Flash Flood in North Carolina, September 5, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
827 PM EDT WED SEP 5 2012

.SYNOPSIS...
DEEP TROPICAL MOISTURE CURRENTLY OVER THE REGION WILL BEGIN TO
PROGRESSIVELY LESSENING THURSDAY INTO FRIDAY. A STRONG COLD FRONT
WILL MOVE OVER THE CAROLINAS THIS WEEKEND FOLLOWED BY HIGH PRESSURE
THAT WILL PROVIDE COOLER AND DRYER WEATHER INTO THE FIRST OF NEXT
WEEK.

&&

.NEAR TERM /THROUGH THURSDAY/...
830 PM EDT UPDATE...NO MAJOR CHANGES NEEDED TO THE GOING FCST. POPS
STILL LOOK GOOD OVER THE NEXT COUPLE HRS...BUT PROBABLY WILL BUMP UP
THE WRN NC MTN ZONES WITH THE OVERNIGHT UPDATE AS AN ACTIVE S/W
APPROACHES THE CWFA FROM THE NW.

530 PM EDT UPDATE...MADE LOWERING CHANGES TO THE POPS/QPF BASED ON
RADAR TRENDS WV IMAGERY AND WRF/ARW OUTPUT. VERY WEAK FORCING ACROSS
THE NRN ZONES AND ACTUAL SUBSIDENCE SOUTH ACROSS THE UPSTATE AND NE
GA HAVE PRECLUDED A SIGFNT CONVECTIVE THREAT IN STRENGTH AND
SPATIALLY THIS EVENING FOR AREAS OUTSIDE THE CENT/NRN NC MTNS AS
WELL AS AREAS SOUTH OF I40. MADE COSMETIC CHANGES TO THE SKY GRIDS
AND MINOR TWEAKS TO TEMPS/TDS...MAINLY ACROSS THE NC MTNS.

THE LATEST WATER VAPOR IMAGERY SHOWS DRIER MID LEVEL AIR SPREADING
ACROSS THE FORECAST AREA SOUTH OF INTERSTATE 40. THE MID LEVEL FLOW
IS QUITE WEAK OVER THE UPSTATE...NE GA AND FAR SRN NC AS A SPUR OF
DEEP LAYER RIDGING HAS PUSHED INTO THE FA FROM THE WEST...FINALLY
DISPLACING THE WEAK...MOIST TROUGH THAT HAD BEEN OVER THE REGION FOR
THE PAST SEVERAL DAYS. THE RAP BRINGS A WEAK SHORT WAVE ACROSS
WESTERN NC THIS EVENING. LLVL CONVERGENCE IS INCREASING OVER THE NC
MTNS AND FOOTHILLS AHEAD OF THE WAVE. THE MODELS ARE SPLIT ON
HOW FAR SOUTHEAST THIS CONVECTION GETS. THE GFS KEEPS IT OVER THE
MTNS AND FOOTHILLS...WHILE THE RAP AND NAM PUSH IT INTO THE NRN
SUBURBS OF CHARLOTTE. WITH THE FINGER OF DRIER AIR PUSHING INTO THE
SRN PIEDMONT I/VE GONE WITH A FORECAST THAT KEEPS THE BEST
CONVECTIVE CHANCES OVER THE MTNS AND FOOTHILLS WITH LOWER POPS ALONG
AND SOUTH OF I-85 AND BACK ACROSS THE SW NC MTNS. THIS IS NOT TO SAY
THAT STORMS WON/T PUSH INTO CHARLOTTE LATER THIS EVENING AS THE
ATMOSPHERE IS NOT CAPPED AND IS MODERATELY UNSTABLE. HOWEVER...THIS
DOES NOT LOOK LIKE THE MOST PROBABLE SOLUTION ATTM.

PATCHY DENSE VALLEY FOG IS EXPECTED OVER THE MTNS TONIGHT. ACROSS
THE FOOTHILLS AND PIEDMONT CONSIDERABLY LESS LOW CLOUD COVER IS
ANTICIPATED OWNING TO A MORE WESTERLY COMPONENT TO THE LLVL WIND
FIELD.

