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Area Forecast Discussion

761
FXUS62 KGSP 042049
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
449 PM EDT FRI SEP 4 2015

.SYNOPSIS...
A COOL AND MOIST WIND FLOW OFF THE ATLANTIC THIS WEEKEND WILL KEEP
CONSIDERABLE CLOUDINESS AND COOLER TEMPERATURES FOR OUR REGION.
EXPECT GRADUAL WARMING EARLY TO MIDDLE OF THE UPCOMING WEEK UNDER AN
UPPER RIDGE. A COLD FRONT AND UPPER TROUGH WILL ARRIVE AT THE END OF
THE WEEK.

&&

.NEAR TERM /THROUGH SATURDAY/...
AS OF 430 PM...MADE SOME UPDATES TO SKY/POPS/WX BASED ON CURRENT
TRENDS. SEEING SOME NON-SEVERE DOWNBURSTS WITH THESE STORMS...
ESPECIALLY WITH THE AID OF THE TCLT TDWR. ISOLATED SEVERE STILL
DEFINITELY POSSIBLE...HAVE SEEN SOME REPORTS OF SEVERE WINDS AND
HAIL TO OUR SOUTH WHERE CAPE/DCAPE VALUES ARE A LITTLE HIGHER.
TRENDS INTO THE EVENING STILL LOOK GOOD BUT WILL UPDATE AS NECESSARY
THROUGH THE EVENING.

AS OF 230 PM...SATELLITE IMAGERY SHOWS ENHANCED CU OVER THE
RIDGETOPS AND RADAR IMAGERY SHOWS THAT DEEP CONVECTION HAS INITIATED
ACROSS PARTS OF NC AND ALONG THE BLUE RIDGE IN VA...AND DOWN ACROSS
THE AREA NEAR THE FALL LINE. COVERAGE SHOULD GRADUALLY EXPAND FROM
THERE. STILL THINK WE COULD SEE A FEW STORMS BECOME STRONG ENUF TO
PRODUCE DAMAGING WIND GUSTS NOW THAT WE HAVE REALIZED THE EXPECTED
ENVIRONMENT WITH DCAPE 1000+ J/KG. TEMPS LOOK GOOD.

THE NEXT 24 HOURS SHOULD SEE THE DEVELOPMENT OF A PSEUDO-WEDGE-LIKE
SCENARIO ACROSS THE WESTERN CAROLINAS. THE UPPER AIR PATTERN REMAINS
CONVOLUTED WITH AN OVERALL HIGH AMPLITUDE RIDGE ACROSS EASTERN NORTH
AMERICA...BUT WITH AN UPPER LOW CENTERED OFF THE SOUTHEAST COAST AND
WEAK CYCLONIC FLOW AT MID LEVELS ACROSS THE FCST AREA. WITHIN THIS
FLOW...A LARGE SURFACE HIGH WILL BE SUPPORTED TO OUR NE...LOOSELY
ANCHORED OVER NEW ENGLAND AND THE MARITIME PROVINCES. WHAT THIS DOES
IS PROVIDE A LOW LEVEL NE FLOW...WHICH IS NOT REALLY DAMMED ACROSS
THE AREA TO OUR N. HOWEVER...THE EASTERLY FLOW AND THE DEVELOPMENT
OF CONVECTIVE PRECIP ACROSS THE PIEDMONT AND FOOTHILLS OF VA AND NRN
NC MAY FORM A COOL POOL THAT OOZES SOUTH AND SOUTHWESTWARD
OVERNIGHT...PERHAPS ALONG CONVECTIVE OUTFLOWS. THIS IN EFFECT WILL
FORM A SORT OF IN-SITU WEDGE INTO THE FCST AREA...WHICH SHOULD
GRADUALLY PUSH S AND SW INTO SATURDAY MORNING. THE PROBLEM IS THE
E/NE FLOW IS FAIRLY DEEP AND THERE WILL NOT BE ANY ISENTROPIC LIFT
OR LLVL WARM ADVECTION OVER THE TOP OF A STABLE BOUNDARY LAYER TO
MAINTAIN IT. AS A RESULT...ON SATURDAY WE ARE LEFT WITH BASICALLY A
COOL AND CLOUDY NE FLOW NEAR THE SFC. THE MODELS PUSH PRECIP
PRODUCTION DOWN INTO NE GEORGIA AND ACROSS THE MTNS BY AFTERNOON.
WILL HEDGE AND KEEP AT LEAST A SLIGHT CHC OVER THE WRN NC PIEDMONT.
THIS WILL MAKE THE HIGH TEMP FCST TRICKY. NOT CONVINCED TEMPS WILL
REMAIN BELOW GUIDANCE...SO HAVE NUDGED THEM UP A BIT OVER THE
PIEDMONT.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
AS OF 200 PM FRI...LITTLE IMPACTFUL CHANGE IS EXPECTED IN THE
OVERALL SFC/UPPER PATTERN OVER THE PERIOD. HIGH PRESSURE OVER THE
NE CONUS WILL WEAKEN SLOWLY AND MOVE OFFSHORE UNDER THE INFLUENCE
OF FALLING HEIGHTS OVER THE MIDWEST. NORTHEASTERLY TO EASTERLY
FLOW WILL CONTINUE INTO THE CAROLINAS ON THE SRN PERIPHERY OF
THE HIGH. THE MAIN EFFECT WILL BE TO MAINTAIN LOW TO MID LEVEL
MOISTURE TRANSPORT FROM THE ATLANTIC...BUT ONLY SLIGHT UPSLOPE
FORCING DUE TO THE WEAK WINDS. POPS ARE WARRANTED MAINLY ON ACCOUNT
OF INSTABILITY...BUT THE USUAL MODEL DIFFERENCES BETWEEN THE GFS
AND NAM ARE SEEN. THE NAM DEPICTS THE EASTERLY WINDS MAINTAINING
A MIXED LAYER ATOP THE BOUNDARY LAYER...FROM WHICH PARCELS WILL
REMAIN BUOYANT OVERNIGHT. GFS SHOWS THIS BEING LESS EFFECTIVE AND
ALSO SHOWS POORER MIDLEVEL LAPSE RATES. NEVERTHELESS SLIGHT CHANCE
NOCTURNAL POPS WITH SLIGHT FAVORITISM TOWARD THE BLUE RIDGE ARE
APPROPRIATE. AFTERNOON CAPE IS EXPECTED TO BE GREATER SUNDAY THAN
MONDAY AND SO THE HIGHEST OVERALL POPS ARE INCLUDED THEN.

