-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KGSP 210514
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
114 AM EDT TUE MAY 21 2013

.SYNOPSIS...
AN APPROACHING UPPER LEVEL TROUGH WILL CONTINUE TO INFLUENCE THE
REGION FOR MOST OF THIS WEEK. SOUTHERLY FLOW WILL KEEP THE AREA IN A
MOIST AND GENERALLY UNSETTLED AIRMASS WITH STORM CHANCES EACH
AFTERNOON. RAIN CHANCES DIMINISH AFTER A COLD FRONT CROSSES THROUGH
BY THE END OF THE WEEK.

&&

.NEAR TERM /THROUGH TODAY/...
-- Changed Discussion --
0515 UTC UPDATE...WINDS WERE UPDATED TO INCORPORATE THE LATEST NAM
DATA. POPS WERE LOWERED BASED ON RADAR TRENDS. VISIBILITY WAS
UPDATED FROM A BLEND OF THE ADJMAV AND ADJMET...PLACING PATCHY FOG
ACROSS MOST OF THE AREA TOWARD DAYBREAK.

AS OF 1030 PM...FOR THE 0230 UTC UPDATE...HAVE CUT BACK POPS FOR THE
NIGHT...TO JUST A SLGT CHC IN THE ERN ZONES. A FAIR AMOUNT OF
CLOUDINESS EXPECTED...WITH PATCHY FOG DEVELOPING OVERNIGHT. TEMPS
WERE BUMPED UP A HAIR...GIVEN THE HIGH DEWPTS.

FOR TUESDAY...A LOOK AT THE 12Z CONVECTIVE ALLOWING MODELS...IT
APPEARS SHWR AND TSTM COVERAGE MAY NOT BE QUITE AS HIGH AS EXPECTED.
SO I CUT BACK POPS A LITTLE BIT...ESP IN THE PIEDMONT. STILL
EXPECTING SCATTERED STORMS...WITH A HIGHER MICROBURST THREAT (CAPE
2500-3500 J/KG IN FCST SNDGS WITH VERY WEAK FLOW SFC-350MB). HIGHS
WILL BE ABOVE NORMAL UNDER BUILDING RIDGE ALOFT.
-- End Changed Discussion --

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...
AS OF 230 PM MONDAY...LOOKS AS IF NOT MUCH HAS CHANGED WITH REGARDS
TO MODEL GUIDANCE IN THE SHORT TERM.  REMNANTS OF UPPER TROF
RESPONSIBLE FOR THE WEATHER ACROSS THE REGION OVER THE LAST FEW DAYS
WILL FINALLY BEGIN TO PUSH OF THE EAST COAST TUESDAY EVENING.  AN
UPPER LEVEL RIDGE WILL BUILD IN ACROSS THE REGION DURING THE
OVERNIGHT HOURS OF WEDNESDAY MORNING.  MODEL SOUNDINGS CONTINUE TO
INDICATE WEAK CAPPING AND MODERATE INSTABILITY THROUGH THE PROFILE
WEDNESDAY.  EXPECTING CONVECTION TO GET GOING ACROSS THE MOUNTAINS
EARLY ON WEDNESDAY AS MECHANICAL FORCING WILL OVERCOME ANY WEAK
INVERSIONS.  CONVECTIVE ACTIVITY WILL SPREAD SOUTHEAST ACROSS
THE I85 CORRIDOR BY MID TO LATE AFTERNOON AS DIURNAL HEATING
DESTABILIZES THE BOUNDARY LAYER FURTHER.  SHEAR PROFILES LOOK RATHER
WEAK ACROSS NORTHEAST GEORGIA AND THE CAROLINAS DURING THIS
PERIOD SO EXPECTING ANY CONVECTION TO BE SOMEWHAT DISORGANIZED
LEADING TO MORE OF A PULSE/CLUSTER TYPE STORM MODE THREAT.  WITH
THAT...THE MAIN THREATS WITH ANY OF THESE STORMS WILL BE BRIEF HEAVY
RAINFALL AND GUSTY WINDS.  TEMPERATURES ON WEDNESDAY WILL DEPEND
SOMEWHAT ON OVERALL COVERAGE OF SHOWER AND THUNDERSTORM ACTIVITY IN
THE AREA.  HOWEVER...WENT WITH SLIGHTLY ABOVE NORMAL LOWS...AND NEAR
NORMAL HIGHS.