A SHORT WAVE CURRENTLY OVER CENTRAL MISSOURI IS EXPECTED TO CROSS
THE FORECAST AREA DURING THE AFTERNOON HOURS ON THURSDAY. THE DEEP
LAYER RH IS CONSIDERABLY HIGHER ON THU THAN TODAY ALONG AND AHEAD OF
THIS WAVE. BY 21 UTC THE FEATURE IS EXPECTED TO BE OVER THE I-77
CORRIDOR AND IT WILL MOVE EAT OF CHARLOTTE BY THE EARLY EVENING. THE
NAM AND GFS BOTH DEVELOP PCPN BY LATE MORNING AHEAD OF THE SHORT
WAVE. THE NAM HAS PCPN ENDING WITH WEAKLY DIVERGENT LLVL FLOW OVER
MOST OF THE FA DEVELOPING BETWEEN 21-00 UTC. THE WAY IT APPEARS
NOW...A BAND OF SHRA AND A FEW TSTMS WILL DEVELOP DEVELOP OVER THE
FOOTHILLS BY LATE MORNING...OR THEY MAY MOVE INTO THE MTNS AS A
REMNANT OF A WEAKENING MCS. THE CONVECTION WILL MOVE ACROSS THE REST
OF THE FOOTHILLS FROM EARLY TO MID AFTN AND EXIT TO THE EAST BY 6 OR
7 PM. THE SHEAR IS WEAK AND CONVECTION ISN/T EXPECTED TO BE HIGHLY
ORGANIZED AND MOST OF THE STORMS SHOULD BE SUB-SEVERE. THIS IS
GENERALLY SUPPORTED BY THE 12 UTC 4KM SPC WRF AS WELL. FOR NOW I/LL
CAP POPS ARE 40 PERCENT...WITH POPS FALLING OFF QUICKLY TOWARD
SUNSET.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
AS OF 2 PM EDT WEDNESDAY...THE CONSENSUS OF THE MODEL GUIDANCE
MAINTAINS THAT THE PLUME OF DEEPER MOISTURE WILL HAVE SHIFTED
EASTWARD TO THE COASTAL PLAIN. IN THE WAKE OF A WEAK UPPER WAVE...
THE ENVIRONMENT SHOULD BE LESS FAVORABLE FOR DEEP CONVECTION. THE
CHANCE FOR SHOWERS AND STORMS SHOULD DROP IN THE EARLY EVENING AS
THE FORCING AND MOISTURE MOVE OFF TO THE EAST...TO THE POINT WHERE
THE FCST IS DRY EAST OF THE MTNS FROM 03Z ONWARD. THERE COULD BE A
FEW LINGERING SHOWERS OVER THE MTNS UNTIL LATE EVENING.

FRIDAY LOOKS LIKE A RELATIVELY QUIET TRANSITION DAY WHILE WE WAIT
FOR THE FORCING AND MOISTURE TO RELOAD. A STRONG SHORT WAVE OVER THE
UPPER MS VALLEY WILL DIG AN UPPER TROF DOWN OVER THE UPPER MIDWEST
AND GREAT LAKES THROUGH THE DAY...WHICH WILL GRADUALLY TURN THE FLOW
ALOFT MORE SW. UPPER FORCING WILL BE MINIMAL. THERE SHOULD BE ENOUGH
LOW LEVEL MOISTURE AND BUOYANCY TO SUPPORT SCATTERED SHOWERS AND
STORMS DEVELOPING OVER THE HIGHER TERRAIN WITH DIFFERENTIAL
HEATING...WITH SOME OF THE STORMS MOVING OUT OVER THE FOOTHILLS AND
PIEDMONT IN THE AFTERNOON AND EARLY EVENING. THE STORMS EAST OF THE
BLUE RIDGE WILL DIE OFF WITH SUNSET...BUT WILL KEEP THE CHANCE OF
PRECIP GOING OVERNIGHT ON THE W SIDE OF THE MTNS IN CASE SOME
ACTIVITY MOVES IN FROM THE WEST AHEAD OF THE NEXT FRONT.

