Area Forecast Discussion
Issued by NWS Greer, SC

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292
FXUS62 KGSP 301512
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
1112 AM EDT SAT APR 30 2016

.SYNOPSIS...
AN UPPER LOW WILL APPROACH THE AREA WITH A FRONTAL PASSAGE MONDAY
MORNING.  A SECOND MAJOR FRONT AHEAD OF A MASS OF COLD AIR FROM
CANADA IS EXPECTED BY THURSDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 1100 AM EDT SATURDAY...LOW STRATUS CONTINUES TO LINGER OVER
THE NC PIEDMONT THIS MORNING LIKELY STUNTING TEMPERATURE TRENDS THIS
AFTERNOON...THEREFORE BLENDED IN LATEST MAV WHICH FAVORED LOWER MAX
TEMPS THAN PREVIOUSLY FORECAST.  OTHERWISE...MID/HIGH CLOUDS OVER
NORTHEAST GA AND THE SC UPSTATE AS WELL AS WESTERN NC ARE STILL
EXPECTED TO GIVE WAY TO DEVELOPING CU AS THE LLVS DESTABILIZE THIS
AFTERNOON.  SPEAKING OF...LATEST HRRR/RAP CONTINUE TO BACK UP OTHER
HIGH RES CAMGUIDE IN THAT CONVECTION WILL INITIATE ALONG THE I85
CORRIDOR THIS AFTERNOON AS HEATING LEADS TO
DESTABILIZATION...EXPANDING FURTHER NORTH OVER THE REMAINDER OF THE
UPSTATE AND WESTERN NC.  WITH THE RETREATING WEDGE BOUNDARY OVER THE
NC PIEDMONT PROVIDING FOCUS FOR A LOCALIZED SRH MAXIMA...THINK THE
POSSIBILITY FOR AN ISOLATED SPIN UP TORNADO REMAINS.  LASTLY...DID
BUMP UP POPS OVER THE UPSTATE AND NC PIEDMONT FOR THE LATE AFTERNOON
HOURS.

HIGH PRESSURE OVER EASTERN CANADA THIS MORNING WILL CONTINUE TO
EXPAND...WHICH IS EXPECTED TO PUSH A BACK DOOR COLD FRONT OF SORTS
FARTHER TO THE SOUTH AND WEST INTO THE WESTERN PART OF NC DURING THE
DAY. ASSUMING THIS HAPPENS...ALL SORTS OF SUBTLE COMPLICATIONS
ARISE. EXTENSIVE LOW CLOUDS BEHIND THE BOUNDARY WOULD KEEP TEMPS
COOLER WELL INTO THE DAY AND DELAY OR PERHAPS EVEN PREVENT MUCH IN
THE WAY OF DESTABILIZATION TO THE NORTH OF IT. MEANWHILE...TO THE
SOUTH OF THE BOUNDARY...WE EXPECT SOME DESTABILIZATION TO OCCUR BY
MIDDAY. HOWEVER...IT MIGHT NOT BECOME AS UNSTABLE AS PREVIOUSLY
THOUGHT...PERHAPS ONLY ON THE ORDER OF 1000-1500 J/KG OF CAPE BASED
ON THE NEW NAM/SREF...WITH THE BETTER CAPE AND LAPSE RATES REMAINING
TO OUR WEST. SHEAR STILL INCREASES LATE IN THE DAY TO SOMETHING ON
THE ORDER OF 35-40 KT SO THERE REMAINS A DISTINCT POSSIBILITY OF
ORGANIZED THUNDERSTORMS. WHAT WE LACK IN INSTABILITY MIGHT BE
COMPENSATED BY A STRONGER SURFACE BOUNDARY WITH BETTER SHEAR AND
STORM RELATIVE HELICITY IN ITS VICINITY. BOTTOM LINE IS WE MIGHT NOT
HAVE QUITE AS GOOD A CHANCE AT SEVERE STORMS ACROSS MUCH OF THE
REGION...STILL ENOUGH TO WARRANT THE MARGINAL IN THE NEW DAY 1
CONVECTIVE OUTLOOK...BUT AT THE SAME TIME WE MIGHT HAVE A SLIGHTLY
BETTER CHANCE AT SEVERE STORMS NEAR THE BOUNDARY. THIS WILL BE
MENTIONED IN THE HWO. THE CONVECTION-ALLOWING MODEL GUIDANCE
DEVELOPS NUMEROUS COVERAGE...SO A RELATIVELY HIGH PRECIP PROBABILITY
WILL BE KEPT...AND IT RAMPS UP THROUGH THE CHANCE RANGE AND INTO THE
LIKELY RANGE THROUGH THE AFTERNOON AND EVENING...AIDED BY THE
PASSAGE OF A SHORT WAVE TROF AND UPPER DIVERGENCE ALOFT. TEMPS WILL
DEPEND ON HOW STRONG THE BOUNDARY IS...WITH PERHAPS NEAR NORMAL
TEMPS OVER THE NW PIEDMONT AND ABOVE NORMAL ELSEWHERE.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
AS OF 300 AM EDT SATURDAY...BY SUNDAY MORNING...UPPER TROUGH
ELONGATES AND MOVES INTO THE UPPER MIDWEST WITH A LARGE AREA OF LOW
SURFACE PRESSURE AROUND IN-OH-KY.  POSITION OF SURFACE LOW WILL
ALLOW BOUNDARY-LAYER MOIST ADVECTION TO CONTINUE OVER THE GSP CWA ON
SOUTHWESTERLY 850MB FLOW THAT BECOMES MORE WESTERLY SUNDAY NIGHT AS
MAJOR COLD FRONT APPROACHES THE APPALACHIANS.  ACTUAL FROPA OCCURS
MONDAY MORNING WITH NORTHWESTERLY FLOW BEHIND FRONT AND ANY DEEP MOISTURE
BEING SCOURED SOUTHEASTWARD.