MEANWHILE...DEEP UPPER LEVEL TROF AND ITS ASSOCIATED SURFACE LOW
WILL CONTINUE TO SLOWLY MOVE TO THE EAST ACROSS THE GREAT LAKES
REGION.  A SURFACE COLD FRONT EXTENDING TO THE SOUTHEAST ACROSS THE
MISSISSIPPI RIVER VALLEY WILL CONTINUE ITS EASTWARD PROGRESSION
TOWARDS THE APPALACHIANS.  FORECAST MODELS SEEM TO SHOW SLIGHTLY
WEAKER INSTABILITY ACROSS THE REGION ON THURSDAY...HOWEVER SHEAR
PROFILES LOOK STRONGER AS THE UPPER LEVEL WAVE PASSES BY AND THE
SURFACE COLD FRONT APPROACHES.  AS THE FRONT MOVES INTO THE AREA ON
THURSDAY AFTERNOON...HEIGHTS WILL FALL ALLOWING SHOWERS AND
THUNDERSTORMS TO INITIATE ACROSS THE HIGHER TERRAINS FIRST...THEN
SPREAD TO THE SOUTHEAST ACROSS THE PIEDMONTS OF THE CAROLINAS.  SOME
OF THESE THUNDERSTORMS COULD BE STRONG TO SEVERE AS UPPER LEVEL
SUPPORT COULD ALLOW FOR SOME ORGANIZATION.  THUS...THE MAIN THREATS
ON THURSDAY WILL BE HEAVY RAINFALL...AND THE POTENTIAL FOR DAMAGING
WIND GUSTS AND SMALL HAIL.  OVERALL TEMPERATURES ON THURSDAY WILL BE
SLIGHTLY ABOVE NORMAL.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
AS OF 100 PM MONDAY...THE LARGE SCALE PATTERN IS WELL AGREED UPON
THROUGH THE PERIOD. A STRONG...YET MOISTURE LIMITED S/W WILL CROSS
OVER THE FCST AREA FRI...WHILE A STRONG MEAN RIDGE AXIS DEVELOPS TO
THE WEST AND DOMINATES THE SENSIBLE WEATHER THROUGH MON.

THERE ARE ENOUGH VARYING IDEAS BTW THE OP MODELS WRT THE AIRMASS
CHANGE AND POSSIBILITY OF A BACK DOOR COLD FRONT SUN TO KEEP POPS ON
THE LOW END SUN/MON. BEFORE THAT...FRI SHOULD SEE THE BEST CHANCE FOR
SHOWERS AND DEEPER CONVECTION...BUT THE UPPER S/W WILL BE BATTLING A
H8/H7 LAYER OF CAA AND A DRYING NE/LY SFC FLOW. SO...WILL KEEP POPS
LOW END CHANCE AND MAINLY ACROSS THE HIGHER TERRAIN DUE TO GOOD MECH
LIFT ENHANCEMENT AND LLVL MOISTURE REMAINING RELATIVELY HIGH. MODEL
SOUNDINGS ARE SHOWING SOME WEAK AFTERNOON INSTABILITY AND PROBABLY
ENOUGH TO SPAWN A FEW CONVECTIVE SHOWERS WITH THE DEEP
OMEGA...HOWEVER TSTMS SHOULD BE HARD TO DEVELOP WITHIN A DEEPLY DRY
ATMOS IN PLACE.

THINGS WILL REMAIN DRY THROUGH THE WEEKEND WITH A LOW CONFIDENCE
BACK DOOR COLD FRONT BUILDING IN FROM THE NORTH SUN. THE ECWMF IS
MORE PRONOUNCED WITH THIS FEATURE BUT THE GFS ALSO HAS A LLVL
THETA/E BOUNDARY IN THE AREA WHICH LOOKS TO LINGER INTO MON. THE
MODELS DIVERGE ONCE AGAIN MON WITH THE AMOUNT OF S/W ENERGY ROUNDING
THE UPSTREAM RIDGE...WITH THE GFS MORE DEFINITIVE ON ENOUGH UPPER
SUPPORT TO GENERATE AT LEAST SCT -SHRA AND TSTMS ACROSS THE HIGH
TERRAIN AND ALONG THE AFOREMENTIONED SFC BOUNDARY. THE ECMWF ON THE
OTHERHAND...KEEPS THE BEST ULVL DYNAMICS WELL TO THE NORTH AND THUS
NO GOOD CHANCE OF PRECIP IN THE CWFA. THUS...POPS LATE SUN THROUGH
MON HAVE BEEN MAINTAINED IN THE SLIGHT RANGE AND LIMITED TO MAINLY
THE WRN ZONES. MAX TEMPS WILL BEGIN THE PERIOD RIGHT AROUND
NORMAL...AND PROBABLY ADJ DOWN A COUPLE DEGREES SUN AND MON WITH THE
CP AIRMASS AND WEAK CAD CONFIG.

