Area Forecast Discussion
Issued by NWS Greer, SC

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000 FXUS62 KGSP 182344 AFDGSP AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC 744 PM EDT SAT APR 18 2015 .SYNOPSIS... A VERY MOIST WARM FRONT WILL LIFT NORTHEAST ACROSS THE AREA TONIGHT INTO SUNDAY. A COLD FRONT WILL THEN CROSS THE AREA ON MONDAY...USHERING IN SEASONABLE TEMPERATURES AND DRY CONDITIONS. && .NEAR TERM /THROUGH SUNDAY/... AS OF 730 PM EDT SATURDAY...ONLY WEAK INSTABILITY LEFT ACROSS THE AREA WITH A DIMINISHING TREND SEEN IN THE INTENSITY OF THE SHOWERS AND FEW THUNDERSTORMS OUT THERE. EVEN THAT THOUGH IS A BIT OF A MISNOMER SINCE WE HAVE NOT SEEN ANY CLOUD-TO-GROUND LIGHTNING IN ANY OF THE ACTIVITY SO FAR THIS EVENING. EXPECT A CONTINUED DIMINISHING TREND FOR THE NEXT COUPLE OF HOURS...RAMPING UP AGAIN AFTER 06Z AND ESPECIALLY AFTER 09Z AS THE WARM FRONT LIFTS NORTH AND ISENTROPIC UPGLIDE INCREASES SIGNIFICANTLY. OVERALL POP TRENDS STILL LOOK OKAY BUT WILL LIKELY NEED TO REFINE THEM WITH THE LATE EVENING UPDATE... MOST LIKELY TO LOWER THEM SOMEWHAT THROUGH THE EARLY OVERNIGHT PERIOD BUT RAISE THEM TOWARD SUNRISE...REALLY SEE NO REASON TO NOT GO 100 PERCENT AS THE DEEPER MOISTURE PUSHES IN. TEMPERATURES LOOK PRETTY GOOD FOR THE MOST PART AND HAVE ONLY UPDATED HOURLY TRENDS BASED ON OBS. AS OF 430 PM EDT SATURDAY...ONLY MINOR TWEAKS TO THE NEAR TERM FORECAST AT THIS TIME. UPDATED HOURLY TEMP/DEWPOINT/SKY TRENDS TO LINE UP WITH OBSERVATIONS. NO CHANGES TO POPS AT THIS TIME AS LATE AFTERNOON AND EVENING TRENDS LOOK GOOD PER RADAR AND HIRES REFLECTIVITY FORECASTS...BUT SOME OF THE LATTER BRING MORE WIDESPREAD PRECIPITATION INTO NORTHEAST GEORGIA AND THE UPSTATE BETWEEN 03-06Z. WILL CONTINUE TO WATCH TRENDS BUT MAY NEED TO BUMP UP POPS FOR THAT TIMEFRAME IN SUBSEQUENT UPDATES. AS OF 300 PM EDT SATURDAY...PRIMARY FCST CONCERN THROUGH THE NEAR TERM PERIOD WILL BE THE POTENTIAL FOR HEAVY RAIN ACROSS MUCH OF THE AREA ON SUNDAY. AS FOR TODAY...EXPECTING CONVECTION TO CONTINUE TO INITIATE...ESPECIALLY ACROSS THE EASTERN UPSTATE AND NC PIEDMONT WHERE BEST DIFFERENTIAL HEATING HAS OCCURRED. LATEST SPC MESO ANALYSIS INDICATES ROUGHLY 500K/KG SBCAPE SPREAD ACROSS THESE ZONES WITH FURTHER EXPANSION EXPECTED ALONG WITH ADDITIONAL DESTABILIZATION. BEYOND THAT...CONVECTION WILL DIMINISH THIS EVENING WITH HEATING LOSS MAKING WAY FOR INCOMING SHORTWAVE AND ASSOCIATED SURFACE WARM FRONT. THUS...FCST FEATURES CHANCE POPS ACROSS MUCH OF THE AREA THIS AFTERNOON TO ACCOUNT FOR ISOLATED/SCATTERED THUNDERSTORMS... WITH FURTHER INCREASES ACROSS NORTHEAST GA...SOUTHWEST NC...AND THE WESTERN UPSTATE OVERNIGHT. EXPECTING THE UPPER VORT MAX TO PASS THROUGH THE REGION AFTER MIDNIGHT WITH ENHANCED H85 LLJ OUT AHEAD. NEARLY 40-50KTS SOUTHERLY H85 FLOW WILL HELP PROMOTE ENHANCED UPSLOPING ALONG THE SOUTHWEST NC...AND WESTERN NC ESCARPMENT. FURTHERMORE...ANTECEDENT