THE UPPER TROF CONTINUES TO DIG OVER THE MIDWEST/OH VALLEY ON
SATURDAY AS THE WAVE DROPS SOUTHEASTWARD. DEEP MOISTURE MOVES IN
AHEAD OF THE WAVE TO BE ACTED UPON BY IMPROVED UPPER FORCING AS WE
WORK THROUGH THE DAY. THE OPERATIONAL RUNS FROM ALL THREE MODELS ARE
IN FAIRLY GOOD AGREEMENT WITH THE TIMING OF THE MAIN VORT AXIS
REACHING THE GREAT VALLEY LATE IN THE DAY. THE NEW GFS SWEEPS THE
UPPER TROF AXIS ACROSS THE WESTERN CAROLINAS OVERNIGHT WHILE THE 00Z
RUN PUSHES IT THROUGH SUNDAY MORNING. EITHER WAY...BOTH MODELS
SUGGEST AN EARLIER TIMING OF THE MAIN FRONTAL PRECIP BAND. WILL
LIMIT THE PRECIP TO THE CHANCE RANGE FOR THE TIME BEING BUT WILL
PROBABLY NEED A LIKELY POP FOR AT LEAST THE MTNS...BUT IT IS A
MATTER OF WHICH PERIOD TO FOCUS ON...SATURDAY AFTERNOON OR SATURDAY
NIGHT. WILL NOT RULE OUT SOME SEVERE STORMS IF THE FRONT WILL MOVE
THROUGH EARLY ENOUGH. AT THIS POINT...THINK MOST OF THE PRECIP WILL
HAVE MOVED EAST OF THE FCST AREA BY SUNRISE ON SUNDAY...WHICH AGREES
WITH THE HPC POSITION OF THE MAIN FRONT ON THE COAST AT THAT TIME.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
AS OF 1130 AM EDT WEDNESDAY...AFTER DEALING WITH A SEMI-TROPICAL AIR
MASS FOR THE LAST TWO WEEKS...IT APPEARS AS THOUGH WE ARE IN FOR A
PERIOD OF SPECTACULAR EARLY AUTUMN-LIKE WEATHER EARLY NEXT WEEK. BUT
FIRST...WE HAVE TO GET A FRONT THROUGH THE CAROLINAS EARLY ON
SUNDAY. THE GFS MOVES THE MAIN SHORT WAVE LOBE ACROSS THE WESTERN
CAROLINAS EARLY SUNDAY MORNING WHILE THE ECMWF HAS THIS FEATURE
CROSSING SUNDAY AFTERNOON. THE TIMING IS FASTER THAN PREVIOUSLY
INDICATED...WITH THE MAIN SFC FRONT POTENTIALLY MOVING ACROSS THE
AREA SATURDAY NIGHT. WILL BACK OFF TO A SLIGHT CHANCE ACROSS THE
REGION ON SUNDAY AS A FEW SHOWERS COULD BE TRIGGERED BY A SECONDARY
FRONT. EITHER WAY...THERE IS A STRONG INDICATION THAT THE PRECIP
WILL BE LONG GONE BY SUNSET SUNDAY...SO THE PRECIP CHANCES ARE
ELIMINATED ALTOGETHER AFTER 00Z MONDAY.

THE UPPER TROF AXIS WILL REACH THE EASTERN SEABOARD BY MONDAY
MORNING AND MOVE SLOWLY OFFSHORE THROUGH MIDWEEK...WHILE AN UPPER
ANTICYCLONE REMAINS PARKED OVER THE SRN PLAINS. THE RESULTING UPPER
RIDGE TO OUR NW WILL BRING DRY HIGH PRESSURE BUILDING IN SUNDAY
NIGHT AND MONDAY. ONCE ESTABLISHED...THIS SURFACE HIGH WILL DOMINATE
OUR WEATHER AS IT MOVES SLOWLY PAST TO THE NORTH THROUGH WEDNESDAY.
THIS SHOULD BRING MOSTLY CLEAR SKY...DRY AIR...HIGH TEMPS IN THE
MID/UPPER 70S AND LOW TEMPS IN THE MID/UPPER 50S. THUS...MONDAY AND
TUESDAY WILL FEEL MORE LIKE LATE SEPTEMBER OR EARLY OCTOBER. THE AIR
MASS SHOULD START TO MODIFY A BIT BY WEDNESDAY.

&&

.AVIATION /01Z THURSDAY THROUGH MONDAY/...
AT KCLT...A VAST AREA OF DEEP SUBSIDENCE WILL PERSIST ACROSS THE
TERMINAL THIS EVENING AND INTO THE OVERNIGHT. THIS HAS PREVENTED
MUCH IF ANY PCPN OR CLOUDS...LET ALONE CONVECTION AROUND THE AREA.
BELIEVE THIS WILL BE MAINTAINED THROUGH THE OVERNIGHT TO A HIGH
DEGREE. THERE WILL BE SOME APPROACHING MLVL ENERGY AND MOISTURE
OVERNIGHT...BUT THIS WILL HAVE TO MAKE IT OVER THE MTNS AND THROUGH
THE AFOREMENTIONED SUBSIDENCE ZONE. SO...DONT ANTICIPATE A SIGFNT
STORM/PCPN THREAT OVERNIGHT. THERE WILL BE ENOUGH MOISTURE TRAPPED
BENEATH THE INVERSION TO ALLOW STCU TO DEVELOP BEFORE DAYBREAK AND
ALL GUIDANCE POINTS TOWARD A MVFR DECK MOST LIKELY PERSISTING TIL
15Z. THE AFTERNOON CONVECTIVE THREAT REMAINS A REAL POSSIBILITY. THE
GREAT VALLEY S/W WILL BE CROSSING THE PIEDMONT NEAR MAX HEATING SO A
PROB390 TS WITH LIMITING VSBY -SHRA IS EXPECTED AFT 18Z.