MAIN FORECAST IMPACT WILL BE THE POSSIBILITY OF THUNDERSTORMS ON
SUNDAY.  CURRENT GFS GIVES LI TO -7 OVER MOST OF THE CWA WITH BULK
SHEAR OF 25 TO 35 KTS SUNDAY AFTERNOON...WITH THINGS MORE
CONVECTIVELY STABLE ON MONDAY AS BL MOISTURE MOVES SOUTHEASTWARD.
OTHER MODELS HAVE LESS INSTABILITY...AND SHEAR FORECAST IS MODERATE
AT BEST.  STILL...IF INSTABILITY CAN TOP 1000 J/KG SUNDAY AFTERNOON
SOME LOW-END SEVERE MAY BE POSSIBLE.  CURRENTLY EXPECT THUNDERSTORM
ACTIVITY PRIMARILY SUNDAY AFTERNOON MOVING GENERALLY WEST TO EAST
ACROSS THE AREA...WITH MONDAY MOSTLY DRY EXCEPT FOR SOME POSSIBLE
UPSLOPE GENERATED SHOWERS ON THE NORTHWEST SIDE OF THE APPALACHIANS.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
AS OF 300 AM SATURDAY...FOLLOWING COLD FROPA ON MONDAY...TUESDAY
THROUGH FRIDAY WILL RETURN TO SEASONALLY NORMAL TEMPERATURES AND
DRYER CONDITIONS.  AREA WILL BE INITIALLY SOUTH OF A LARGE UPPER LOW
AND SOME SHORTWAVE ENERGY MAY ROTATE AROUND THE LOW ACROSS THE
AREA TO GIVE SOME SCATTERED SHOWERS TUESDAY OR WEDNESDAY.  AS THE
EASTERN CONUS UPPER LOW DEVELOPS...A LATE SEASON SHOT OF ARCTIC AIR
DESCENDS FROM CANADA ACROSS THE GREAT LAKES AND REACHES THE
APPALACHIANS BY THURSDAY...BRINGING THE NEXT CHANCE OF SIGNIFICANT
PRECIPITATION THURSDAY AND FRIDAY.  LATEST GFS IS RATHER COLD WITH
THE THURS.-FRI. SYSTEM WITH 500-1000MB THICKNESS DOWN TO 531 DAM AS
FAR SOUTH AS NORTH CAROLINA FRIDAY MORNING WITH 850MB TEMPERATURES
DOWN TO -2C. WOULD STILL EXPECT SURFACE TEMPERATURES TO REMAIN IN
THE MID TO UPPER 30S AT WORST OVER HIGHER TERRAIN...PRECLUDING FOR
THE MOMENT A CHANCE FOR SNOW.  HOWEVER...SYSTEM WILL COOL HIGHS ON
FRIDAY TO 10 DEGREES BELOW NORMAL...WITH A RAPID RECOVERY TO NORMAL
ON SATURDAY.

