Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 280727 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 327 AM EDT THU AUG 28 2014 .SYNOPSIS...A BACK DOOR COLD FRONT WILL TRACK SOUTHWARD FROM VIRGINIA INTO NORTH CAROLINA THROUGH TONIGHT...THEN STALL OUT ACROSS THE REGION THROUGH FRIDAY NIGHT. && .NEAR TERM /TODAY AND TONIGHT/... AS OF 315 AM THURSDAY... SUBSIDENCE IN THE WAKE OF AN UPPER LEVEL TROUGH PROGRESSING OFFSHORE NEW ENGLAND TODAY WILL RESULT IN SFC PRESSURE RISES OVER THE OH VALLEY AND MID-ATLANTIC...DRIVING A WEAK BACK DOOR COLD FRONT FROM THE MASON-DIXON LINE SOUTHWARD TO THE VA/NC BORDER LATE THIS AFT/EVE INTO TONIGHT. CENTRAL NC WILL BE SITUATED ON THE NORTH/NNE PERIPHERY OF AN UPPER LEVEL RIDGE CENTERED OVER NORTHERN FL TODAY...WITH THE PRIMARY BELT OF WESTERLIES LOCATED FURTHER NORTH IN THE MID- ATLANTIC. AS A RESULT...FORCING FOR ASCENT TODAY WILL LARGELY BE CONFINED TO WEAK/SHALLOW CONVERGENCE ALONG THE WEST-EAST BACK-DOOR COLD FRONT PROGRESSING SLOWLY SOUTHWARD TO ALONG OR NEAR THE NC/VA BORDER. A MOIST UPSTREAM AIRMASS (925MB TD 17-19C/H85 TD ~15C) OVER WV/VA WILL ADVECT SOUTH/SE INTO THE NORTHERN PIEDMONT THIS AFTERNOON...DISPLACING THE DRY/STABLE AIRMASS PREVIOUSLY IN PLACE AND CONTRIBUTING TO AS MUCH AS 1000-1500 J/KG OF MLCAPE DURING PEAK HEATING. IN FACT...A WEST-EAST INSTABILITY GRADIENT IS ANTICIPATED TO DEVELOP VERY NEAR THE HWY 64 CORRIDOR THIS AFTERNOON...WITH THE BEST DESTABILIZATION NEAR THE VA BORDER AND THE WEAKEST NEAR THE SC BORDER. MODEL GUIDANCE REMAINS SPLIT WITH REGARD TO WHETHER OR NOT CONVECTION WILL DEVELOP...THOUGH THE BULK OF 00Z GUIDANCE SHOWS SOME DEVELOP IN VCNTY OF THE VA BORDER. WITH THE ABOVE IN MIND...WILL INDICATE A CHANCE OF CONVECTION FROM HWY 64 NORTH TO THE VA BORDER BETWEEN 18-04Z (1PM-MIDNIGHT)...WITH THE BEST CHANCE ACROSS THE NC/VA BORDER COUNTIES. DEEP-LAYER SHEAR WILL BE WEAK AT <15 KT... AND THERE WILL BE NO POTENTIAL FOR ORGANIZED CONVECTION... THOUGH STEEP 0-3 KM LAPSE RATES...MODERATE DCAPE(750-1000 J/KG)...AND PWAT VALUES ~1.75" SUGGEST A LOW-END POTENTIAL FOR BRIEF SEVERE DOWNBURSTS CONDITIONAL UPON THE PRESENCE OF DEEP CONVECTION. HIGHS TODAY SLIGHTLY ABOVE NORMAL IN THE LOWER 90S...WARMEST SOUTH. LOWS TONIGHT RANGING FROM THE MID 60S TO NEAR 70F. -VINCENT && .SHORT TERM /FRIDAY AND FRIDAY NIGHT/... AS OF 315 AM THURSDAY... RIDGING ALOFT WILL PRECLUDE MUCH IN THE WAY OF UPPER LEVEL FORCING FRI/FRI NIGHT...HOWEVER...THE FRONTAL ZONE STALLING OVER THE REGION WILL INTRODUCE A LARGE DEGREE OF UNCERTAINTY WITH REGARD TO TEMPERATURES AND PRECIP CHANCES...INCLUDING A POTENTIAL FOR WEAK HYBRID CAD AND ELEVATED CONVECTION. AT THIS TIME WILL MAKE LITTLE CHANGE TO THE PREVIOUS FORECAST...THOUGH