ELSEWHERE...ONGOING ISOL/SCT CONVECTION WILL PERSIST ACROSS THE
NW/RN ZONES TONIGHT AND INTO THE EARLY MORNING HRS. ENOUGH SO...THAT
VCTS WAS INCLUDED AT KAVL AND KHKY TIL 03Z. THE MTNS COULD
EXPERIENCE A PROLONGED PERIOD OF SCT PRECIP OVERNIGHT WITH AN
APPROACHING S/W AND AN AMENDMENT MAY BE NEEDED AROUND MIDNIGHT.
WITH THE PRECIP MAINLY ACROSS THE HIGHER TERRAIN TODAY...THE NC MTN
VALLEYS AND FOOTHILLS SHOULD HAVE THE BEST SHOT AT IFR MORNING
FG...PERISTING THROUGH 15Z OR SO. AT THE SC UPSTATE TERMINALS...MVFR
TO POSSIBLY IFR STCU WILL BE THE FLIGHT RESTRICTION CONCERN THROUGH
MID MORNING. SCT CONVECTION WILL BEGIN IN THE MTNS RATHER
EARLY...AROUND 16Z...AND SHIFT EAST OVER THE EASTERN TERMINALS
THROUGH THE AFTERNOON.

OUTLOOK...GRADUAL DRYING FROM THE WEST IS EXPECTED ON THURSDAY
AFTERNOON...WITH DEEPER DRYING ON FRIDAY. A COLD FRONT WILL APPROACH THE
REGION FROM THE NORTHWEST ON SATURDAY AND MOVE EAST OF THE AREA ON
SUNDAY.

AN EXPERIMENTAL AVIATION FORECAST CONSISTENCY TABLE SHOWING THE
PERCENTAGE OF GUIDANCE MEMBERS AGREEING WITH THE SCHEDULED ISSUANCE
CATEGORY IS AVAILABLE AT FOLLOWING LINK (USE LOWER CASE)...

WWW.WEATHER.GOV/GSP/AVIATION/TABLES.HTM

&&

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

&&

SYNOPSIS...PM
NEAR TERM...MCAVOY/SBK
SHORT TERM...PM
LONG TERM...PM
AVIATION...SBK

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
445 PM EDT WED SEP 5 2012

GAZ010-017-018-026-028-029-NCZ033-035>037-048>053-056>059-062>065-
068>072-082-501>510-SCZ001>014-019-062045-
RABUN-HABERSHAM-STEPHENS-FRANKLIN-HART-ELBERT-AVERY-ALEXANDER-
IREDELL-DAVIE-MADISON-YANCEY-MITCHELL-SWAIN-HAYWOOD-BUNCOMBE-CATAWBA-
ROWAN-GRAHAM-NORTHERN JACKSON-MACON-SOUTHERN JACKSON-TRANSYLVANIA-
HENDERSON-CLEVELAND-LINCOLN-GASTON-MECKLENBURG-CABARRUS-UNION NC-
CALDWELL MOUNTAINS-GREATER CALDWELL-BURKE MOUNTAINS-GREATER BURKE-
MCDOWELL MOUNTAINS-EASTERN MCDOWELL-RUTHERFORD MOUNTAINS-
GREATER RUTHERFORD-POLK MOUNTAINS-EASTERN POLK-OCONEE MOUNTAINS-
PICKENS MOUNTAINS-GREENVILLE MOUNTAINS-GREATER OCONEE-
GREATER PICKENS-GREATER GREENVILLE-SPARTANBURG-CHEROKEE-YORK-
ANDERSON-ABBEVILLE-LAURENS-UNION SC-CHESTER-GREENWOOD-
445 PM EDT WED SEP 5 2012

THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTHEAST GEORGIA...PIEDMONT
NORTH CAROLINA...WESTERN NORTH CAROLINA AND UPSTATE SOUTH CAROLINA.

.DAY ONE...TONIGHT.

SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS WILL DEVELOP
ACROSS THE REGION THIS AFTERNOON AND LINGER INTO THIS EVENING.
THE PRIMARY THREAT FROM ANY THUNDERSTORMS WILL BE LOCALLY HEAVY
RAINFALL. IF THUNDERSTORMS REPEATEDLY AFFECT THE SAME
LOCATION...FLASH FLOODING OF STREAMS OR URBAN FLOODING OF POOR
DRAINAGE AREAS COULD RESULT. CLOUD TO GROUND LIGHTNING STRIKES
WILL ALSO BE POSSIBLE WITH ANY THUNDERSTORMS.

.DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY.

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.SPOTTER INFORMATION STATEMENT...

NONE.