-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KGSP 232352
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
752 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 FRIDAY/...
-- Changed Discussion --
AS OF 800 PM...OVERALL COVERAGE OF SHOWERS AND THUNDERSTORMS HAS
DECREASED ACROSS THE UPSTATE AND THE NC PIEDMONT.  SOME NEW SHOWER
ACTIVITY HAS TRIED TO GET GOING ACROSS THE HIGHER TERRAINS BUT LOOKS
TO BE STRUGGLING AS DAYTIME HEATING INDUCED INSTABILITY FALLS.
LATEST SURFACE OBSERVATIONS DO SHOW A FAIRLY EVIDENT WIND SHIFT TO
THE NNW ACROSS THE HIGHER TERRAINS AS THE BELOW MENTIONED FRONTAL
BOUNDARY MAKES ITS WAY INTO THE CWA. OTHERWISE...TOUCHED UP
TEMPERATURES AND MADE NO OTHER CHANGES TO THE FCST.

REMAINDER OF PREVIOUS DISCUSSION...

THE CONVECTION WILL BE SHUNTED EAST BY THE VEERING FLOW AHEAD OF THE
MAIN COLD FRONT...WHICH IS EXPECTED TO CROSS THE FCST AREA THIS
EVENING AS A SHORT WAVE ROUNDS THE BOTTOM OF THE UPPER TROF AND
SWINGS IT EASTWARD...CARRYING MOST OF THE CLOUDS AND SHOWERS WITH
IT. EXPECT SOME LINGERING SHOWER CHANCE ON THE TN BORDER THROUGH
FRIDAY MORNING WITH MOISTURE BEING FORCED UPWARD BY THE INCREASING
NW FLOW.

ON FRIDAY...IT LOOKS LIKE A BREEZY DAY WITH A DECENT PRESSURE
GRADIENT AS HIGH PRESSURE BUILDS IN FROM THE NW. AN EVEN MORE
DYNAMIC SHORT WAVE WILL ROUND THE BASE OF THE TROF AND CARRY THE
TROF AXIS FARTHER EAST DURING THE DAY...BUT THIS WAVE HAS NO
MOISTURE TO WORK WITH THIS FAR SOUTH. MIXING WILL BE RELATIVELY
SHALLOW BECAUSE OF A STRONG INVERSION...WHICH WILL PREVENT THE
TAPPING OF THE MUCH STRONGER WINDS AROUND 850 MB. THUS WE SHOULD
AVOID ANY WIND ADVISORY ISSUES. HIGH TEMPS WILL BE A FEW CATEGORIES
COOLER THAN TODAY.
-- End Changed Discussion --

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
AS OF 210 PM THURSDAY...A STRONG UPPER LOW WILL MOVE UP THE EAST
COAST FRIDAY NIGHT AS AN UPPER RIDGE BUILDS OVER THE CENTRAL PART OF
THE COUNTRY. THIS WILL ALLOW A COOL NW FLOW TO CONTINUE ACROSS THE
SOUTHERN APPALACHIANS THROUGH THE WEEKEND.  AT THE SURFACE...A RIDGE
OF HIGH PRESSURE WITH CANADIAN ORIGIN IS FORECAST TO EXTEND FROM THE
GREAT LAKES SOUTHWARD TO FLORIDA FRIDAY NIGHT. THE HIGH WILL CONTROL
THE WEATHER THROUGH SATURDAY PRODUCING GENERALLY CLEAR AND
SEASONALLY COOL WEATHER. IN FACT...SOME SHELTERED MOUNTAIN VALLEYS
COULD SEE SOME SOME SCATTERED FROST FRI NIGHT PARTICULARLY IN THE
LITTLE TN VALLEY.