MAX TEMPS WILL BE A DEGREE OR TWO BELOW CLIMO SUNDAY BUT REBOUND
TO ABOUT CLIMO ON MONDAY WITH AIRMASS MODIFICATION. MIN TEMPS WILL
BE SLIGHTLY ABOVE CLIMO WITH NOCTURNAL CLOUD COVER.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
AT 150 PM EDT FRIDAY...STARTING MONDAY NIGHT THE 500MB PATTERN HAS
A SHALLOW RIDGE OVER THE SOUTHEAST STATES WITH A TROUGH AXIS OVER THE
WESTERN DAKOTAS AND PROGRESSING EAST IMPINGING ON THE SE RIDGE.  THE
RIDGE WILL FLATTEN OUT THROUGH MID WEEK THEN THE TROUGH DIGS OVER
THE SE WITH AXIS BECOMING ESTABLISHED JUST WEST OF THE APPALACHIANS
BY LATE FRIDAY.  RIDGE WILL BUILD AT WEEKS END OVER THE FAR WEST.

AT THE SURFACE...HIGH PRESSURE WILL BE OVER THE CAROLINAS AND MID
ATLANTIC TUESDAY THEN DRIFTS EAST AS COLD FRONT CROSSES THE MISS
VALLEY. THIS COLD FRONT REACHES THE OHIO VALLEY BY THURSDAY AND WILL
BE CROSSING OUR AREA BY FRIDAY.  SCATTERED CONVECTION WILL INCREASE
AS THE FRONT GETS CLOSER LATE IN THE WEEK.

OVERALL...THE INSTABILITY WILL BE MODEST AT BEST.  CAPE
VALUES AROUND 1000 ON THE GFS FOR PIEDMONT GA AND SC LATE
TUESDAY...WEAKER VALUES WED AND 700 TO 1000 THURSDAY AND FRIDAY.
WIND SPEEDS IN THE LOW TO MID LEVELS NOT EXCESSIVE AS H8GHER WINDS
AT THE END OF THE WEEK IN THE TROUGH.  EVEN THEN THE 700MB WIND WILL
BE AROUND 20 KTS WHICH IS FAIRLY LIGHT.

TEMPS A LITTLE ABOVE NORMAL IN MID WEEK...DIPPING TO A LITTLE BELOW
NORMAL NEXT WEEKEND WITHIN THE TROUGH.

