Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 241713
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
113 PM EDT FRI APR 24 2015

.SYNOPSIS...
COOL AND DRY HIGH PRESSURE MOVES OUT OVER THE ATLANTIC
TONIGHT AS LOW PRESSURE MOVES EAST FROM THE CENTRAL PLAINS STATES. A
WARM FRONT WILL LINGER OVER OUR AREA FOR MUCH OF THE WEEKEND
PROVIDING A FOCUS OF SHOWERS AND A FEW STORMS. DRY HIGH PRESSURE
WILL MOVE IN AGAIN FOR THE START OF THE WORK WEEK...WITH ANOTHER LOW
PRESSURE SYSTEM CROSSING THE SOUTHEAST BY MIDWEEK.

&&

.NEAR TERM /THROUGH SATURDAY/...
1715 UTC UPDATE...SKY COVER WAS REDUCED AGAIN PER SATELLITE IMAGERY.
WINDS WERE UPDATED FROM A BLEND OF THE LATEST NAM AND ADJMAV...
PRESERVING GUSTS AT HIGHER ELEVATIONS OVERNIGHT AS S FLOW ALOFT
INCREASES AHEAD OF AN APPROACHING MID LEVEL SYSTEM.
TEMPERATURES...DEW POINTS...AND AERAS OF PRECIPITATION WERE
ADJUSTED PER THE LATEST MODEL DATA.

AS OF 640 AM...I WILL ISSUE A QUICK UPDATE TO ADJUST HOURLY
TEMPERATURES AND DEWPOINTS TO OBSERVATIONS. IN ADDITION...LATEST IR
SATELLITE IMAGES INDICATED A BAND OF HIGH CLOUDS LIFTING NORTH OF
THE FORECAST AREA...LEAVING SKY MOSTLY CLEAR. I WILL EDIT SKY GRIDS
TO LOWER SKY VALUES THROUGH MID MORNING.

AS OF 315 AM...GUSTY HIGH ELEVATION WINDS SHOULD STEADILY WEAKEN
EARLY THIS MORNING AS THE CENTER OF A LARGE AREA OF LOW PRESSURE
SLIDES OVER THE SOUTHERN APPALACHIANS. BY SUNRISE...TEMPERATURES IN
THE LOW 30S SHOULD BE COMMON ACROSS THE NC MTNS...NO CHANGES TO NPW.

HIGH CLOUDS SHOULD STREAK ACROSS THE REGION THROUGH THE
DAY...ESPECIALLY AS THE CORE OF A 140KT H3 JET PASSES OVER THE NC
ZONES THIS MORNING. OVERALL...CONDITIONS WILL FEATURE SCT TO BKN
HIGH CLOUDS...DRY AFTERNOON HUMIDITY...AND LIGHT WINDS. USING A
BLEND OF MOS...I WILL FORECAST HIGH TEMPERATURES TO RANGE FROM THE
MID 60S WITHIN THE MTN VALLEYS TO NEAR 70 EAST.

TONIGHT...DEEP LOW PRESSURE WILL TRACK FROM THE CENTRAL PLAINS TO
THE CENTRAL CONUS DURING THE NIGHT. AN ASSOCIATED WARM FRONT WILL
LIFT NORTHWARD...BUT SHOULD REMAIN WELL SOUTH OF THE FORECAST AREA
BY 12Z SAT. HOWEVER...A WIDE SWATH OF H3 JET DIVERGENCE WILL DEVELOP
OVER THE REGION LATE TONIGHT. IN ADDITION...MOISTURE AND LIFT
ASSOCIATED WITH THE WARM FRONT WILL LIKELY INCREASE ACROSS THE NC
MTNS AFTER 6Z SATURDAY. I WILL INDICATE A RAPID INCREASE IN POPS
FROM 6Z-12Z SAT. BY SUNRISE SAT...MOST OF THE CWA SHOULD SEE RAIN
DEVELOP. I WILL FORECAST AFTERNOON POPS TO RANGE FROM 80-90 PERCENT
ACROSS THE MTNS TO CHC POPS EAST OF I-77. LOW TEMPERATURES ARE
FORECAST TO RANGE FROM THE MID 50S WITHIN THE SAVANNAH RIVER VALLEY
TO THE U40S NORTH OF I-40..

