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Area Forecast Discussion

691
FXUS62 KGSP 231512
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
1012 AM EST SUN NOV 23 2014

.SYNOPSIS...
MOISTURE WILL INCREASE RAPIDLY IN ADVANCE OF A COLD FRONT THAT WILL
MOVE OVER THE REGION ON MONDAY. A COASTAL LOW IS THEN EXPECTED TO
DEVELOP OFF THE SOUTHEAST COAST AND MOVE UP THE ATLANTIC SEABOARD ON
WEDNESDAY AND THURSDAY. HIGH PRESSURE WILL SPREAD BACK OVER THE AREA
BY THE END OF THE WEEK.

&&

.NEAR TERM /THROUGH TONIGHT/...
1000 AM EST SUNDAY UPDATE...LATEST RADAR TREND INDICATES THAT
WIDESPREAD BANDS OF MODERATE RAIN WITH ISOLATED HEAVY RAIN WERE
PUSHING NORTHWARD ACROSS OUR CWA AS A SERIES OF UPPER DISTURBANCES
RIPPLE THROUGH THE REGION IN SWLY FLOW ALOFT. HENCE...HAVE INCREASED
POPS TO 90-100% RANGE FOR ENTIRE CWA THIS MORNING THRU THE AFTERNOON
HOURS. ALSO...HAVE UPDATE QPF PER LATEST WPC GUIDANCE...SUGGESTING
1.5-2 INCHES OF RAINFALL ACROSS MUCH OF THE CWA WITH HIGHER AMOUNTS
OVER THE BLUE RIDGE ESCARPMENT. THE LATEST 12Z NAM CONTINUES TO
HIGHLIGHT PERSISTENT IN-SITU CAD THROUGHOUT THE DAY...BEFORE ERODING
BY AROUND 00Z-03Z. THEREFORE...SOME DEGREE OF ELEVATED BUOYANCY WILL
HAVE BEEN PUSHED FARTHER NORTH INTO AT LEAST SOUTHERN HALF OF THE
CWA BY THIS EVENING. HENCE...WILL CONTINUE TO MENTION SLIGHT TO LOW
END CHANCE FOR THUNDER MAINLY OVER THE PIEDMONT.

AS OF 640 AM...A LULL IN THE MORE WIDESPREAD PRECIP ASSOCIATED
WITH WARM UPGLIDE IS NOW MOVING INTO THE WRN THIRD OF THE
CWFA...WITH THE WARM FRONT NOW POSITIONED FROM AROUND MACON GA EAST
TO CHARLESTON. THERE IS SOME MORE INTENSE RAIN FALLING ALONG THE
FRONT WHICH MAY BE ROBBING MOISTURE FROM OUR AREA AND ACCOUNTING FOR
THE LULL. NONETHELESS LATEST GUIDANCE STILL SUPPORTS POPS INCREASING
FROM MIDDAY TO LATE AFTN AS THE WARM FRONT NEARS AND ELEVATED
INSTABILITY COMES INTO PLAY. TEMPS ARE LIKELY TO REMAIN NEARLY
STEADY THRU MID MRNG DUE TO WET BULB EFFECTS OFFSETTING ANY SOLAR
WARMING.

AS OF 315 AM...COOL HIGH PRESSURE CENTERED OFF THE OUTER BANKS WILL
CONTINUE TO MOVE EAST TODAY. HOWEVER AS A DEEP TROUGH MOVES INTO THE
MISSISSIPPI VALLEY AND IS PRECEDED BY A SRN-STREAM SHORTWAVE OVER
THE GULF SOUTH...DEEP SW FLOW WILL PROVIDE AMPLE MOISTURE AND WARM
AIR ADVECTION INTO NRN GA AND THE CAROLINAS. INCREASINGLY WIDESPREAD
RAINFALL WILL RESULT OVER THE CWFA THIS MORNING...AND A BRIEF IN-
SITU CAD EVENT WILL DEVELOP. TEMPS TODAY WILL BE 7 TO 10 DEGREES
BELOW CLIMO UNDER CLOUDY AND DAMP CONDITIONS. IT APPEARS TEMPS WILL
BE WARM ENOUGH TO PRECLUDE ANY PTYPE ISSUES EVEN IN THE HIGH
ELEVATIONS. TO OUR WEST HEIGHTS WILL FALL THRU THE DAY WITH THE
LEADING SHORTWAVE LIFTING FROM THE GULF COAST TODAY TO THE UPPER
OHIO VALLEY AT DAYBREAK MONDAY...WITH THE LARGER TROUGH NOT PUSHING
MUCH PAST THE MS RIVER BY THAT TIME. THE LEADING WAVE WILL BRING A
WARM FRONT THRU THE AREA THIS EVENING WHICH HAS SOME POTENTIAL TO
ERODE THE CAD. HOWEVER THE GFS AND ESPECIALLY THE NAM SUGGEST THAT
THE WEDGE WILL REMAIN ENTRENCHED AT THE SFC THRU THE NIGHT
TONIGHT...THOUGH VERY SHALLOW.