&&

.AVIATION /21Z FRIDAY THROUGH WEDNESDAY/...
AT KCLT...ADJUSTED THE TEMPO TIMING TO START AT 20Z WITH
A PREVIOUS AMD AS SEVERAL TSRA ARE DEVELOPING AROUND THE
AIRPORT. LOW-END DOWNBURSTS EVIDENT ON TDWR SO ADDED VRB GUSTS
TO THE TEMPO. CONVECTIVE INTENSITY DOES LOOK LIKE IT IS DROPPING
SOMEWHAT SO WILL TAKE THE TEMPO THROUGH 23Z AND MAKE ADDITIONAL
ADJUSTMENTS THEREAFTER BASED ON TRENDS. AFTER A ROUND OF STORMS
MOVES THRU THE AREA...WILL KEEP ONLY CONVECTIVE DEBRIS ACROSS
THE REGION OVERNIGHT...WITH A LIGHT NE WIND. THE MODEL GUIDANCE
SUGGESTS BRINGING A LOW CLOUD DECK DOWN FROM THE NE IN THE EARLY
MORNING HOURS. A SIMILAR EVOLUTION OCCURS IN MOST SITUATIONS LIKE
THIS...WITH PLENTY OF MOISTURE...AND EASTERLY COMPONENT TO THE LOW
LEVEL FLOW...WITH HIGH PRESSURE TO THE NE. THE LAMP GUIDANCE WAS
GENERALLY ACCEPTED... BUT CEILINGS WERE KEPT TO MVFR ACROSS THE WRN
PIEDMONT OF NC. LATER TAFS WILL HAVE TO ADDRESS THE POSSIBILITY
OF IFR CEILINGS AROUND DAYBREAK...BUT FOR NOW IT IS MORE LIKELY
TO STAY MVFR. ON SATURDAY...LOW CLOUD BASE WILL GRADUALLY RISE TO
VFR AS BOUNDARY LAYER DEEPENS. ADDITIONAL SHOWERS OR STORMS SHOULD
DEVELOP WITH HEATING AROUND MIDDAY.

ELSEWHERE...SCATTERED STORMS WILL FORM FIRST ACROSS THE HIGHER
TERRAIN AND THEN SLOWLY DEVELOP DOWN ACROSS THE REST OF THE FCST
AREA THRU EARLY EVENING. CONFIDENCE ONLY HIGH ENUF AT KAVL/KHKY TO
INCLUDE A TEMPO FOR NOW. THE UPSTATE SC TAF SITES WERE LIMITED TO
VCTS/PROB30 BASED ON LACK OF COVERAGE IN CONVECTION ALLOWING MODELS.
THERE COULD BE A ROUND OF SHOWERS IN THE EARLY MORNING SATURDAY AS
A BACK DOOR SFC BOUNDARY MOVES DOWN FROM THE NE...BUT CONFIDENCE
NOT HIGH ENUF TO MENTION. EXPECT VALLEY FOG TO FORM AT KAVL IN THE
PRE- DAWN HOURS...MOST LIKELY RESTRICTING VSBY TO MVFR...UNLESS
PRECIP FALLS THERE TODAY. THINK LOW CLOUD CEILING WILL REACH KHKY
SATURDAY MORNING...BUT GET HUNG UP MOVING SW AS SUN STARTS TO
WARM ATMOSPHERE AFTER 12Z. WIND SHOULD REMAIN N TO NE THRU THE
PERIOD. MORE SHOWERS CAN BE EXPECTED WITH HEATING ON SATURDAY.

OUTLOOK...A MOIST EASTERLY FLOW WILL CONTINUE THRU SUNDAY...BRINGING
A GOOD CHANCE OF IFR/MVFR RESTRICTIONS SATURDAY NIGHT AND SUNDAY
MORNING. LIGHT PRECIP WILL REMAIN POSSIBLE. THE THREAT FOR
RESTRICTIONS WILL CONTINUE THRU SUNDAY NIGHT. DRY AIR WORKING IN
FROM THE E ON MONDAY WILL BEGIN TO DIMINISH THE THREAT FOR EARLY
NEXT WEEK.

CONFIDENCE TABLE...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       HIGH 100%     HIGH 100%     MED   65%     HIGH  85%
KGSP       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAVL       HIGH 100%     HIGH 100%     HIGH  80%     HIGH  95%
KHKY       HIGH 100%     HIGH 100%     MED   66%     HIGH  83%
KGMU       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAND       HIGH 100%     HIGH 100%     HIGH  83%     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

&&

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

&&

$$
SYNOPSIS...DEO
NEAR TERM...PM/TDP
SHORT TERM...WIMBERLEY
LONG TERM...DEO
AVIATION...PM/TDP



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