CONDITIONS FROM PREVIOUS DAYS RAINFALL OVER THE UPSTATE AND PORTIONS OF NORTHEAST GA WILL YIELD ENHANCED FLOODING CONCERNS AS A RESULT OF NEARLY 2-4 INCHES OF QPF IN THE FCST. THUS...OPTED TO ISSUE FLOOD WATCH FOR THESE ZONES STARTING TONIGHT AT MIDNIGHT RUNNING THROUGH 8PM SUNDAY. IN ADDITION...EXPECTING CONVECTION ALONG THE WARM FRONT AS IT SWEEPS THROUGH ON SUNDAY. MODELS INDICATE NEARLY 500-1000J/KG SBCAPE WITH MODEST SHEAR AT 30-40KTS BULK. THUS...FCST FEATURES CAT POPS THROUGH THE DAY ON SUNDAY WITH HEAVY RAIN MENTION DUE TO WIDESPREAD SHOWERS/THUNDERSTORMS. NEW SPC DAY2 OUTLOOK PLACES THE REGION IN THE SLIGHT RISK FOR SEVERE WEATHER...WHICH SEEMS TO FIT WELL WITH THE FCST. THEREFORE EXPECTING ADDITIONAL THREATS OF DAMAGING WINDS...AND PERHAPS ANY ISOLATED TORNADO ON SUNDAY. TEMPERATURES THROUGH THE PERIOD WILL REMAIN ABOVE NORMAL. && .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/... AS OF 215 PM SATURDAY...PRECIP CHANCES WILL BE WINDING DOWN SUNDAY EVENING...AS WARM CONVEYOR BELT ZONE SHIFTS INTO THE MID-ATLANTIC. LOW LEVEL MOISTURE WILL LINGER THROUGH THE OVERNIGHT WITHIN DEEP SW FLOW...AND TOKEN SLIGHT/LOW CHANCE POPS WILL BE RETAINED THROUGH MONDAY MORNING. A POTENTIALLY VERY INTERESTING SITUATION THEN EVOLVES FOR MONDAY AFTERNOON...AS HEIGHT FALLS AND ATTENDANT FRONTAL ZONE OVERSPREAD THE REGION FROM THE TENN VALLEY. IT SEEMS THAT THE MODELS ARE DEPICTING AN INCREASINGLY WELL-FORCED FRONT WITH EACH SUCCESSIVE RUN...SWINGING A FAIRLY SUBSTANTIVE SHORT WAVE TROUGH ACROSS THE AREA DURING THE AFTERNOON. IN RESPONSE...THE LOW LEVEL FLOW IS MORE BACKED THAN IN EARLIER RUNS...RESULTING IN BETTER SHEAR PROFILES AS WELL AS IMPROVED FRONTAL CONVERGENCE. SHORT TERM MODELS CONTINUE TO DEPICT ROBUST BUOYANCY...AS ADVECTION OF ELEVATED MIXED LAYER AIR INTO THE SOUTHEAST SUPPORTS MID-LEVEL LAPSE RATES OF AROUND 7 C/KM. BASED UPON A MODEL AND ENSEMBLE CONSENSUS...CAPE OF AROUND 2000 J/KG APPEARS LIKELY ACROSS MUCH OF THE AREA DURING THE AFTERNOON. A CONSIDERABLE DEGREE OF DRY MID-LEVEL AIR...MODEST FORCING...AND DEEP LAYER SHEAR IN EXCESS OF 40 KTS MAY LEAD TO MORE IN THE WAY OF SCATTERED/DISCRETE CELLS AS OPPOSED TO LINEAR ORGANIZATION. VERY DRY AIR ALOFT AND A RELATIVELY DRY/DEEP BOUNDARY LAYER WOULD SUPPORT DAMAGING WINDS AND LARGE HAIL AS THE PRIMARY THREATS. POPS LINGER INTO MONDAY EVENING...AS THE MAIN UPPER TROUGH AXIS LINGERS TO OUR WEST...AND FRONTAL BOUNDARY TAKES A BIT OF TIME PUSHING EAST OF THE AREA. THE REMAINDER OF THE PERIOD WILL BE DRY AND SEASONABLY MILD...AS CP HIGH PRESSURE OVERSPREADS THE AREA IN THE WAKE OF THE FRONT. