Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 272034
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
434 PM EDT SAT SEP 27 2014

.SYNOPSIS...
COOL HIGH PRESSURE WILL PERSIST ACROSS THE REGION TONIGHT AND WEAKEN
ON SUNDAY. A LOW PRESSURE SYSTEM WILL MOVE NORTHWARD FROM THE GULF
OF MEXICO SUNDAY NIGHT AND MONDAY AND SPREAD RAIN ACROSS THE REGION.
THE LOW SHOULD MOVE SLOWLY AWAY TO THE NORTHEAST MONDAY NIGHT AND
TUESDAY WITH COOL HIGH PRESSURE BUILDING IN FROM THE NORTHEAST ON
WEDNESDAY. A COLD FRONT WILL ARRIVE FROM THE NORTHWEST LATE IN THE
WEEK.

&&

.NEAR TERM /THROUGH SUNDAY/...
AS OF 430 PM EDT SATURDAY...MOSTLY SUNNY SKIES AND TEMPERATURES IN
THE LOWER/MID 70S CONTINUE TO PREVAIL AMONGST DRIER NORTHEASTERLY
FLOW.  ON THE CONTRARY...WEDGE BOUNDARY DRAPED SOUTH OF INTERSTATE
85 IN PROXIMITY TO THE SAVANNAH RIVER VALLEY CONTINUES TO YIELD
PARTLY/MOSTLY CLOUDY SKIES.  MADE SLIGHT TWEAKS TO THE SKY FORECAST
TO ACCOUNT FOR MOST RECENT SATELLITE AND OBSERVATIONAL TRENDS.  ALSO
ADJUSTED TEMPERATURES AND DEWPOINTS IN THE EXTREME NEAR TERM TO
REFLECT LATEST OBS FROM AROUND THE REGION.

PREVIOUS DISCUSSION...

AS OF 220 PM...DRY AIR CONTINUES TO MIX IN FROM THE NE ALIGNED IN A
PERSISTENT SFC WEDGE. THE MAJORITY OF HIGHER MOISTURE AND CLOUD
COVER HAS PUSHED OUT OF THE CWFA WITH LINGER BKN/OVC CONDS ACROSS
THE SW/RN ZONES. TEMPS WILL CONTINUE TO RISE A FEW MORE DEGREES
THROUGH MAX HEATING...REMAINING A CAT OR SO BELOW NORMAL.

A QUIET OVERNIGHT PERIOD IS IN STORE WITH A SUBTROPICAL HIGH
REMAINING OVER THE SE CONUS. RAP SOUNDINGS ARE SHOWING A DECOUPLED
ATMOS OVERNIGHT...ALLOWING MINS TO REACH NORMAL LEVELS NORTH AND A
COUPLE DEGREES ABOVE NORMAL SOUTH WITH LINGERING CLOUDS. THERE COULD
BE SOME MTN VALLEY FG DEVELOPING ARND SUNRISE...BUT TDDS WILL REMAIN
FAIRLY HIGH FOR MUCH OF A DENSE THREAT.