THE NAM/GFS/SREF ALL INDICATE ELEVATED INSTABILITY ABOVE THE WEDGE
INVERSION ALONG AND BEHIND THE WARM FRONT AND THIS IS THE PRIMARY
REASON WE WILL INCLUDE THUNDER IN THE FORECAST. A FEW SREF MEMBERS
DO RETREAT THE WEDGE BOUNDARY NWD ENOUGH TO BRING SBCAPE VALUES OF
UNDER 100 J TO ABOUT I-85. EVEN SMALL BUOYANCY WILL BE ENOUGH TO
CAUSE SOME CONCERN FOR DMGG WIND OR A BRIEF SPIN-UP TOR DUE TO THE
TREMENDOUS SHEAR PARAMETERS THAT WILL BE PRESENT. THIS DUE TO THE
VERY STRONG LOW TO MIDLEVEL FLOW FORCED BY THE SHORTWAVE IN
CONJUNCTION WITH BACKED NEAR-SFC FLOW. DEEP LAYER HODOGRAPHS ARE
VERY IMPRESSIVE AND LOOPING...PRODUCING HELICITIES OF SEVERAL
HUNDRED M2/S2 AT THEIR PEAK THIS AFTN. EFFECTIVE SRH PLOTS ARE A
MORE MUTED 100-200 M2/S2 ON ACCOUNT OF THE MINIMAL BUOYANCY AT THAT
TIME. BY THE TIME THAT SBCAPES EXHIBIT THEIR SMALL PEAK...THE SHEAR
NUMBERS ARE ALREADY PAST THEIR PRIME. SPC HAS LARGELY CARVED US OUT
OF ANY RISK AREA IN THE DAY 1 GRAPHIC...BUT DID INCLUDE OUR SRNMOST
ZONES IN A 5 PERCENT WIND THREAT AREA...THESE AREAS SEEING THE BEST
CHANCE OF DESTABILIZING BEFORE THE SHEAR/HELICITY WEAKEN. WE WILL
NONETHELESS BE CLOSELY MONITORING THE CONVECTIVE SITUATION THIS
AFTERNOON PARTICULARLY SOUTH OF THE WEDGE FRONT.

