Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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936 FXUS62 KRAH 021140 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 740 AM EDT MON MAY 2 2016 .SYNOPSIS... WARM SOUTHWEST FLOW WILL PREVAIL AHEAD OF A SLOW-MOVING COLD FRONT...AND ASSOCIATED WAVES OF LOW PRESSURE...THAT WILL APPROACH FROM THE NORTHWEST AND CROSS THE MIDDLE ATLANTIC STATES LATE TUE THROUGH EARLY WED. && .NEAR TERM /THROUGH TONIGHT/... AS OF 425 AM MONDAY... LOW PRESSURE NOW OVER THE OH VALLEY WILL MIGRATE SLOWLY EAST INTO THE NORTHERN MIDDLE ATLANTIC STATES...WHERE IT WILL MERGE WITH AN APPALACHIAN LEE TROUGH THAT WILL EXTEND SOUTHWESTWARD THROUGH THE WEST-CENTRAL CAROLINAS TODAY. A PRECEDING EFFECTIVE WARM FRONT OVER SOUTH-CENTRAL VA (MODULATED BY EARLIER STRONG TO SEVERE CONVECTION OVER THE VIRGINIAS) WILL RETREAT QUICKLY NORTH ACROSS VA OWING TO THE WARMTH JUST ABOVE THE SURFACE LAYER...WHICH WILL BE REALIZED QUICKLY AS LOW CLOUDS ERODE AND DIABATIC HEATING/ MIXING OCCURS. ASIDE FROM THE LEE TROUGH...A COUPLE OF OTHER OUTFLOW BOUNDARIES OVER THE FAR NORTHERN NC PIEDMONT AND SOUTHERN PIEDMONT AND SANDHILLS MAY SERVE AS FOCI FOR CONVECTIVE DEVELOPMENT TODAY...THE LATTER OF WHICH MAY SUPPORT THE MAINTENANCE AND/OR RE-DEVELOPMENT OF SHOWERS AND STORMS THERE THROUGH MIDDAY...AS AN UPSTREAM MCV OVER GA APPROACHES FROM THE SW. SOME SUBSIDENCE BEHIND THIS FEATURE MAY INITIALLY OTHERWISE INHIBIT CONVECTIVE DEVELOPMENT UNTIL EARLY-MID AFTERNOON. OTHERWISE...THE EROSION OF LOW CLOUDS WILL CAUSE TEMPERATURES TO WARM THROUGH THE 70S...TO PROBABLE HIGHS IN THE LOWER 80S...WHICH IN CONJUNCTION WITH SURFACE DEWPOINTS IN THE 60S...WILL YIELD A MODERATELY UNSTABLE AIR MASS OVER CENTRAL NC THIS AFTERNOON. LIKE SUNDAY...HOWEVER...THE MAIN BELT OF THE NORTHERN STREAM MID- UPPER FLOW AND ASSOCIATED SHORTWAVE TROUGHS WILL REMAIN TO THE NORTH OF CENTRAL NC TODAY...SO ONLY LOW AMPLITUDE IMPULSES IN THE SOUTHERN STREAM AND/OR MCV/S FROM UPSTREAM CONVECTION WILL BE AVAILABLE TO ENCOURAGE FORCING FOR ASCENT. THE RESULT WILL LIKELY BE RELATIVELY SCATTERED...MULTI-CELLULAR STORMS THIS AFTERNOON...CAPABLE OF PRODUCING STRONG WIND GUSTS AND PERHAPS SOME MARGINALLY SEVERE HAIL. TONIGHT: A COUPLE OF NORTHERN STREAM SHORTWAVE TROUGHS STRETCHING FROM OK TO THE FOUR CORNERS REGION WILL CAUSE MID LEVEL HEIGHTS TO FALL ACROSS THE CENTRAL APPALACHIANS/OH VALLEY/NORTHERN MIDDLE ATLANTIC STATES BY THIS EVENING. FORCING FOR ASCENT --IN THE ASSOCIATED DIVERGENT RIGHT ENTRANCE