Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 242030
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
430 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/...
AS OF 430 PM EDT FRIDAY...CURRENT FCST REMAINS ON TRACK THEREFORE NO
SIG CHANGES NEEDED/MADE.  CYCLOGENESIS OVER THE SOUTHERN PLAINS
CONTINUES TO PUSH A DEVELOPING WARM FRONT THROUGH THE LOWER
MISSISSIPPI RIVER VALLEY AT THIS TIME...WHILE WESTERLY MID/UPPER
LEVEL FLOW STREAMS CIRRUS ATOP THE TN VALLEY INTO THE SOUTHERN APPS.
FURTHER SKY COVER WILL BUILD IN TONIGHT AS WEAK UPGLIDE REGIME
SLIDES EAST...AND STRENGTHENS.  THUS...DID TWEAK SKY COVER A BIT TO
ACCOUNT FOR MOST RECENT TRENDS.  OTHERWISE TEMPERATURE/DEWPOINT
TRENDS LOOK TO BE IN GOOD AGREEMENT WITH THE CURRENT FCST.  WILL
REEVALUATE POP TIMING WITH NEWER GUIDANCE LATER THIS EVENING.

PREVIOUS DISCUSSION...

AT 230 PM EDT FRIDAY...AN UPPER RIDGE WAS LOCATED OVER THE MS RIVER
VALLEY THIS AFTERNOON...WITH TROUGHS OFF THE EAST COAST AND OVER THE
ROCKIES. SURFACE HIGH PRESSURE CENTERED OVER THE OH RIVER VALLEY
EXTENDED TO THE CAROLINA AND GA COASTS THIS AFTERNOON. MEANWHILE A
COLD FRONT WAS LOCATED OVER CENTRAL FL AND THE NW GULF OF MEXICO.
CLOUDS COVER REMAINED LIMITED MAINLY TO THE MS RIVER VALLEY...WITH
SOME HIGH LEVEL CLOUDS SPILLING OVER THE RIDGE IN SW TN AND N GA.

THE UPPER RIDGE IS EXPECTED TO PROGRESS TO THE SOUTHERN APPALACHIANS
BY SATURDAY MORNING...WHILE AN UPPER LOW CLOSES OFF IN THE TROUGH
UPSTREAM AND CROSSES THE CENTRAL PLAINS. THIS LOW FILLS NEAR THE
SOUTHERN GREAT LAKES LATE SATURDAY...WHILE THE UPPER RIDGE
DEAMPLIFIES...LEADING TO ZONAL FLOW OVER THE SE.

THE SURFACE BOUNDARY IN THE GULF STARTS TO RETURN N AS A WARM FRONT
OVERNIGHT...AND MOIST SOUTHWESTERLY ISENTROPIC UPGLIDE OVER THE
BOUNDARY WILL SUPPORT LOWERING CLOUDS AND PRECIPITATION OVER OUR
AREA. A LIGHT LOW LEVEL SOUTHERLY FLOW WILL ADD SOME UPSLOPE FLOW
INTO THE SOUTHERN APPALACHIANS. UPGLIDE MAXIMIZES BY MIDDAY
SATURDAY...WHEN THE GREATEST PRECIPITATION IS EXPECTED. RAINFALL
AMOUNTS COULD EXCEED ONE INCH IN THE UPPER SAVANNAH RIVER VALLEY AND
THE SOUTH FACING BLUE RIDGE ESCARPMENT ON SATURDAY...BUT AT THIS
TIME FLOODING APPEARS TO BE A LIMITED HAZARD BASED ON THE SOMEWHAT
EXTENDED DURATION OF THE EVENT.