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
AS OF 315 AM FRIDAY...WE HAVE SOME SIGNIFICANT WEATHER POSSIBLE
THROUGH THE SHORT TERM. WE BEGIN AT 12Z SATURDAY WITH INSITU DAMMING
DEVELOPING ACROSS THE PIEDMONT AS A MIDLEVEL LOW TRAVERSES THE UPPER
MIDWEST....DRAGGING A SURFACE LOW AND ATTENDANT FRONT WITH IT. WEAK
MIDLEVEL RIDGING DOMINATES THE SYNOPTIC FIELDS EARLY SATURDAY BUT
WITH WIDESPREAD ISENTROPIC UPGLIDE OVER THE COLD DOME. SHORT-TERM
GUIDANCE IN PRETTY GOOD AGREEMENT WITH THE SURFACE LOW WORKING ITS
WAY ACROSS THE OHIO VALLEY AND EAST SOUTHEAST INTO THE SOUTHERN
APPALACHIANS BY SUNDAY MORNING. THE TRACK OF THE SURFACE LOW
INCREASES THE PRESSURE GRADIENT...ESPECIALLY WITH THE EROSION OF THE
DAMMING...QUITE A BIT. ADD TO THIS THE MEAN UPPER TROUGH PLUS THE
SHORTWAVE WITH A SPLIT FLOW PATTERN ALOFT AND WE HAVE ALL SORTS OF
INGREDIENTS THAT ARE COMING TOGETHER. IN A NUTSHELL...EXPECT SHOWERS
TO DEVELOP AND SPREAD NORTHEAST ACROSS THE AREA SATURDAY
MORNING...WITH HIGHEST POPS AROUND MIDDAY. VERY STABLE LOW LEVEL
AIRMASS IN THE COLD DOME WILL KEEP INSTABILITY DOWN AND TEMPERATURES
BELOW NORMAL...BUT OBVIOUSLY THIS WILL NOT BE THE CASE SOUTH OF THE
WEDGE FRONT. OF COURSE...THIS DOES NOT EVEN BEGIN TO TELL THE WHOLE
STORY FOR THE SHORT TERM. LET US START WITH THE RAIN/QPF SITUATION.

AS THE MOISTURE OVERSPREADS THE AREA FRIDAY NIGHT INTO SATURDAY
MORNING...PRECIPITABLE WATER VALUES INCREASE SIGNIFICANTLY...WELL
ABOVE THE 90TH PERCENTILE FOR CLIMATOLOGY FOR THE AREA AND THIS TIME
OF THE YEAR. NAM IS A LITTLE MORE ROBUST WITH MOISTURE FLUX AND
MOISTURE TRANSPORT VECTORS MAKING IT A LITTLE FARTHER INTO THE NORTH
CAROLINA PIEDMONT...BUT GFS KEEPS BEST MOISTURE FLUX JUST A HAIR
SOUTH OF OUR AREA INTO CENTRAL SOUTH CAROLINA WHERE THE BEST UPPER
DIFFLUENCE REMAINS FROM THE JET STREAK PUSHING ACROSS ALOFT. ONCE
THE CONVECTIVE ELEMENT IS ADDED TO QPF ESPECIALLY SOUTH OF THE WEDGE
FRONT...ABSOLUTELY SEE WHY WPC HAS INCLUDED THE UPSTATE IN A SLIGHT
RISK OF EXCESSIVE RAINFALL FOR DAY2. STORM TOTAL QPF VALUES FOR NOW
RANGE BETWEEN 1-1.5 INCHES BUT WITH THE GROUND FAIRLY MOIST FROM
SUCCESSIVE RAINFALL EVENTS THE PAST WEEK OR SO...COULD BE ENOUGH TO
CAUSE SOME PROBLEMS. AT ANY RATE...WILL INTRODUCE THE POTENTIAL FOR
HEAVY RAIN IN THE HWO.