WE HAVE BEEN WATCHING HYDROLOGIC THREATS WITH THIS EVENT FOR SEVERAL
DAYS AND THINKING REMAINS LARGELY THE SAME. THE UPGLIDE ITSELF IS NOT
ESPECIALLY IMPRESSIVE UNTIL THE MOISTURE ADVECTION AND LIFT ARE AIDED
BY THE SHORTWAVE THIS AFTERNOON. THIS IS WHEN OUR RAINFALL RATES WILL
BE GREATEST. OVERALL QPF NUMBERS FROM VARIOUS MODELS ARE IN THE SAME
BALLPARK. THE FAST MOTION OF THE LOW IS LARGELY RESPONSIBLE FOR
KEEPING HYDRO THREATS TO A MINIMUM...BUT THE RECENT DRY WEATHER OF
COURSE SUGGESTS SOILS HAVE RELATIVELY HIGH CAPACITY.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
AS OF 320 AM SUNDAY...A COLD FRONT WILL PROGRESS ACROSS THE AREA AT
A RATHER DELIBERATE PACE ON MONDAY...LIKELY TAKING A GOOD CHUNK OF
THE DAY BEFORE TO COMPLETELY CLEAR THE FORECAST AREA. THE NAM
CONTINUES TO INSIST ON DEVELOPING SOME RATHER SUBSTANTIAL CAPE
MONDAY AFTERNOON ACROSS THE PIEDMONT. AND WHILE I DO HAVE DOUBTS
THAT THE UPPER 60S DEWPOINTS THAT THE NAM IS ADVERTISING WILL BE
REALIZED...IT STANDS TO REASON THAT INSTABILITY WILL BE MORE
SIGNIFICANT THAN THAT ADVERTISED BY THE GFS...WHICH IS PROBABLY
SWEEPING THE FRONT ACROSS THE AREA TOO QUICKLY. HAVING SAID THAT...
FORECAST SOUNDINGS ARE QUITE WARM IN THE MID-LEVELS...AND IT/S NOT
AT ALL CLEAR THAT AIR PARCELS WILL BE ABLE TO OVERCOME THAT...
ESPECIALLY IN THE ABSENCE OF SIGNIFICANT FORCING. THEREFORE...WILL
CONTINUE TO HOLD POPS DOWN IN THE SLIGHT CHANCE RANGE (FOR SHOWERS)
MONDAY AFTERNOON. THE BIGGER STORY WILL BE THE WINDS...AS FORECAST
SOUNDINGS DEPICT A VERY DEEP MIXED LAYER...WITH AS MUCH AS 60 KTS
(!) OF SW FLOW AT THE TOP OF THE BL. WITH ONLY A 4-6 MB SURFACE
GRADIENT ADVERTISED ACROSS THE FORECAST AREA...I DOUBT WE/LL REALIZE
ANYTHING APPROACHING THAT DEGREE OF GUSTINESS. NEVERTHELESS...GUSTS
AS HIGH AS 40 MPH CAN BE EXPECTED MONDAY AFTERNOON.

A RETURN TO COOLER AND DRIER CONDITIONS IS EXPECTED BY EARLY TUESDAY
BEFORE UNCERTAINTY ENTERS THE FORECAST EQUATION BY TUE NIGHT AND
WED. THE MAIN ISSUE IS WITH SOUTHERN STREAM ENERGY LIFTING OUT OF
THE TEXAS...AND ACROSS THE SOUTHEAST DURING THIS TIME FRAME. EACH
MODEL HAS ITS OWN IDEA OF HOW THE PATTERN WILL EVOLVE...PARTICULARLY
WITH HOW A NORTHERN STREAM WAVE DROPPING OUT OF THE NORTHERN ROCKIES
WILL INTERACT WITH THE SOUTHERN STREAM WAVE/POTENTIAL FOR PHASING OF
THE TWO STREAMS. THE ECMWF REMAINS THE MOST AGGRESSIVE IN KEEPING
THE STREAMS SEPARATED THROUGH WED...WITH THE SOUTHERN STREAM WAVE
TAKING ON A NEG TILT ORIENTATION...AND INSTIGATING STRONG
CYCLOGENESIS ACROSS THE EASTERN GULF TUE NIGHT...THEN TRACKING IT
VERY CLOSE TO THE SOUTHEAST ATLANTIC COAST. MEANWHILE...THE GFS IS A
LITTLE FARTHER OFF SHORE AND NOT AS DEEP WITH THE CYCLONE...WHILE
THE NAM DEPICTS NOTHING MORE THAN A WEAK WAVE OFF THE COAST BY EARLY
WED. IT/S ALSO SOMEWHAT DISCONCERTING THAT THERE APPEARS TO BE ONLY
A SINGLE SREF MEMBER THAT IS EC-LIKE. NEVERTHELESS...BASED UPON THE
FACT THAT EVEN A GFS SOLUTION DEVELOPS SOME PRECIP INTO OUR EASTERN
ZONES TUE NIGHT...WILL CONTINUE TO ADVERTISE SOLID CHANCE POPS
ACROSS THE NC PIEDMONT BY WED MORNING...WITH LOW CHANCE AS FAR WEST
AS THE BLUE RIDGE. WILL CONTINUE TO FEATURE MOUNTAIN RAIN/SNOW
POSSIBILITIES...WITH GENERALLY ALL LIQUID OUTSIDE THE MTNS.
HOWEVER...IT SHOULD BE STATED THAT IF THE SITUATION EVOLVES IN AN
ECWMF-LIKE MANNER...SIGNIFICANT SNOWFALL WILL BE POSSIBLE ACROSS
PORTIONS OF THE MTNS LATE TUE NIGHT INTO WED.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
AS OF 345 AM...THE MEDIUM RANGE APPEARS LARGELY COOL...DRY...AND
UNEVENTFUL...AS A LOW AMPLITUDE LONG WAVE TROUGH BECOMES ESTABLISHED
OVER THE EASTERN CONUS. THE ECMWF DEPICTS PROGRESSION OF A COLD
FRONT INTO THE REGION BY THE END OF THE WEEK...ASSOCIATED WITH AN
AREA OF HEIGHT FALLS OVERSPREADING THE GREAT LAKES. HOWEVER...IT IS
NOT AT ALL CLEAR IF SUFFICIENT MOISTURE WILL BE AVAILABLE TO SUPPORT
PRECIP CHANCES OVER OUR AREA.