REGION OF THE UPPER JET-- WILL SPREAD EAST OF THE APPALACHIANS THIS EVENING...THEN PIVOT EAST ACROSS VA/NC TONIGHT. THESE FEATURES WILL INTERACT WITH A CONTINUED UNSEASONABLY MOIST AND CONDITIONALLY UNSTABLE AIR MASS AND SUPPORT THE MAINTENANCE AND POSSIBLE UPSCALE GROWTH INTO LARGER CLUSTERS OF SHOWERS AND STORMS OVERNIGHT. THE ASSOCIATED GREATEST SEVERE WEATHER THREAT WOULD LIKELY OCCUR OVER THE WESTERN PIEDMONT DURING THE EVENING...WHEN INSTABILITY WILL STILL BE RELATIVELY MAXIMIZED AND OVERLAP IN SPACE AND TIME WITH STRENGTHENING UVV/S AND MID LEVEL FLOW/SHEAR ACCOMPANYING THE APPROACHING SHORTWAVE TROUGHS. DAMAGING WIND WILL POSE THE PRIMARY THREAT...THOUGH SOME SEVERE HAIL WOULD ALSO BE POSSIBLE IN ANY STORMS THAT REMAIN RELATIVELY DISCRETE. HEAVY RAIN AND LOCALIZED FLOODING...PARTICULARLY IN URBAN AREAS...WILL ALSO BE OF CONCERN GIVEN THE PRESENCE OF AN UNSEASONABLY MOIST AIRMASS CHARACTERIZED BY PRECIPITABLE WATER VALUES OF 1.5-1.75". HUMID OTHERWISE...WITH LOWS IN THE 60S. && .SHORT TERM /TUESDAY AND TUESDAY NIGHT/... AS OF 400 AM MONDAY... A SHORTWAVE WILL DIVE SOUTH THROUGH THE PLAINS STATES ON TUESDAY AND THEN SWING ACROSS THE DEEP SOUTH TUESDAY NIGHT.... FINALLY GIVING THE STATIONARY FRONT CURRENTLY STRETCHING FROM EAST TX TO THE LOWER GREAT LAKES A KICK TO THE EAST. THE FRONT IS PROGGED TO CROSS NC DURING THE DAY ON TUESDAY...THOUGH THE GFS IS SLIGHTLY FASTER THAN THE ECMWF/NAM IN BRINGING THE FRONT THROUGH CENTRAL NC BY LATE AFTERNOON. THE ECMWF AND NAM FAVOR A FRONTAL PASSAGE LATER IN THE EVENING. DEEP SHEAR IS INCREASINGLY FAVORABLE FOR ORGANIZED CONVECTION TUESDAY AFTERNOON...WITH 40-50KT OF UNIDIRECTIONAL 0-6KM BULK SHEAR AS THE UPPER JET SHIFTS EAST. CAPE IS LITTLE MORE UNCERTAIN GIVEN 1) THE POTENTIAL FOR ONGOING PRECIP OVER WESTERN NC EARLY TUESDAY TO INHIBIT DESTABILIZATION OVER THE WESTERN PIEDMONT AND 2) WESTERLY FLOW AND DEEPER MIXING TO LOWER DEWPOINTS (AGAIN OVER THE WESTERN PIEDMONT). GFS FORECAST SOUNDINGS SUGGEST100- 1500JKG MLCAPE IN THE COASTAL PLAIN. THUS...THE COASTAL PLAIN IS FAVORED FOR STRONGER STORMS TUESDAY AFTERNOON. HIGHS WILL BE IN THE MID AND UPPER 70S. THE SLOWER PASSAGE OF THE COLD FRONT SHOW IN THE ECMWF SUGGESTS THE POTENTIAL FOR LINGERING SHOWERS IN THE COASTAL PLAIN TUESDAY NIGHT AS A WEAK WAVE OF LOW PRESSURE DEVELOPS ALONG THE FRONT WHILE IT APPROACHES THE COAST. LOWS 54-59. