108  
FXUS62 KGSP 250745  
AFDGSP  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC  
345 AM EDT SAT APR 25 2015  
   
SYNOPSIS
 
 
AN AREA OF LOW PRESSURE WILL MOVE EAST FROM THE CENTRAL PLAINS  
STATES INTO THE SOUTHEAST TODAY. 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 TONIGHT/
 
 
AS OF 340 AM...HIGH DEBRIS CLOUDS WILL LIKELY INCREASE ACROSS THE  
CWA THROUGH SUNRISE. FURTHER WEST...WATER VAPOR LOOP INDICATED A  
CLOSED MID LEVEL LOW OVER THE CENTRAL PLAINS...DRIFTING TO THE EAST-  
NORTHEAST. SOUTH EAST OF THE MID LEVEL LOW...A LARGE BAND OF  
CONVECTION WAS TRACKING EAST ACROSS NRN MS/AL. THIS ACTIVITY LARGELY  
ORIGINATED NEAR THE SFC WARM FRONT AND WITHIN A BELT OF UPPER JET  
DIVERGENCE. HOWEVER...THE CONVECTION IS TRACKING QUICKLY  
EAST...OUTPACING THE MOST SUPPORTIVE ENVIRONMENT. AS A  
RESULT...RECENT IR SATELLITE IMAGES INDICATED A STEADY WARMING TREND  
OF CLOUD TOP TEMPERATURES...LIGHTNING COUNTS WERE ALSO DECREASING.  
THESE WEAKENING TRENDS MATCH UP WELL WITH THE CONVECTION ALLOWING  
MODELS...WHICH INDICATES THAT THE DEEP CONVECTION WILL STEADY WEAKEN  
AS IT TRACKS EAST ALONG THE I-20 CORRIDOR. A LARGE AREA RAIN IS  
FORECAST TO OCCUR ACROSS THE CWA THIS MORNING...PRIMARILY WEST OF I-  
26. I WILL INDICATE THE RAINFALL TO FALL FROM SHRA WITH SOME  
EMBEDDED THUNDERSTORMS. SEVERE WEATHER ASSOCIATED WITH THE MORNING  
ACTIVITY APPEARS MINIMAL.  
 
BY MID DAY...THE BULK OF THE DECAYING CONVECTION IS EXPECTED TO PUSH  
EAST OF THE REGION. DURING THE AFTERNOON...THE PRIMARY CHALLENGE  
WILL BE THE PLACEMENT OF A WEAK IN-SITU AREA OF COLD AIR DAMMING.  
BASED OFF THE LATEST NAM...I WILL INDICATE THAT THE NC FOOTHILLS AND  
NRN PIEDMONT WILL SEE LIGHT NE WINDS AND THICK AND PERSISTENT CLOUD  
COVER. HOWEVER...AREAS TO THE SOUTH AND WEST MAY EXPERIENCE DEEP  
SOUTHWEST FLOW AND DIURNALLY WARMING TEMPERATURES. IN FACT...THE NAM  
SHOWS A HORSESHOE SHAPED AREA OF SB CAPE ACROSS THE MTNS AND  
PORTIONS OF THE UPPER SAVANNAH RIVER VALLEY AND SC UPSTATE EAST OF I-  
85. IN ADDITION...IMPRESSIVE SHEAR VALUES DEVELOP WITHIN THE  
UNSTABLE AREAS...PEAKING DURING THE MID EVENING HOURS. HEILICITY  
BETWEEN SFC-1KM IS FORECAST TO RANGE AROUND 300 M2/S2...EHI VALUES  
FROM 2.5-4...AND SWEAT VALUES GENERALLY AROUND 350. HOWEVER...CIN  
VALUES FROM THE NAM SOUNDING RANGE FROM 30 TO 50 J/KG THIS EVENING.  
MODELS INDICATE THAT CONVECTION COVERAGE WILL BE AT A MINIMUM DURING  
THE LATE AFTERNOON AND EVENING.  
 
TONIGHT...A WELL DEVELOPED SFC LOW WILL SLIDE EAST ACROSS  
KY/TN...CROSSING THE SOUTHERN APPALACHIANS AROUND MIDNIGHT. AT THE  
MID LEVELS...THE CLOSED LOW FROM THIS MORNING WILL DEVELOP INTO A  
POSITIVE TILTED TROF AND WILL RIPPLE EAST OVER THE SFC LOW. NEAR TERM  
MODELS INDICATE THAT A MCS WILL LIKELY DEVELOP AHEAD OF THE LOW  
CENTER...STEERING FLOW SHOULD RESULT IN THE SYSTEM TO TRACK EAST-  
SOUTHEAST TONIGHT. BASED ON EXPERIENCE...I ANTICIPATE THAT THE MCS  
SPEED WILL BE GREATER THAN THE MODEL CONSENSUS. THIS ADJUSTMENT  
WOULD PLACE THE MCS OVER THE NC MTNS DURING THE EARLY MORNING HOURS  
SUNDAY. BY THAT TIME...THE GREATEST SHEAR AND INSTABILITY VALUES  
HAVE ENDED...LEAVING WEAK TO MODERATE VALUES ACROSS THE MTNS AND  
SC/GA LINE. THIS WILL LIKELY SUPPORT THE MCS REACHING THE MTNS...BUT  
STEADILY WEAKENING AS THE ACTIVITY APPROACHES THE FOOTHILLS.  
HOWEVER...LOW WBZ AND LINGERING SHEAR AND INSTABILITY WOULD LIKELY  
YIELD SEVERAL SEVERE WIND AND HAIL EVENTS AS THE CONVECTION ARRIVES.  
WE WILL CONTINUE TO HIGHLIGHT IN THE HWO.  
 