ADDITIONAL MODIFICATIONS MAY BE NEEDED PER FURTHER MODEL/DATA ANALYSIS THIS MORNING. THE MOST LIKELY CHANGES WOULD BE A DROP IN HIGH TEMPS ACROSS THE N/NW PIEDMONT AND AT LEAST A LOW CHANCE (20%) OF CONVECTION IN MOST LOCATIONS. -VINCENT && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... AS OF 255 AM THURSDAY... UPPER RIDGING IS EXPECTED TO RESIDE OVER THE SOUTHEASTERN STATES SAT- MON... THEN BUILD WESTWARD INTO THE SOUTHERN PLAINS NEXT WEEK. GUIDANCE WAS FASTER THIS RUN WITH THE HEIGHT FALLS AND THE ATTENDANT SURFACE COLD FRONT NOW SLATED INTO THE REGION FROM THE NORTHWEST ON WEDNESDAY AFTERNOON AND EVENING. HIGHLIGHTS IN THE LONG TERM WILL BE INCREASING HEAT...HUMIDITY... AND THE CHANCES OF SHOWERS AND THUNDERSTORMS. THIS PATTERN WILL LIKELY LAST THROUGH MID-WEEK WHEN A COLD FRONT IS NOW SLATED TO ARRIVE WED PM. THE FRONT SHOULD SETTLE SOUTH OF THE STATE THURSDAY ALLOWING FOR COOLING FROM THE NE AGAIN THU-FRI. UNTIL THEN... EXPECT PARTLY SUNNY TO PARTLY CLOUDY SKIES THIS WEEKEND. A CHANCE OF PM THUNDERSTORMS ESPECIALLY BY SUNDAY. HIGHS RETURN TO NEAR NORMAL (86-92) BOTH SAT-SUN. LOWS 65-70. MON-WED... A CHANCE OF MAINLY AFTERNOON AND EVENING THUNDERSTORMS EACH DAY... WITH THE BEST CHANCE EXPECTED WEDNESDAY. HIGHS 86-92. LOWS IN THE UPPER 60S TO MID 70S. TEMPS WILL COOL BACK INTO THE LOWER TO MID 80S BY THURSDAY WITH THE NE FLOW LEVEL FLOW. ALSO... THE CHANCE OF THUNDERSTORMS WILL SHIFT WITH THE FRONT INTO FAR SE NC AND SC. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... AS OF 135 AM THURSDAY... 24-HR TAF PERIOD: VFR CONDITIONS AND CALM OR LIGHT AND VARIABLE WINDS WILL PREVAIL THROUGH THE MAJORITY OF THE TAF PERIOD. THE ONLY POSSIBLE EXCEPTION WILL BE A POTENTIAL FOR ISOLD TO SCT CONVECTION AT THE LEADING EDGE OF A BACKDOOR COLD FRONT MOVING SOUTH/SE INTO THE REGION LATE TODAY/TONIGHT...OR ROUGHLY FROM 21Z THIS AFT THROUGH 06Z FRI MORNING...WITH THE BEST CHANCE ALONG OR JUST SOUTH OF THE VIRGINIA BORDER...INCLUDING THE INT/GSO/RDU/RWI TERMINALS. LOOKING AHEAD: THE FRONT WILL STALL AND SLOWLY BEGIN TO DISSIPATE OVER THE REGION FRI/FRI NIGHT. CONFIDENCE REMAINS BELOW NORMAL IN THIS PERIOD...AND IT IS POSSIBLE THAT VFR CONDITIONS PREVAIL EVERYWHERE FRI/FRI NIGHT...OR THAT SUB-VFR CONDITIONS ASSOC/W LOW CEILINGS AND ISOLD ELEVATED CONVECTION COULD BE PRESENT LATE FRI/FRI NIGHT. OTHERWISE...CLIMATOLOGICAL CONDITIONS APPEAR LIKELY SAT THROUGH EARLY NEXT WEEK...I.E. A POTENTIAL FOR ISOLD AFT/EVE CONVECTION AND EARLY MORNING FOG/STRATUS. -VINCENT && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...VINCENT NEAR TERM...VINCENT SHORT TERM...VINCENT LONG TERM...PWB AVIATION...VINCENT

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