FXUS62 KGSP 231758
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
158 PM EDT THU MAY 23 2013

.SYNOPSIS...
AN UPPER LEVEL TROUGH WILL SWING THROUGH THE REGION TOMORROW...
BRINGING COOLER AND DRIER CONDITIONS FOR THE WEEKEND. TEMPERATURES
WILL MODERATE BY THE BEGINNING OF NEXT WEEK... WITH A CHANCE OF
AFTERNOON SHOWERS AND THUNDERSTORMS EACH DAY CONCENTRATED OVER THE
MOUNTAINS.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF EARLY AFTERNOON...MOST OF THE CONVECTIVE DEVELOPMENT HAS TAKEN
PLACE JUST EAST OF THE CLT METRO AREA. OTHERWISE...KGSP RADAR IS
NEARLY QUIET ACROSS THE FCST AREA. SEEING SOME ENHANCEMENT TO THE CU
NEAR THE BLUE RIDGE...SO AM NOT READY TO GIVE UP ON THE PRECIP
CHANCES FOR THE AFTERNOON...PER THE OLD RUN OF THE 4KM WRF. TEMPS
LOOK OKAY.

PREVIOUS DISCUSSION...

AT 300 AM EST THURSDAY...AN UPPER TROUGH AXIS WILL CROSS THE OH AND
TN VALLEYS TODAY...CROSS THE SOUTHERN APPALACHIANS THIS EVENING...
THEN MOVES SLOWLY OVER THE EASTERN SEABOARD TONIGHT. THE REMAINS OF
A COLD FRONT WILL CONTINUE TO CROSS OUR AREA EARLY TODAY..EXITING
THE AREA TO THE EAST THIS AFTERNOON. A STRONG COLD FRONT WILL
APPROACH FROM THE NW TODAY...REACHING THE MOUNTAINS BY THIS
EVENING...AND CROSSING THE ENTIRE AREA SHORTLY AFTER MIDNIGHT.

THE ATMOSPHERE WILL DESTABILIZE AGAIN TODAY...WITH MODEST WIND
SHEAR...AND SUFFICIENT DRY AIR ALOFT FOR DOWNDRAFT PRODUCTION. MODEL
TIME HEIGHTS SHOW THE BEST LIFT BETWEEN 18 AND 21Z. A VEERING LOW
LEVEL FLOW WILL DRY OUT THE LOWER LEVELS BEFORE LIFT IS MAXIMIZED IN
THE FOOTHILLS...BUT DEEPER MOISTURE WILL REMAIN IN THE PIEDMONT...
WHERE HEAVIER RAINFALL CAN BE EXPECTED. MAXIMUM TEMPERATURES WILL BE
SLIGHTLY ABOVE NORMAL TODAY.

AS THE COLD FRONT CROSSES THE ARE THIS EVENING...WINDS WILL VEER NW.
MODEL TIME HEIGHTS SHOW SOME LOW LEVEL MOISTURE PERSISTING EAST OF
THE BLUE RIDGE...DESPITE A DOWNSLOPE FLOW...AND LIGHT RAINFALL MAY
PERSIST INTO THE EVENING. POST FRONTAL PRECIPITATION WILL BE BEST
AND MOST PROLONGED ALONG THE TN BORDER WITH NC...WHERE A PRONOUNCED
MOIST NW UPSLOPE FLOW WILL SET UP AND LINGER OVERNIGHT. MINIMUM
TEMPERATURES WILL BE NEAR NORMAL.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
AS OF 210 AM THURSDAY...AXIS OF AN UPPER TROF SWINGS ACROSS THE AREA
FRI. AT THE SFC...PRESSURE GRADIENT TIGHTENS ACROSS THE AREA AS A
COLD FRONT SLOWLY MOVES EAST AND HIGH PRESSURE SLOWLY MOVES IN FROM
THE WEST. LOW LEVEL MOISTURE LINGERS OVER THE MTNS DURING THE
MORNING...KEEPING SOME SCT RAIN SHOWERS THERE IN THE NWLY UPSLOPE
FLOW. THE SHRA DISSIPATE BY AFTERNOON AS THE MOISTURE DIMINISHES.
THE WIND AND COOLER AIR MASS WILL BE THE BIGGER STORIES. MODERATELY
STRONG LOW LEVEL WINDS MOVE IN AND MIXING SHUD BE ABLE TO TAP THOSE
WINDS DURING THE DAY. ALTHOUGH A WINDY DAY IS EXPECTED...GUSTS SHUD
REMAIN BELOW WIND ADV LEVELS. WITH THE COOLER AIR MASS MOVING IN...
HIGHS WILL BE 5 TO 10 DEGREES BELOW NORMAL.