 
   
SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/
 
 
AS OF 330 AM SATURDAY...THE SHORT TERM BEGINS WITH TRYING TO TIME  
THE EXIT OF THE SURFACE LOW OFF THE CAROLINA COAST. SHORT TERM  
GUIDANCE IN PRETTY GOOD AGREEMENT WITH EAST-WEST ELONGATED TROUGH  
STRETCHING WESTWARD INTO OUR AREA FROM THE CENTER OF THE SURFACE LOW  
IN THE ATLANTIC AT THE START OF THE SHORT TERM. SPLIT FLOW PATTERN  
CONTINUES ALOFT...WITH SHORTWAVE ALSO PUSHING OFFSHORE AND MEAN  
TROUGH TAKING OVER. STROGN DOWNSLOPING COMPONENT ON SUNDAY WILL LEAD  
TO HIGHS INCREASING TO WELL ABOVE NORMAL VALUES ACROSS THE UPSTATE  
AND PORTIONS OF THE PIEDMONT BEFORE THE ACTUAL COLD FRONT PUSHES  
THROUGH...THOUGH ESPECIALLY THE NORTHERN MOUNTAINS WILL BE ON THE  
COOL SIDE. MIDLEVEL MOISTURE HANGS ON ENOUGH TO KEEP SOME CLOUDINESS  
AND PERHAPS SOME LINGERING SHOWERS ACROSS THE AREA THROUGH THE EARLY  
PART OF THE DAY...BUT WE SHOULD START SEEING A CLEARING TREND  
OVERNIGHT SUNDAY NIGHT INTO MONDAY.  
 
SHOULD SEE A PRETTY MARKED DIFFERENCE IN HIGHS ON MONDAY BEHIND THE  
FRONT WITH DEEP-LAYER CAA ACROSS THE AREA...THOUGH THE MAGNITUDE OF  
THE CAA DECREASES THROUGH THE DAY. ANOTHER SHORTWAVE AXIS PUSHES  
DOWN THE UPPER TROUGH...STILL FIRMLY IN PLACE ACROSS THE EASTERN  
PORTION OF THE COUNTRY...LATE MONDAY INTO EARLY TUESDAY. THIS TROUGH  
MANAGES TO FINALLY PUSH THE DEEP UPPER LOW FARTHER OUT INTO THE  
ATLANTIC BUT WILL USHER IN COOLER HIGH PRESSURE FOR THE BEGINNING OF  
THE EXTENDED.  
 

 
   