LOW TO MID LEVEL ATL MOISTURE BEGINS PUSHING IN EARLY SUN MORNING
AND THE SRN ZONES WILL SEE THE BEST CLOUD COVER AFT DAYBREAK. THE
NAM IS MORE AGGRESSIVE WITH THIS SCENARIO THAN THE GFS...WHICH KEEPS
THE DEEPER MOISTURE ALIGNED ALONG THE WEDGE BNDRY. SRN FA SOUNDINGS
SHOW NEARLY SATURATED COLUMNS BY THE AFTERNOON AS A WEAK COASTAL LOW
BEGINS ADVECTING GOM MOISTURE INTO THE REGION. STILL...THE MAIN FLOW
WILL BE ATL...WHICH WILL INSTIGATE GOOD MECH LIFT ACROSS THE SRN
ESCARPMENT...NE GA AND SW NC MTNS BY THE AFTERNOON. POPS WILL RISE
QUICKLY INTO THE HIGH CHANCE AND LOW/END LIKELY RANGE SUN AFTERNOON
ACROSS THE FAR WEST...HOWEVER PRECIP AMOUNTS WILL REMAIN FAIRLY LOW
THROUGH THE NRT PERIOD AS INSTABILITY REMAINS LOW KEEPING CONVEC
PRECIP WELL SOUTH. POPS WILL REMAIN NIL OUTSIDE THE MTNS AND SW/RN
ZONES. MAX TEMPS WILL FEATURE A SW/NE COOL/WARM GRADIENT DUE TO
CLOUD COVER AND DIFF HEATING.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
AS OF 245 PM SATURDAY...AN H5 SHORTWAVE WILL LIFT FROM THE GULF
COAST TO THE EXTREME SRN APPALACHIANS SUNDAY NIGHT AND MERGE WITH A
NRN STREAM SHORTWAVE SWINGING EAST THROUGH THE OH VALLEY. AT LOWER
LEVELS...AN INCREASINGLY MOIST SOUTHEAST UPSLOPE FLOW WILL SET UP
OVER THE FORECAST AREA SUNDAY NIGHT TO ALLOW PRECIPITATION TO BECOME
WIDESPREAD BY DAYBREAK MONDAY...ACROSS AT LEAST THE SW HALF OF THE
AREA. DEEP LAYER FORCING WILL THEN CROSS THE REGION MON WITH THE
MERGING UPPER WAVES AND PASSING UPPER LEVEL JET DIVERGENCE. AN 850
MB LOW CENTER WILL LIKELY RIDE NE THROUGH THE SOUTHEAST PART OF THE
FORECAST AREA...SETTING UP ADDITIONAL WEAK FRONTOGENESIS OVER THE
AREA. THESE FEATURES WILL BE VERY SLOW TO MOVE E OR NE TO THE COAST
MON NIGHT THROUGH TUE...WITH CONTINUED LIGHT PRECIPITATION WRAPPING
BACK OVER THE REGION ON THE W SIDE OF THE LOW.

THE MAIN LIMITING FACTORS FOR HEAVY QPF ARE A FAIRLY DRY BOUNDARY
LAYER IN THE NC PIEDMONT AT ONSET...AS WELL AS THE STRONG POTENTIAL
FOR GULF COASTAL CONVECTION AND CONVECTION NEAR A WEAK WEDGE
BOUNDARY IMPEDING THE INFLUX OF MOISTURE. SO..THE QUESTION IS NOT
WHETHER IT WILL RAIN...BUT HOW MUCH. THE 12Z GFS IS PRODUCING A
BULLSEYE OF NEARLY 6 INCHES ALONG THE INTERSTATE 85 CORRIDOR...BUT
IT FEATURES LITTLE TO NO GULF COASTAL CONVECTION. THIS SOLUTION
SEEMS UNLIKELY...AND THE FORECAST WILL CONTINUE TO LEAN HEAVILY ON A
MODEL BLEND WITH A HEAVY NOD TOWARD THE DRIER SREF MEAN AND LATEST
ECM QPF. IN ADDITION...WITH A WEAK SURFACE CAD LAYER PERSISTING OR
REDEVELOPING AS THE RAIN ARRIVES...INSTABILITY SHOULD BE LARGELY
SHUNTED SOUTH OF THE REGION FOR THE UPCOMING EVENT. WITH 850 TO 500
MB LAPSE RATES RATHER WEAK OVER THE SFC HIGH...RAINFALL RATES SHOULD
NOT BE HIGH ENOUGH TO SUPPORT FLASH FLOODING. DURATION RAINFALL
ALONG MAIN STEMS AND THE MOST FLOOD PRONE AREAS WILL NEED TO BE
MONITORED...BUT ANTECEDENT CONDITIONS ARE UNFAVORABLE FOR MUCH OF A
HYDRO THREAT GIVEN EXPECTED QPF/DURATION. STILL...AN HWO MENTION
SEEMS WARRANTED FOR AT LEAST THE HEAVY RAINFALL MONDAY. A SMALLER
THAN MOS DIURNAL RANGE ON TEMPS WILL BE FEATURED WITH CAD CONDITIONS.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
AS OF 230 PM EDT SAT...A WEAK UPPER RIDGING WILL BUILD IN FROM THE
WEST TUE NIGHT INTO WED AS A DEAMPLIFYING SHORTWAVE PUSHES OFF THE
ATLANTIC COAST. THE UPPER RIDGE AXIS SHOULD CROSS THE REGION
THU...WITH SWLY FLOW ALOFT SETTING UP BY FRI IN LIGHT OF A DEEP
UPPER TROUGH APPROACHING FROM THE CENTRAL CONUS. THIS TROUGH IS
EXPECTED TO MOVE ACROSS THE EASTERN STATES LATE SAT INTO SUN.