INSTABILITY WILL INCREASE ON SATURDAY...BECOMING INCREASINGLY
SUFFICIENT FOR THUNDERSTORMS IN THE AFTERNOON OVER THE S AND W OF
THE AREA. WITH SHEAR INCREASING...THE CHANCE OF SEVERE THUNDERSTORMS
WILL INCREASE AS WELL. TEMPERATURES WILL EXHIBIT A LIMITED DIURNAL
RANGE...AS CLOUD COVER AND INCREASING MOISTURE LIMIT COOLING AND
HEATING.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
AS OF 205 PM FRIDAY...THE MAIN SHORT TERM CONCERN REVOLVES AROUND
THE CONTINUED POTENTIAL FOR STRONG/SEVERE CONVECTION ACROSS THE AREA
INTO SATURDAY NIGHT. UNFORTUNATELY...CONSIDERING THE RATHER WEAKLY
FORCED NATURE OF THE ENVIRONMENT...CONVECTIVE TRENDS ACROSS OUR AREA
ARE NONE-TOO-OBVIOUS. IT SEEMS A VIRTUAL CERTAINTY THAT ORGANIZED
CONVECTION WILL DEVELOP DURING SAT AFTERNOON/EVENING ACROSS THE OHIO
VALLEY. HOWEVER...THERE WILL BE COMPETITION BETWEEN THE MEAN CLOUD-
BEARING FLOW (DUE WESTERLY) AND THE ORIENTATION OF INSTABILITY AND
LOW LEVEL FORCING (WARM FRONT/CAPE AXIS EXTENDING SOUTHEAST FROM THE
OHIO VALLEY) IN DETERMINING THE TRACK OF ANY ORGANIZED CONVECTIVE
COMPLEX. IN FACT...THERE IS NO CLEAR CONSENSUS AMONG THE LATEST
MESOSCALE AND SHORT TERM MODELS IN THIS REGARD. THE HIGHEST POPS
(PRIMARILY LIKELY) FOR SAT EVENING WILL BE MAINTAINED ACROSS THE
WESTERN MTNS AND OUR FAR NORTHERN ZONES. ASSUMING AN MCS DOES MANAGE
TO DROP INTO THE CWA TOMORROW EVENING...A CONSENSUS OF MODEL
INSTABILITY PROGS INDICATES IT WILL NOT FIND THE MOST ACCOMMODATING
ENVIRONMENT ACROSS OUR NORTHERN ZONES...AS ESSENTIALLY NO MODEL
DEPICTS POSITIVE SURFACE-BASED BUOYANCY DEVELOPING NORTH OF
(ROUGHLY) I-85 IN THE PIEDMONT/FOOTHILLS. (THERE IS HOWEVER A DECENT
AMOUNT OF ELEVATED CAPE DEPICTED). THEREFORE...OUTSIDE OF THE
POTENTIAL FOR A DOWNBURST-PRODUCING MCS TO SLAM INTO THE NC MTNS
BEFORE WEAKENING AND/OR BECOMING LESS ROOTED IN THE BOUNDARY
LAYER...THE SEVERE WEATHER POTENTIAL ACROSS OUR NORTHERN ZONES
APPEARS LESS THAN STELLAR.

THE MORE INTERESTING ACTION TOMORROW NIGHT COULD BE FARTHER SOUTH...
AS WARM FRONT SLOWLY LIFTS NORTH AND SPREADS UNSTABLE AIR INTO OUR
SOUTHERN PIEDMONT/SOUTHWEST MTNS AREA. SOME OF THE MESO-MODELS ARE
HINTING AT THE POTENTIAL FOR SCATTERED DISCRETE CONVECTION TO
DEVELOP WITHIN A DESTABILIZING/RECOVERING AIR MASS ACROSS NORTH
GEORGIA AND THE TENN VALLEY BY LATE AFTERNOON...WHICH COULD MAKE A
RUN INTO THE CWA DURING THE EVENING. AGAIN...THIS IS FAR FROM A SLAM
DUNK IN LIGHT OF A GENERALLY WEAKLY FORCED WARM SECTOR ENVIRONMENT.
HOWEVER...THE POTENTIAL IS CERTAINLY THERE...AND IN LIGHT OF THE
RATHER VOLATILE ENVIRONMENTAL WARM SECTOR CONDITIONS THAT ARE
EXPECTED (SBCAPE 1500-2000 J/KG...DEEP LAYER SHEAR OF 70
KTS)...SUPERCELLS WOULD BE LIKELY...WITH PERHAPS A THREAT FOR AN
ISOLATED TORNADO NEAR THE WARM FRONT.