&&

.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
AT KCLT...WARM AIR AND MOISTURE ADVECTION WILL GRADUALLY
INCREASE TODAY BRINGING LOWERING CIGS AND REINFORCING COLD AIR
DAMMING OVER THE AREA. GUIDANCE FAIRLY SOLID ON CIG LOWERING TO LIFR
BY LATE AFTN AS RAIN CONTINUES AND FORCING PEAKS ALOFT WITH THE
PASSAGE OF A S/W TROF. THE WEDGE BOUNDARY IS MOST LIKELY TO STAY S
OF KCLT UNTIL THIS EVENING...BUT ISOLD ELEVATED CONVECTION COULD
RESULT IN TS EARLIER IN THE AFTN. BEST CHANCE HOWEVER IS AROUND
SUNSET WITH THE BOUNDARY NEARING WHILE SOME DIURNAL BUOYANCY STILL
EXISTS. THOUGH SOME DRYING WILL OCCUR SUNDAY EVENING...THERE IS NOT
A GOOD SCOURING MECHANISM FOR THE CAD AND ONLY SLIGHT IMPROVEMENT TO
CIG IS EXPECTED OVERNIGHT. WINDS NE UNTIL GOING SE THIS EVENING AS
WEDGE STARTS TO WEAKEN. HAVE INCLUDED LLWS MENTION THIS AFTN AND
EVENING WHERE SUGGESTED BY MODEL WINDS...WITH VERY STRONG FLOW
PRESENT ABOVE THE PBL AHEAD OF THE SHORTWAVE.

ELSEWHERE...TRENDS MUCH LIKE KCLT. HAVE INCLUDED A LOW-END TSRA
MENTION IN THE LATE AFTN AT THE SC SITES AS THEY ARE MOST LIKELY TO
BE AFFECTED BY ACTIVITY NEAR THE WEDGE BOUNDARY. LLWS IS INCLUDED
MAINLY INVOF THE SHORTWAVE PASSAGE...BUT AT KAVL THE FLOW REMAINS
STRONG ENOUGH TONIGHT TO INCLUDE THRU 12Z. WIND CHANGES OVERALL ARE
SIMILAR IN PROGRESSION WITH THE EXCEPTION OF KAVL WHICH WILL SEE
SE WINDS THROUGHOUT THE PERIOD.

OUTLOOK...BRIEF DRYING FOLLOWS A COLD FRONT MON-TUE...WITH RAIN
AND/OR WINTRY PRECIP POSSIBLY RETURNING WED. MORE SETTLED WEATHER
WILL RETURN TO END THE WEEK.

CONFIDENCE TABLE...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       MED   69%     HIGH  97%     MED   75%     MED   67%
KGSP       HIGH  94%     HIGH 100%     HIGH  80%     MED   77%
KAVL       HIGH  94%     HIGH  89%     HIGH  86%     MED   66%
KHKY       HIGH  91%     HIGH 100%     HIGH 100%     LOW   59%
KGMU       HIGH  94%     HIGH  97%     HIGH  89%     MED   73%
KAND       HIGH  94%     HIGH  97%     MED   63%     LOW   46%

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...JDL
NEAR TERM...JOH/WIMBERLEY
SHORT TERM...JDL
LONG TERM...JDL
AVIATION...WIMBERLEY



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