HIGH PRESSURE WILL CONTINUE TO BUILD INTO THE ARE FRI NITE AS NWLY
FLOW DEVELOPS ALOFT. THE GUSTY WINDS SHUD COME TO AN END DURING THE
EVENING. HOWEVER...THE COLD AIR MASS WILL DROP LOWS TO AROUND 15
DEGREES BELOW NORMAL. IN FACT...TEMPS IN THE NC MTN VALLEYS WILL BE
COLD ENUF FOR FROST. IF THE WIND CAN DIMINISH ENUF...WHICH LOOKS
QUITE POSSIBLE FOR THE SWRN VALLEYS...THEN A FROST ADV WOULD BE
NEEDED.

NWLY FLOW ALOFT REMAINS OVER THE AREA SAT AND SAT NITE AS A FEW WEAK
SHORT WAVES MOVE THRU. AT THE SFC...A WEAK BACK DOOR COLD FRONT
DROPS INTO THE AREA LATE SAT OR SAT NITE. ALTHOUGH MOISTURE
INCREASES SIGNIFICANTLY AND THERE IS SOME FORCING...THE FCST REMAINS
DRY. HIGHS REMAIN NEARLY STEADY OUTSIDE OF THE MTNS...BUT REBOUND A
FEW DEGREES OVER THE MTNS SAT. LOWS SAT NITE ALSO INCREASE...BUT
REMAIN ABOUT 10 DEGREES BELOW NORMAL.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
AS OF 145 PM THURSDAY...THE 06Z GFS AND 00Z ECMWF ARE NOW SOMEWHAT
AT ODDS IN REGARD TO THE UPPER LEVEL FLOW PATTERN. THE MAIN
DIFFERENCE IS THAT THE GFS IS QUICKER MOVING THE ANOMOLOUSLY STRONG
UPPER LOW THROUGH NEW ENGLAND AND OUT OF THE PICTURE. THIS ALLOWS
THE FLOW TO DEAMPLIFY OVER THE SE BY MONDAY WITH AN UPPER RIDGE
BUILDING OVER THE REGION BY MIDWEEK. THE ECM ON THE OTHER HAND KEEPS
THE SE U.S. IN A NW FLOW REGIME TO THE EAST OF AN UPPER HIGH IT
BUILDS OVER THE LOWER MISSISSIPPI VALLEY. THIS ALLOWS WEAK IMPULSES
TO TRANSLATE ACROSS THE REGION IN THE NW FLOW MAKING FOR A RATHER
UNSETTLED PERIOD UNTIL LATE WEEK WHEN THE ECM FINALLY BUILDS THE
RIDGE TO THE EAST. THE LATEST GFS ENSEMBLE SUPPORTS THE GFS
OPERATIONAL RUN.