ON SUNDAY...AN UPPER IMPULSE IN THE NW FLOW ALOFT WILL CROSS THE
AREA ALONG WITH A WEAK BACKDOOR SURFACE FRONT. LOW LEVEL MOISTURE
WILL BE LACKING...BUT MODEL TIME HEIGHTS SHOW MID AND UPPER LEVEL
MOISTURE INCREASING WHICH MEANS AN INCREASE IN CLOUD COVER FOR SAT
NIGHT INTO SUNDAY. MODEL QPF RESPONSE IS QUITE MINIMAL ON SUNDAY AND
ANYTHING THAT DOES FALL WOULD PROBABLY BE JUST A FEW SPRINKLES OVER
THE MOUNTAINS.

USED A MODEL BLEND FOR TEMPS WHICH ADVERTISES WELL BELOW AVERAGE
TEMPERATURES FOR THE ENTIRE WEEKEND. MIN TEMPS FRI NIGHT WILL FALL
INTO THE UPPER 30S OVER MANY MOUNTAIN LOCATIONS WITH 40S COMMON
ELSEWHERE. MAX TEMPS WILL BE IN THE 70S...EXCEPT MID 60S TO AROUND
70 MOUNTAIN VALLEYS.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
AS OF 145 PM THURSDAY...THE 06Z/12Z 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
HOLDS STATIONARY 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 RUNS.

WPC HAS FAVORED THE ECM IN ITS MEDIUM RANGE. HOWEVER...I HAVE HEDGED
THE FORECAST TOWARD A BLEND OF OUR GSP 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. IF THE GFS SOLUTION VERIFIES...MAX
TEMPS WOULD BE ABOUT A CATEGORY HIGHER EACH DAY.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --
AT KCLT...EXPECTING VFR CONDITIONS TO PREVAIL THROUGH THE ENTIRE TAF
PERIOD.  LOOKS AS IF THERE MIGHT BE ENOUGH CONVECTIVE DEBRIS IN THE
REGION TO KEEP CIGS AT AROUND THE 12-15KFT LEVEL THROUGH MOST OF THE
EVENING BEFORE SCATTERING OUT BY DAYBREAK.  WINDS WILL SHIFT TO A
MORE NNW COMPONENT AROUND THE 04Z TIMEFRAME AND SHOULD STAY AROUND
6-7KTS GENERALLY.  OTHERWISE...WINDS WILL INCREASE AROUND 14Z FRIDAY
MORNING AS SFC PRESSURE GRADIENT TIGHTENS AND LOW LEVEL WINDS MIX
DOWN.  THUS...WENT WITH PREVAILING 12KTS WITH 25KT GUSTS FOR THE
REMAINDER OF THE TAF PERIOD.

ELSEWHERE...VFR CONDITIONS CAN BE EXPECTED AT ALL TAF SITES THROUGH
THE PERIOD AND CIGS WILL IMPROVE THROUGHOUT THE EVENING.  AS
WITH KCLT ABOVE...WINDS WILL BE THE GREATEST CONCERN ON FRIDAY AS
850MB FLOW ALOFT MIXES DOWN.  THUS...ALL SITES HAVE GROUPS WITH
PREVAILING WINDS IN THE 10-12KT RANGE WITH GUST APPROACHING 25KTS.
KAVL LOOKS TO HAVE THE HIGHEST WIND POTENTIAL AS FLOW CHANNELS
THROUGH THE FRENCH BROAD VALLEY AND IS EXPECTED TO GUST NEAR 30KTS.

OUTLOOK...VFR THROUGH THE PERIOD.

CONFIDENCE TABLE...

            00-06Z        06-12Z        12-18Z        18-24Z
KCLT       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGSP       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAVL       HIGH  98%     MED   79%     HIGH  91%     HIGH 100%
KHKY       HIGH 100%     HIGH  94%     HIGH 100%     HIGH 100%
KGMU       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAND       HIGH 100%     HIGH 100%     HIGH 100%     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
-- End Changed Discussion --

&&

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

&&

$$
SYNOPSIS...LEV
NEAR TERM...CDG/PM
SHORT TERM...LG
LONG TERM...LG
AVIATION...CDG