WHILE THE GFS AND EC STILL HAVE CONSIDERABLE DISAGREEMENT...THEY ARE
ALIGNED BETTER THAN YESTERDAY. IN FACT...THE EC LOOKS QUITE SIMILAR
TO THE GFS FRIDAY MORNING WITH 850MB TEMPS OF AROUND -2C REACHING
THE APPALACHIANS.  A MAJOR DIFFERENCE BETWEEN GFS AND EC IS THAT THE
EC HAS A MUCH STRONGER SHORT WAVE ROTATING AROUND THE NORTHEAST
LOW...GIVING STRONGER CHANCES FOR LIGHT RAIN WED.-THURS.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
AT KCLT AND KHKY...IFR HAS REACHED KHKY...AND IS ON THE DOORSTEP AT
KCLT. A WEAK BACK DOOR FRONT WILL DRIFT DOWN FROM THE NE AND SHOULD
BRING THE MVFR/IFR CLOUD DECK ACROSS KCLT AT THE START OF THE
PERIOD. THE LOW CLOUDS SHOULD LIFT/ERODE IN THE MIDDLE OF THE DAY
AND GIVE WAY TO CONVECTIVE CLOUDS IN THE MID/LATE AFTERNOON...AT
WHICH TIME SHOWERS/THUNDERSTORMS BECOME A CONCERN. THIS WILL BE
HANDLED WITH A TEMPO WITH THE TIMING BASED ON A CONSENSUS OF THE
GUIDANCE. WIND SHOULD COME AROUND TO SE AS THIS HAPPENS. EITHER
WAY...EXPECT FLIGHT RESTRICTIONS INSIDE THE PRECIP AREAS. CEILING
MAY COME BACK DOWN LATE SATURDAY EVENING BUT THIS DETAIL HAD TO BE
LEFT OUT OF THE TAFS FOR NOW.

ELSEWHERE...HOW FAR TO THE SW THE LOW CLOUDS WOULD MOVE BEFORE
RUNNING UP AGAINST A WARMING BOUNDARY LAYER AFTER SUNRISE IS OPEN TO
DEBATE. A CONSENSUS OF THE GUIDANCE WOULD BRING AT LEAST A BKN/OVC
LOW CLOUD DECK DOWN ACROSS THE NC PIEDMONT AND FOOTHILLS WITH A
CLOUD BASE IN THE MVFR RANGE...BUT IT WOULD NOT REACH ANY OF THE
UPSTATE TAF SITES...SO VFR CONDITIONS WILL PREVAIL AT LEAST THRU
MID-MORNING...AND WILL CONTINUE OUTSIDE OF SHOWERS/STORMS THAT
DEVELOP IN THE AFTERNOON/EVENING. WILL HANDLE CONVECTION WITH A
TEMPO GROUP IN THE LATE AFTERNOON AND WILL TRANSITION TO A PROB30 IN
THE EVENING WHEN COVERAGE IS EXPECTED TO DIMINISH. WIND SHOULD BE
LIGHT NE OVERNIGHT...THEN SE DURING THE DAY.

OUTLOOK...UNSETTLED WEATHER/OCCASIONAL CONVECTION/RESTRICTIONS ARE
EXPECTED TO CONTINUE INTO EARLY NEXT WEEK...AS A FRONT BECOMES
STALLED NEAR THE AREA. DEFINITIVE DRYING DOES NOT APPEAR LIKELY
UNTIL AT LEAST MID-WEEK.

CONFIDENCE TABLE...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       MED   74%     HIGH  85%     MED   79%     HIGH  83%
KGSP       HIGH 100%     HIGH 100%     HIGH  80%     HIGH 100%
KAVL       HIGH 100%     HIGH  94%     MED   79%     MED   61%
KHKY       MED   72%     LOW   53%     HIGH  98%     HIGH 100%
KGMU       HIGH 100%     HIGH 100%     MED   73%     HIGH  94%
KAND       HIGH 100%     HIGH 100%     MED   78%     MED   65%

THE PERCENTAGE REFLECTS THE NUMBER OF GUIDANCE MEMBERS AGREEING
WITH THE SCHEDULE TAF ISSUANCE FLIGHT RULE CATEGORY.  COMPLET HOURLY
EXPERIMENTAL AVIATION FORECAST CONSISTENCY TABLES AND ENSEMBLE FORECASTS
ARE AVAILABLE AT THE FOLLOWING LINK:

WWW.WEATHER.GOV/GSP/AVIATION

&&

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

&&

$$
SYNOPSIS...WJM
NEAR TERM...CDG/PM
SHORT TERM...WJM
LONG TERM...WJM
AVIATION...PM



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