AT THE SURFACE...A SURFACE WEDGE WILL SET UP ACROSS THE REGION BY
TUE NIGHT AND PERSIST THROUGH THU...YIELDING CONSIDERABLE LLVL
CLOUDINESS AND COOL NE WINDS DURING THIS PERIOD. NE WINDS SHOULD
VEER TO THE SE BY THU NIGHT AS THE SURFACE RIDGE WEAKENS AND SHIFT
EAST WHILE AN ACTIVE COLD FRONT APPROACHES FROM THE MISSISSIPPI
VALLEY. BOTH THE NEW 12Z GFS/ECMWF AGREE THAT THE FRONT WILL REACH
THE SOUTHERN APPALACHIANS BY AROUND SAT 00Z AND OFF TO OUR EAST BY
AROUND 12Z SAT. THE THREAT FOR SEVERE WEATHER LOOKS MINIMAL GIVEN
THE LATE OF GOOD UPPER FORCING/BOUNCY. IN FACT...MODELS HAVE TRENDED
A BIT DRIER AS THE FRONT WEAKENS AS IT CROSS THE FORECAST AREA.
HENCE...HAVE INHERITED LOW END CHANCE POPS DURING THE FROPA.
CONDITIONS SHOULD DRY OUT BY SAT AFTERNOON AS THE FRONT PUSHES
OFFSHORE AND A COOL DRY HIGH PRESSURE BUILDS IN FROM THE NW.
TEMPERATURES WILL REMAIN 3-7 DEGREES BELOW NORMAL THROUGHOUT THE
MEDIUM RANGE PERIOD.

&&

.AVIATION /21Z SATURDAY THROUGH THURSDAY/...
AT KCLT...VFR CONDS EXPECTED THROUGH THE PERIOD. FEW TO SCT CU WILL
PERSIST THIS AFTERNOON...BUT REMAIN ABV 030. LOW TO MID LEVEL ATL
MOISTURE FLUX WILL COMMENCE OVERNIGHT WHICH WILL BRING IN
CLOUDS...BUT NOT ENOUGH FOR A RESTRICTIVE CIG MENTION. THESE CLOUDS
WILL ALSO OFFSET MUCH A VSBY CONCERN ARND DAYBREAK.

ELSEWHERE...VFR CONDS WILL PERSIST THROUGH MOST OF THE TAF PERIOD
WITH LINGER LOW VFR CU THROUGH THE AFTERNOON. ATL MOISTURE FLUX WILL
BRING IN MVFR CLOUDS FAIRLY LATE...ARND DAYBREAK WHICH SHOULD AFFECT
ALL TERMINALS THROUGHOUT THE MORNING...WITH CIGS LIFTING TO VFR
PROBABLE NEAR THE END OF THE TAF PERIOD.

OUTLOOK...HIGH LEVELS MOISTURE WILL RETURN FOR THE START OF THE WORK
WEEK ASSOCIATED WITH AN AREA OF UNSETTLE WEATHER MOVING NORTH OUT OF
THE GULF OF MEXICO. PERIODS OF CONTINUOUS MODERATE TO HEAVY RAINFALL
WILL AFFECT THE AREA MON/TUE PRODUCING LOW CIGS AND VSBY.

CONFIDENCE TABLE...

            20-02Z        02-08Z        08-14Z        14-20Z
KCLT       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGSP       HIGH 100%     HIGH 100%     HIGH  94%     MED   69%
KAVL       HIGH 100%     HIGH  88%     MED   67%     MED   60%
KHKY       HIGH 100%     HIGH 100%     HIGH  84%     MED   75%
KGMU       HIGH 100%     HIGH 100%     HIGH  94%     MED   69%
KAND       HIGH 100%     HIGH 100%     HIGH  93%     MED   78%

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...JOH
NEAR TERM...CDG/SBK
SHORT TERM...HG
LONG TERM...JOH
AVIATION...SBK






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