OTHER SIGNIFICANT ITEM OF NOTE IS POTENTIAL FOR SEVERE CONVECTION
ALONG/SOUTH/WEST OF THE WEDGE FRONT THAT SETS UP. PINPOINTING THE
LOCATION OF THE WEDGE FRONT AT THIS POINT IN THE FORECAST IS
DIFFICULT AT BEST BUT WE CAN AT LEAST GET AN OVERALL IDEA OF THE
EVOLUTION OF CONVECTION DURING THE DAY. SBCAPE VALUES DO NOT START
CREEPING UP INTO THE FORECAST AREA UNTIL SATURDAY AFTERNOON AS THE
APPROACH OF THE SURFACE LOW STARTS PUSHING THE WEDGE FRONT A LITTLE
FARTHER NORTH AND EAST...BUT EVEN THEN THE DIABATIC COOLING
COMPONENT OF THE RAIN FALLING INTO THE SURFACE DRY LAYER SHOULD KEEP
THE WEDGE PRETTY STRONG. GFS IS QUITE A BIT STRONGER WITH THE
DAMMING THAN THE NAM AND HOLDS OFF UNTIL AFTER 18Z SATURDAY TO BRING
ANY SBCAPE IN TO THE REGION...BUT THE MORE BULLISH /AS USUAL/ NAM
BRINGS IN SBCAPE VALUES UPWARDS OF 2000 J/KG INTO EXTREME WESTERN
ZONES BY 00Z SUNDAY. THIS DOES DIMINISH WITH A DIURNAL TREND AS THE
WEDGE TRIES TO REINFORCE ITSELF...DEFLECTING THE HIGHER CAPE VALUES
INTO THE UPSTATE BUT STILL ENOUGH TO BE OF CONCERN. NOT ONLY THE
INSTABILITY HOWEVER BUT THE SHEAR IS QUITE IMPRESSIVE. GFS AND NAM
IN REMARKABLE AGREEMENT BRINGING 70-80KT DEEP LAYER SHEAR ACROSS THE
SOUTHERN APPALACHIANS SATURDAY AFTERNOON AND EVENING...ONLY
WEAKENING VALUES SOMEWHAT OVERNIGHT BUT STILL WIDESPREAD 50-60KT BY
SUNDAY MORNING. LOW LEVEL HELICITY...ENHANCED BY THE AGEOSTROPHIC
ADJUSTMENT OF THE DAMMING...ALSO GETS PRETTY DARN HIGH. PERFECT
PROGGING THE NAM...TAKE SOUTHERN ANDERSON COUNTY 00Z SUNDAY FOR
EXAMPLE...WITH AROUND 800 J/KG SBCAPE...75KT 0-6KM SHEAR...AND OVER
500 M2/S2 0-3KM HELICITY. THIS MIGHT BE AN EXTREME EXAMPLE BUT IT
DOES AT LEAST GIVE SOME IDEA OF WHAT WE ARE LOOKING AT. AS SUCH...
THE DAY2 OUTLOOK FROM SPC TRIES TO DEPICT AN APPROXIMATE LOCATION OF
THE WEDGE FRONT...WITH A GOOD PORTION OF THE FORECAST AREA IN A
SLIGHT RISK AND EVEN EXTREME WESTERN PORTIONS IN ENHANCED. THIS IS
DEFINITELY SOMETHING WE WILL NEED TO NARROW DOWN OVER THE NEXT
COUPLE OF FORECASTS.

DEEP UPPER TROUGH DOMINATES THE EASTERN SEABOARD IN THE WAKE OF THE
SURFACE LOW THAT PUSHES OFF THE CAROLINA COAST ON SUNDAY. DEEP LAYER
CAA CONTINUES THROUGH THE END OF THE SHORT TERM KEEPING TEMPERATURES
BELOW-NORMAL DESPITE THE EROSION OF THE DAMMING. SURFACE GRADIENT
REMAINS STRONG AND WINDS WILL LIKELY PICK UP ON SUNDAY ESPECIALLY IN
THE MOUNTAINS. OF COURSE...WITH THE LOW LEVEL JET AND STRONG
GRADIENT THROUGH THE ENTIRE WEEKEND...ONLY THE STRONG LOW-LEVEL
INVERSION ASSOCIATED WITH THE DAMMING WILL KEEP THAT MOMENTUM FROM
TRANSFERRING TO THE SURFACE ON SATURDAY AND SATURDAY NIGHT. HIGHLY
BUSTABLE TEMPERATURE FORECASTS THROUGH THE WEEKEND AND AS TYPICAL
WILL ENTIRELY DEPEND ON THE LOCATION OF THE WEDGE FRONT.