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... AS OF 230 PM SATURDAY...WEDNESDAY THROUGH SATURDAY...A LARGE MID LEVEL CLOSED LOW IS EXPECTED TO ROTATE OVER THE U.S./CANADA BORDER FROM THE GREAT LAKES EAST...AS A RIDGE REMAINS ACROSS THE SOUTHERN PLAINS. THE HEIGHTS ACROSS THE CWA SHOULD PERSIST GENERALLY ZONAL THROUGH THE MEDIUM RANGE. THE PERIOD SHOULD BEGIN DRY WITH POPS INCREASING TO CHC TO SCHC THURSDAY AND FRIDAY. THE GREATEST CHC FOR RAIN WILL OCCUR ON SATURDAY AS SFC LOW PRESSURE SYSTEM ORGANIZES OVER THE SOUTHERN MISSISSIPPI RIVER VALLEY AND TRACKS EAST...WITH A WARM FRONT RUNNING ACROSS THE SOUTHERN APPALACHIANS. I WILL INDICATES SOLID CHC POPS WITH LIKELY ACROSS THE MTN RIDGES. AS THE CASE OVER THE PAST SEVERAL DAYS...THE GFS IS MUCH WETTER THAN THE ECMWF...I WILL USE A BLEND. TEMPERATURES THROUGH THE PERIOD SHOULD RANGE WITHIN SEVERAL DEGREES OF NORMAL. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... AT KCLT...EXPECT VFR THIS EVENING AND EARLY OVERNIGHT IN ADVANCE OF THE WARM FRONT LIFTING NORTH ON SUNDAY. CIGS WILL START LOWERING BETWEEN 04-06Z AS THE COLUMN SATURATES...WITH MVFR CIGS/VSBY AND VCSH. BY 10Z OR SO...EXPECT WIDESPREAD SHRA ACROSS THE REGION WITH CIGS LOWERING TO IFR DUE TO THE ABUNDANT MOISTURE. WILL LIKELY SEE SOME PERIODS OF +SHRA BUT LEFT TAF AS SHRA FOR NOW. KEPT TREND OF INTRODUCING PROB30 TSRA AT 16Z AS CONVECTION DEVELOPS IN THE WARM SECTOR. EXPECT THAT CIGS AND VSBY MAY BE UP AND DOWN DEPENDING ON HOW HEAVY THE RAIN IS BUT OVERALL GENERALLY IFR CONDITIONS EXPECTED ONCE THE SHRA OVERSPREADS. ELSEWHERE...GENERALLY SIMILAR TREND TO KCLT THOUGH WITH LESS CONFIDENCE IN TSRA AT KAVL/KHKY AND MORE CONFIDENCE AT UPSTATE TAFS. HAVE KEPT TREND OF VCTS RATHER THAN PROB30 TSRA AT UPSTATE TAFS FOR SUNDAY MIDDAY. AS THE WARM FRONT APPROACHES OVERNIGHT...SHOULD SEE ONLY MINOR TIMING DIFFERENCES AS FOR EXAMPLE CIGS LOWER FIRST AT KAND AND SPREADING EAST FROM THERE. AGAIN...BEST TSRA CHANCES REMAIN ACROSS THE UPSTATE CLOSER TO THE BETTER INSTABILITY AND AT SOME POINT MAY HAVE TO GO PREVAILING TSRA DEPENDING ON DESTABILIZATION TRENDS. OUTLOOK...A COLD FRONT WILL SWEEP THROUGH THE REGION ON MONDAY WITH STRONG TO SEVERE THUNDERSTORMS BEING A POSSIBILITY. RESTRICTIONS WILL BE LIKELY AT TIMES ON MONDAY FOR ALL SITES. HIGH PRESSURE WILL BUILD IN BEHIND THE COLD FRONT LEADING TO LOWER PRECIP/RESTRICTION CHANCES. CONFIDENCE TABLE... 23-05Z 05-11Z 11-17Z 17-18Z KCLT HIGH 100% LOW 52% HIGH 93% MED 70% KGSP HIGH 93% HIGH 100% HIGH 100% HIGH 80% KAVL HIGH 100% MED 76% HIGH 100% HIGH 100% KHKY HIGH 100% MED 64% HIGH 100% MED 70% KGMU HIGH 93% HIGH 100% HIGH 93% HIGH 80% KAND HIGH 85% MED 64% HIGH 93% HIGH 90% 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...FLOOD WATCH FROM MIDNIGHT EDT TONIGHT THROUGH SUNDAY EVENING FOR GAZ010-017-018-026-028. NC...FLOOD WATCH FROM MIDNIGHT EDT TONIGHT THROUGH SUNDAY EVENING FOR NCZ033-035>037-049-050-053-056-057-062>065-068>072- 501>510. SC...FLOOD WATCH FROM MIDNIGHT EDT TONIGHT THROUGH SUNDAY EVENING FOR SCZ001>010. && $$ SYNOPSIS...CSH NEAR TERM...CDG/TDP SHORT TERM...JDL LONG TERM...NED AVIATION...TDP

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