WEAKENING SURFACE LOW IS EXPECTED TO TRACK NEAR THE NC/VA LINE SAT
NIGHT INTO SUNDAY...WITH TRAILING COLD FRONT MOVING ACROSS THE AREA
DURING THE MORNING INTO THE AFTERNOON. A BIT OF SBCAPE MAY CREEP
INTO THE PIEDMONT AHEAD OF THE FRONT BY EARLY AFTERNOON...WHILE NW
FLOW COULD SUPPORT SHOWER DEVELOPMENT ACROSS THE FAR WESTERN MTNS.
POPS FOR SCATTERED CONVECTION ARE THEREFORE WARRANTED IN MAINLY
THOSE AREAS. THE REMAINDER OF THE PERIOD WILL BE QUIET AND DRY...
WITH TEMPS ABOUT 5 DEGREES BELOW CLIMO.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
AS OF 145 PM FRIDAY...THE MODEL GUIDANCE HAS CONTINUED THE TREND OF
DELAYING THE ARRIVAL OF THE NEXT SYSTEM DURING THE MIDDLE OF NEXT
WEEK. THE LATEST GFS KEEPS THE UPPER RIDGE AXIS TO THE WEST MONDAY
NIGHT AND DOES NOT MAKE IT INTO THE WESTERN CAROLINAS UNTIL MAYBE
VERY LATE IN THE DAY ON TUESDAY. THE 00Z RUN OF THE ECMWF WAS
SIMILAR IN SHOWING LITTLE SUPPORT OR MOISTURE FOR PRECIP UNTIL MAYBE
LATE TUESDAY AFTERNOON. WITH THAT IN MIND...WILL IMPOSE YET ANOTHER
DELAY WITH THE ARRIVAL OF PRECIP CHANCES FROM THE SOUTHWEST UNTIL
LATE TUESDAY AFTERNOON...AND WILL SLOW THE INCREASE IN PRECIP
CHANCES TUESDAY NIGHT. NOT MUCH IN THE NEW MODEL GUIDANCE TO SUGGEST
ANY CHANGES TO THE FCST...UNLESS THE 12Z RUN OF THE ECMWF TRENDS
FASTER LIKE THE GFS. HOWEVER...IN SPITE OF THE MODEL DIFFERENCES
WITH THE EVOLUTION OF THE UPPER LOW AND TRACK/STRENGTH OF THE
SURFACE LOW MOVING ACROSS THE DEEP SOUTH ON WEDNESDAY AND
THURSDAY... THE WEAKER/FASTER GFS STILL MANAGES TO HOLD ONTO SOME
LIGHT PRECIP ACROSS THE WESTERN CAROLINAS AS ITS SHEARING/POSITIVELY
TILTED UPPER TROF MOVES OVERHEAD THRU THURSDAY EVENING. THUS...FEW
CHANGES WERE MADE. PRECIP PROB WAS KEPT IN THE CHANCE RANGE AND
TEMPS WERE KEPT SEASONALLY COOL...ABOUT 10 DEG BELOW NORMAL. THE
MODEL SOLUTIONS COME INTO AGREEMENT BY THURSDAY NIGHT WITH MOVING
THE PRECIP OUT. FRIDAY SHOULD BE DRY WITH HIGH PRESSURE BUILDING IN
FROM THE WEST AND TEMPS STARTING A REBOUND TOWARD NORMAL.

&&

.AVIATION /21Z FRIDAY THROUGH WEDNESDAY/...
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...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       HIGH 100%     HIGH 100%     HIGH 100%     LOW   44%
KGSP       HIGH 100%     HIGH 100%     HIGH 100%     MED   62%
KAVL       HIGH 100%     HIGH 100%     HIGH 100%     MED   70%
KHKY       HIGH 100%     HIGH 100%     HIGH 100%     LOW   38%
KGMU       HIGH 100%     HIGH 100%     HIGH 100%     LOW   58%
KAND       HIGH 100%     HIGH 100%     HIGH  91%     MED   66%

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...DEO
NEAR TERM...CDG/JAT
SHORT TERM...JDL
LONG TERM...PM
AVIATION...JAT


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