&&

.LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
AS OF 345 AM FRIDAY...LUCKILY THE EXTENDED BEGINS FAIRLY QUIET IN
THE WAKE OF THE MESS OVER THE WEEKEND. SURFACE HIGH PRESSURE
CENTERED ACROSS THE UPPER MIDWEST RIDGES INTO THE SOUTHEAST BY
MONDAY MORNING WITH STRONG NORTHWEST FLOW ALOFT CONTINUING THE CAA
PATTERN. THE NEXT SYSTEM...IN THE FORM OF A CLOSED MIDLEVEL LOW
TRAVERSING THE SOUTHERN PLAINS...IN PHASE WITH BUT NOT ABSORBED BY A
TROUGH DIGGING OUT OF CANADA...BEGINS TO IMPACT OUR AREA BY MIDWEEK.
SURFACE LOW DEVELOPS ACROSS THE NORTHERN GULF AND MOVES INTO THE
DEEP SOUTH LATE TUESDAY AND EARLY WEDNESDAY...AGAIN SPREADING
PRECIPITATION ACROSS THE SOUTHEAST AHEAD OF IT. GFS AND ECMWF IN
FAIRLY DECENT AGREEMENT ON OVERALL MASS FIELDS AT LEAST THROUGH
WEDNESDAY...BUT DIVERGE BY THE END OF THE PERIOD WITH THE GFS
PUSHING THE SURFACE LOW ON OUT INTO THE ATLANTIC WITH THE MIDLEVEL
CLOSED LOW BEING ABSORBED BY THE LONGWAVE TROUGH PUSHING THROUGH THE
EASTERN PART OF THE COUNTRY...PUSHING ALL PRECIP OUT OF THE FORECAST
AREA WEDNESDAY NIGHT. MEANWHILE THE ECMWF KEEPS THE MIDLEVEL LOW AN
ENTITY UNTO ITSELF WHILE DEEPENING THE SURFACE LOW ACROSS THE
COASTAL CAROLINAS. DEFORMATION ZONE SETS UP ACROSS THE SOUTHERN
APPALACHIANS KEEPING PRECIPITATION ACROSS THE AREA INTO LATE
THURSDAY. TRIED TO TAKE A COMPROMISE ESPECIALLY WITH WPC SUGGESTION
OF LEANING TOWARD ENSEMBLES. TEMPERATURES REMAIN BELOW-AVERAGE FOR
THIS TIME OF YEAR ESPECIALLY ON WEDNESDAY WITH HIGHS A GOOD 10
DEGREES OR SO BELOW SEASONAL NORMALS.

&&

.AVIATION /17Z FRIDAY THROUGH TUESDAY/...
AT KCLT...VFR. GUIDANCE BRINGS THE CIG DOWN TO LOW VFR OVERNIGHT AS
LOW LEVEL MOISTURE INCREASES OVER A WARM FRONT TO THE SOUTH. A
GUIDANCE BLEND DOES NOT SUPPORT A VSBY RESTRICTION OVERNIGHT...BUT
WITH RAIN CHANCES INCREASING...THE WIND EXPECTED TO BE RATHER
LIGHT...IT WOULD NOT BE SURPRISING TO SEE A VSBY RESTRICTION CREEP
INTO LATER FORECASTS. LIGHT WINDS WILL BACK FROM NW TO SW THIS
AFTERNOON...THEN TO S LATE TONIGHT.

ELSEWHERE...GUIDANCE LOWERS CIGS TO LOW VFR OVERNIGHT AS LOW LEVEL
MOISTURE SPREAD NORTH OVER A WARM FRONT TO THE S. SC SITES AND KAVL
ARE EXPECTED TO SEE MVFR CIGS BY DAWN....WHICH PERSIST THROUGH MIDDAY
SATURDAY. WITH RAINFALL EXPECTED TO SET UP TONIGHT...GUIDANCE
SUPPORTS MVFR VSBY AT KAVL BEFORE DAWN...AND THIS TREND MAY SPREAD TO
OTHER SITES IN LATER FORECASTS IF WINDS REMAIN LIGHT AS EXPECTED.
UNDER THE CURRENT SCENARIO...KAVL VSBY WOULD IMPROVE IN THE MORNING.
LIGHT WINDS NW WILL BACK TO SW THIS AFTERNOON...IF THEY HAVE NOT
ALREADY...AND TO S THIS EVENING.

OUTLOOK...A LOW PRESSURE SYSTEM WILL MOVE ACROSS THE MID ATLANTIC
STATES THIS WEEKEND...AND WIDESPREAD RAINFALL MAY SUPPORT
RESTRICTIONS. A COLD FRONT WILL SWEEP ACROSS THE REGION SUNDAY
NIGHT...FOLLOWED BY DRY HIGH PRESSURE THROUGH TUESDAY. ANOTHER LOW
PRESSURE SYSTEM MAY TRACK ACROSS THE DEEP SOUTH WEDNESDAY AND
THURSDAY.

CONFIDENCE TABLE...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGSP       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAVL       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KHKY       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGMU       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAND       HIGH 100%     HIGH 100%     HIGH 100%     MED   70%

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...TDP
NEAR TERM...JAT/NED
SHORT TERM...TDP
LONG TERM...TDP
AVIATION...JAT



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