LONG TERM /TUESDAY THROUGH FRIDAY/
 
 
AS OF 345 AM SATURDAY...HIGH PRESSURE CENTERED ACROSS THE NORTHERN  
PLAINS AND SOUTHERN CANADA WILL BE RIDGING INTO THE SOUTHEAST AS WE  
START THE EXTENDED...WITH THE AFOREMENTIONED UPPER SHORTWAVE DIGGING  
AROUND THE WESTERN PERIPHERY OF THE LOW OVER THE NORTHWEST ATLANTIC.  
MEANWHILE...A CUTOFF LOW OVER THE SOUTHERN PLAINS WILL BEGIN TO  
SLIDE EAST AS THE WAVETRAIN FINALLY PROGRESSES. HEIGHT FALLS AHEAD  
OF THE UPPER LOW WILL INDUCE CYCLOGENESIS ACROSS THE NORTHERN  
GULF...SPREADING MOISTURE INTO THE LOWER MISSISSIPPI VALLEY AND  
SOUTHEAST THROUGH THE MIDDLE OF THE WORK WEEK. BOTH THE GFS AND  
ECMWF HAVE BEEN TRENDING THE SURFACE LOW FARTHER SOUTH WITH  
SUBSEQUENT RUNS AND FOR THIS MORNING IT IS LITTLE DIFFERENT...WITH  
THE LOW PUSHING EAST ACROSS THE FLORIDA PANHANDLE UNTIL WEDNESDAY  
NIGHT AND ONLY LIFTING SLIGHTLY INTO SOUTH GEORGIA. THE ECMWF HAS  
SPED THE SYSTEM UP WITH A DEEPER NORTH-SOUTH ELONGATED TROUGH PHASED  
WITH THE GREAT LAKES TROUGH...WHEREAS THE GFS HAS TRENDED A LITTLE  
SLOWER AND TAKES A LITTLE LONGER TO PHASE THE FORMER CUTOFF LOW WITH  
THE LARGER SYSTEM. IN ANY CASE...CLOUDY AND COOL CONDITIONS WILL  
PREVAIL THROUGH THE MIDDLE OF THE WEEK...WITH HIGHS ESPECIALLY  
WEDNESDAY A GOOD 10 DEGREES BELOW SEASONAL NORMALS. VERY HARD TO  
TELL AT THIS POINT...WITH THE DIFFERENCES IN THE MODELS PLUS LACK OF  
RUN-TO-RUN CONTINUITY...WHEN THE PRECIP WILL MOVE OUT. IN GENERAL  
HAVE CONTINUED TREND OF MID-RANGE POPS...WITH A FEW LIKELIES SOUTH  
ON WEDNESDAY...WITH A SLOW DRYING TREND THURSDAY INTO THURSDAY  
EVENING. MORE DIVERGENCE IN EXTENDED GUIDANCE AT THE END OF THE  
PERIOD BUT OVERALL EXPECT A DRYING TREND TOWARD THE END OF THE WORK  
WEEK WITH TEMPERATURES TRENDING BACK TOWARDS CLIMO.  
 

 
   
AVIATION /08Z SATURDAY THROUGH WEDNESDAY/
 
 
AT KCLT...HIGH DEBRIS CLOUDS WILL LIKELY INCREASE ACROSS THE NC  
PIEDMONT THROUGH SUNRISE. LLVL MOISTURE SHOULD RAPIDLY INCREASE FROM  
THE SOUTHWEST DURING THE MID TO LATE MORNING HOURS...RESULTING IN  
MVFR CEILINGS AND LIGHT SHRA. CEILINGS WILL CONTINUE TO LOWER INTO  
THE AFTERNOON AS LLVL WINDS INCREASES...DEEPENING THE LLVL FORCING.  
BASED ON THE TIMING OF THE NAM...SBCAPE AND STEEPENING LAPSE RATES  
WILL DEVELOP DURING THE LATE AFTERNOON AND EARLY EVENING. I WILL  
HIGHLIGHT THE 22Z TO 1Z WINDOW WITH A PROB30 FOR TSRA. BY LATE  
EVENING...FORECAST SOUNDINGS BECOME MOIST THROUGH A DEEP LAYER AND  
MODEL CLOUDS ARE EASILY WITHIN IFR. WINDS DURING THE 6Z TAF WILL  
REMAIN FROM THE SOUTH-SOUTHWEST...RESULTING FROM THE APPROACH OF A  
WELL ORGANIZED SFC LOW OVER KY/TN.  
 
ELSEWHERE...CONDITIONS WILL GENERALLY FOLLOW THE TREND DISCUSSED FOR  
CLT ABOVE. HOWEVER...INCREASES IN MOISTURE AND INSTABILITY WILL  
REACH KAND SEVERAL HOURS EARLIER THAN KCLT. THE DEEP MOISTURE AND  
FORCING WILL SPREAD SW TO NE...REACHING KHKY AROUND THE SAME PERIOD  
AS KCLT. KHKY MAY DEVELOP WINDS FROM THE EAST-NORTHEAST AROUND  
SUNRISE...PERSISTING THROUGH THE REST OF THE TAF PERIOD.  
ELSEWHERE...WINDS WILL REMAIN FROM THE SOUTH-SOUTHWEST.  
THUNDERSTORMS SHOULD DEVELOP OVER THE REGION DURING THE LATE  
AFTERNOON TO EARLY EVENING PERIOD. THE GREATEST OVERLAP OF SHEAR AND  
INSTABILITY WILL GENERALLY OCCUR ALONG AND WEST OF I-26...SUPPORTING  
DAMAGING WINDS AND HAIL. I WILL HIGHLIGHT THE PERIOD OF DEEP  
CONVECTION WITH A PROB30.  
 
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...  
 
07-13Z 13-19Z 19-01Z 01-06Z  
KCLT HIGH 100% MED 75% MED 73% MED 73%  
KGSP HIGH 100% MED 66% HIGH 86% HIGH 83%  
KAVL HIGH 100% MED 75% MED 74% HIGH 100%  
KHKY HIGH 100% MED 69% MED 73% HIGH 81%  
KGMU HIGH 100% MED 69% MED 74% HIGH 86%  
KAND HIGH 100% MED 62% HIGH 82% HIGH 98%  
 
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...NED  
SHORT TERM...TDP  
LONG TERM...TDP  
AVIATION...NED  
 
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