HPC HAS FAVORED THE ECM IN ITS MEDIUM RANGE. HOWEVER...I HAVE HEDGED
THE FORECAST TOWARD A BLEND OF OUR OFFICIAL/GFS/ECM WHICH WOULD PUT
A DIURNAL HARMONIC TO THE FORECAST FAVORING SCATTERED AFTERNOON
CONVECTION MAINLY OVER THE MOUNTAINS. THE EXCEPTION IS MON AFTERNOON
WHEN SOME SCATTERED SHOWERS/THUNDERSTORMS MAY ALSO OCCUR OVER THE NC
FOOTHILLS/PIEDMONT AS THE BACKDOOR FRONT THAT SLIPPED THROUGH THE
AREA ON SUN RETURNS NORTH AS A WARM FRONT. TEMPS WILL BEGIN THE
PERIOD A LITTLE BELOW CLIMO...THEN WARM TO ABOUT A CATEGORY ABOVE
CLIMO BY THE END OF THE WEEK.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
AT KCLT...MVFR CIG RESTRICTION SHOULD NOT LAST MORE THAN ONE OR TWO
HOURS AS THE BOUNDARY LAYER WARMS AND DEEPENS...LIFTING ANY CLOUD
BASES UP IN TO THE 035 TO 050 RANGE. THINK THAT AT LEAST A TEMPORARY
LOW CLOUD VFR CEILING WILL CONTINUE THROUGH THE AFTERNOON GIVEN
SUFFICIENT MOISTURE IN THE BOUNDARY LAYER. HAVE HELD ONTO A TWO HOUR
WINDOW OF THUNDERSTORMS PER THE 4KM WRF MODEL IN THE 21Z TO 23Z TIME
RANGE...BUT CONFIDENCE IS DROPPING. THE FRONT SHOULD FINALLY CROSS
THE REGION DURING THE MIDDLE PART OF THE EVENING WHICH WILL CLEAR
OUT THE LOW CLOUDS AND SHIFT THE WIND FROM SW TO NW...BY 04Z AT THE
LATEST. CONDITIONS SHOULD BE VFR FROM THAT POINT ONWARD. ON
FRIDAY...ONCE THE WINDS COME UP DURING MID MORNING...EXPECT FREQUENT
GUSTS FROM THE NW.

ELSEWHERE...SIMILAR CHANCES FOR MVFR CEILING UNTIL 20Z OR SO WHEN
THE BOUNDARY LAYER LIFTS THE CLOUD BASES ABOVE 030. OTHERWISE...
LESS OF A CHANCE FOR CONVECTIVE ACTIVITY. ONLY KHKY WARRANTS A
VICINITY THUNDERSTORM FOR THE TIME BEING. EXPECT VFR CONDITIONS
AFTER FRONTAL PASSAGE...WHICH SHOULD BE LATE AFTERNOON TO EARLY
EVENING. SOME UPSLOPE SHOWERS COULD OCCUR ALONG THE TN BORDER...BUT
THIS IS NOT EXPECTED TO AFFECT KAVL. WINDS SHOULD BE STRONG AND
GUSTY FROM THE NW ON FRIDAY.

OUTLOOK...VFR THROUGH THE PERIOD.

CONFIDENCE TABLE...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       HIGH  85%     HIGH 100%     HIGH 100%     HIGH 100%
KGSP       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAVL       HIGH 100%     MED   79%     HIGH  81%     HIGH  95%
KHKY       HIGH  89%     HIGH 100%     HIGH  83%     HIGH 100%
KGMU       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAND       HIGH 100%     HIGH 100%     HIGH  94%     HIGH 100%

THE PERCENTAGE REFLECTS THE NUMBER OF GUIDANCE MEMBERS AGREEING
WITH THE SCHEDULED TAF ISSUANCE FLIGHT RULE CATEGORY. COMPLETE
HOURLY EXPERIMENTAL AVIATION FORECAST CONSISTENCY TABLES AND ENSEMBLE
FORECASTS ARE AVAILABLE AT THE FOLLOWING LINK: (MUST BE LOWER CASE)

WWW.WEATHER.GOV/GSP/AVIATION_TABLES

&&

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

&&

$$
SYNOPSIS...RWH
NEAR TERM...JAT/PM
SHORT TERM...RWH
LONG TERM...RWH
AVIATION...PM