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... AS OF 400 AM MONDAY... AN OMEGA BLOCK PATTERN IS FORECAST TO DEVELOP FROM MID TO LATE WEEK...WITH A STRONG UPPER RIDGE OVER THE WESTERN CONUS AND AN UPPER LOW DROPPING SLOWLY OUT OF CANADA AND ACROSS THE MID-ATLANTIC STATES AND CAROLINAS. MEDIUM RANGE ENSEMBLES HAVE ADVERTISED THIS UPPER LOW AS 2-3 STD BELOW NORMAL. H10-H85 THICKNESSES DROPPING TO AROUND 1340M BY FRIDAY MORNING SUPPORT THE IDEA OF HIGHS SEVERAL DEGREES BELOW NORMAL...WITH THE COOLEST MEX GUIDANCE INDICATING MID 60S TO LOWER 70S THURS/FRI. MULTIPLE DISTURBANCES WITH THE UPPER LOW AND PERIODIC DCVA COMBINED WITH A POCKET OF COLD AIR ALOFT WILL SUPPORT SHOWERS AND EVEN A FEW ISOLATED STORMS (MAINLY AFTERNOON) THROUGH FRIDAY. THE UPPER LOW SHOULD BEGIN TO FILL AND MOVE EAST BY SATURDAY...WITH RIDGING OVER THE DEEP SOUTH AND NORTHWESTERLY FLOW ACROSS NC. TEMPS SHOULD BEGIN TO CLIMB BACK TO AND ABOVE NORMAL BY THE END OF NEXT WEEKEND. && .AVIATION /12Z TUESDAY THROUGH FRIDAY/... AS OF 740 AM MONDAY... AREAS OF STRATUS...IN MOIST/HUMID SSW LOW LEVEL FLOW BELOW A TEMPERATURE INVERSION AROUND 1500 FT...WITH COVERAGE MORE VARIABLE THAN IN RECENT DAYS...WILL LIFT THROUGH MVFR RANGE AND SCATTER AS THE INVERSION BREAKS BETWEEN 13-16Z. SUBSEQUENT DIURNAL HEATING OF THE AFOREMENTIONED HUMID AIR MASS WILL DESTABILIZE THE ATMOSPHERE AND SUPPORT SCATTERED STORMS THAT WILL LIKELY DEVELOP ALONG AN APPALACHIAN LEE SURFACE TROUGH...AND REMNANT OUTFLOW AND/OR DIFFERENTIAL BOUNDARIES FROM EARLIER CONVECTION AND CLOUDS...THIS AFTERNOON. THE RELATIVE BEST CHANCE OF SHOWERS AND STORMS WILL OCCUR THIS EVENING AT TRIAD TAF SITES...OWING TO ADDED SUPPORT ALOFT BY THE NEARBY PASSAGE OF A COUPLE OF DISTURBANCES ACROSS THE LOWER OH VALLEY AND CENTRAL APPALACHIANS. SHOWERS AND STORMS WITH HEAVY RAIN MAY LINGER IN SOME AREAS IN A CONTINUED UNSEASONABLY MOIST AND WEAKLY UNSTABLE AIR MASS OVERNIGHT. OTHERWISE...AREAS OF LOW STRATUS ARE EXPECTED TO DEVELOP ONCE AGAIN IN A CONTINUED HUMID AIR MASS LATE TONIGHT-EARLY TUE. OUTLOOK: ABOVE AVERAGE CHANCES OF SHOWERS AND STORMS WILL PERSIST UNTIL A COLD FRONT CROSSES THE MIDDLE ATLANTIC STATES AND MOVES OFFSHORE LATE TUE THROUGH EARLY WED. A POLAR VORTEX WILL MEANWHILE MIGRATE FROM THE ARCTIC CIRCLE TO THE MIDDLE ATLANTIC STATES THROUGH THE END OF THE WEEK...WITH RESULTANT ENHANCED AFTERNOON CLOUDINESS AND SHOWERS/STORMS...AS DIURNAL HEATING OCCURS BENEATH VERY COLD TEMPERATURES ALOFT ATTENDING THE VORTEX. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...26 NEAR TERM...26 SHORT TERM...22 LONG TERM...22 AVIATION...26

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