&&

.AVIATION /05Z TUESDAY THROUGH SATURDAY/...
-- Changed Discussion --
AT KCLT...NO PRECIPITATION WILL BE CARRIED OVERNIGHT AS HEATING HAS
ABATED AND THE ATMOSPHERE HAS STABILIZED. A BLEND OF MODEL GUIDANCE
SUPPORTS ONLY LOW VFR VSBY IN FOG TOWARD DAWN...DESPITE MOIST LOW
LEVELS. CONFIDENCE ON CIGS IS QUITE LOW DUE TO MODEL VARIANCE.
CLOUDS HEIGHTS WILL GENERALLY BE BASED ON CURRENT OBSERVATIONS...
WITH A DAYBREAK DIP TO MVFR...WHICH IS SUPPORTED BY THE MIDDLE OF
THE ROAD MET GUIDANCE. INSTABILITY WILL SUPPORT ROBUST CONVECTION ON
TUESDAY...MAINLY IN THE AFTERNOON...BUT COVERAGE WILL BE RATHER
LIMITED AS UPPER LEVEL SUPPORT DRIFTS TO THE EAST. LIGHT SOUTH WINDS
WILL PERSIST.

ELSEWHERE...RADAR TRENDS SUPPORT LEAVING PRECIPITATION OUT OF THE
TAFS OVERNIGHT IN A STABILIZING ATMOSPHERE. A GUIDANCE BLEND
SUPPORTS A DAYBREAK IFR CIG AT KAVL...LOW VFR AT KGSP...AND MVFR AT
ALL OTHER SITES. CONFIDENCE IS LOW ON CIGS...ESPECIALLY AT
DAYBREAK...AS MODEL GUIDANCE VARIES CONSIDERABLY...AND THERE IS
STILL SOME DOUBT CLOUD COVERAGE WILL INCREASE ENOUGH BY DAWN IN THE
WAKE OF A DEPARTING UPPER SYSTEM. MVFR CIGS WILL BE CARRIED TOWARD
DAYBREAK AT KGMU...KGSP AND KHKY...WHICH IS SUPPORTED BY LESS
EXTREME MET GUIDANCE. ALTHOUGH CONVECTIVE COVERAGE WILL BE LIMITED
TUESDAY AS UPPER FORCING DEPARTS TO THE EAST...INSTABILITY WILL
SUPPORT VIGOROUS UPDRAFTS. LIGHT SOUTH WINDS ARE EXPECTED.

OUTLOOK... CEILING AND VISIBILITY RESTRICTIONS IN SCATTERED SHOWERS
AND THUNDERSTORMS AGAIN WEDNESDAY AND THURSDAY AFTN/EVE. GENERALLY
VFR WEATHER FRIDAY AND SATURDAY.

CONFIDENCE TABLE...

            05-11Z        11-17Z        17-23Z        23-05Z
KCLT       HIGH  83%     MED   78%     HIGH 100%     HIGH 100%
KGSP       HIGH  85%     MED   74%     HIGH 100%     HIGH 100%
KAVL       LOW   57%     LOW   56%     LOW   57%     LOW   58%
KHKY       MED   72%     HIGH  84%     HIGH 100%     HIGH 100%
KGMU       HIGH  85%     HIGH  87%     HIGH 100%     HIGH 100%
KAND       MED   70%     HIGH  87%     HIGH 100%     HIGH 100%

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
-- End Changed Discussion --

&&

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

&&

$$
SYNOPSIS...LEV
NEAR TERM...ARK/JAT
SHORT TERM...CDG
LONG TERM...SBK
AVIATION...JAT