Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 241852 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 252 PM EDT THU JUL 24 2014 .SYNOPSIS... A COLD FRONT WILL MOVE SOUTHWARD INTO SOUTHERN VIRGINIA LATE THIS AFTERNOON...PROGRESS INTO CENTRAL NORTH CAROLINA TONIGHT...THEN STALL NEAR THE SOUTH CAROLINA BORDER ON FRIDAY. && .NEAR TERM /THROUGH TONIGHT/... AS OF 245 PM THURSDAY... THOUGH MID-LEVEL LAPSE RATES REMAIN WEAK AT ~5.5 C/KM...A VERY MOIST BOUNDARY LAYER ALONG WITH STRONG INSOLATION WILL YIELD MODERATE DIURNAL DESTABILIZATION TODAY WITH MLCAPE VALUES GENERALLY RANGING FROM 1000-2000 J/KG...HIGHEST IN THE EAST WHERE LOW-LEVEL MOISTURE AND SUNSHINE ARE MOST ABUNDANT AND LOWEST WEST. MODERATE INSTABILITY AND LITTLE OR NO CIN ALLOWED CONVECTION TO DEVELOP IN VICINITY OF THE PRE-FRONTAL SFC TROUGH OVER THE COASTAL PLAIN EARLIER THIS MORNING (12-16Z)...CREATING AN EFFECTIVE FRONTAL ZONE ORIENTED ROUGHLY WEST-EAST ALONG OR NEAR HWY 64. AS OF 18Z... DISSIPATING CLOUD COVER FROM PRIOR CONVECTION HAS ALLOWED FULL HEATING /INSOLATION/ FOR SEVERAL HOURS OVER THE NORTHERN AND WESTERN PIEDMONT...SIGNIFICANTLY WEAKENING THE EFFECTIVE FRONT. HOWEVER... CONVECTION PERSISTS OVER THE COASTAL PLAIN...AND THE EFFECTIVE FRONT PERSISTS EAST OF THE TRIANGLE. THE PRESENCE OF THE AFOREMENTIONED `EFFECTIVE FRONT` CONTINUES TO INCREASE UNCERTAINTY WITH REGARD TO THE CONVECTIVE FORECAST THROUGH THIS EVENING. WITH THE ABOVE IN MIND...CONFIDENCE IS TOO LOW TO PROVIDE MORE SPECIFIC DETAILS ABOUT CONVECTIVE COVERAGE AND EVOLUTION THIS AFT/EVE. HOWEVER...FORECAST REASONING REMAINS THE SAME. GIVEN CYCLONIC FLOW ALOFT (CYCLONIC SHEAR VORTICITY AND DPVA ASSOC/W SMALL AMPLITUDE SHORTWAVES)...A SFC-925 MB TROUGH SINKING SOUTHWARD INTO THE REGION NEAR PEAK HEATING...ABUNDANT MOISTURE...AND MODERATE INSTABILITY...EXPECT ABOVE NORMAL CHANCES FOR CONVECTION LATE THIS AFT THROUGH THIS EVENING...WITH CONVECTION POSSIBLY PERSISTING THROUGH SUNRISE FRIDAY MORNING SOUTH AND EAST OF THE TRIANGLE. SEVERE THREAT: DEEP LAYER SHEAR ON THE ORDER OF 25-35 KT WILL SUPPORT MULTICELL CLUSTERS AND PERHAPS AN ISOLD SUPERCELL THIS AFT/EVE. EXPECT A POTENTIAL FOR A FEW SEVERE STORMS WITH A PRIMARY THREAT OF DAMAGING WINDS...PRIMARILY SOUTH OF HWY 64 WHERE DCAPE VALUES BEGIN TO INCREASE ABOVE 500 J/KG...APPROACHING 1000 J/KG NEAR THE SC BORDER. DIME TO QUARTER SIZE HAIL WILL ALSO BE POSSIBLE WITH ANY ROBUST UPDRAFTS. THOUGH WEAK MID-LEVEL LAPSE RATES WILL REDUCE THE POTENTIAL FOR HAIL > 1.00 IN...LARGE HAIL UP TO THE SIZE OF GOLFBALLS WOULD BE POSSIBLE IN ANY ROBUST UPDRAFT THAT ACQUIRES ROTATION...ESP SOUTH OF HWY 64 WHERE MLCAPE VALUES WILL APPROACH 2000-2500 J/KG DURING PEAK HEATING. -VINCENT && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... AS OF 320 AM THURSDAY... FOR FRI/FRI NIGHT: LITTLE CHANGED NEEDED TO EXISTING FORECAST. STILL EXPECT THE FRONT TO HANG UP ACROSS THE SE CWA AND ALONG THE CENTRAL/WRN NC/SC BORDER... AS THE 850 MB TROUGH HOLDS TO ITS NW ACROSS FAR NRN NC. THIS WILL KEEP ENOUGH LOW LEVEL MOISTURE WHICH... IN CONJUNCTION WITH LINGERING PW VALUES NEAR 1 INCH... NECESSITATES A MENTION OF ISOLATED TO SCATTERED SHOWERS AND STORMS OVER THE FAR SE CWA... FOCUSED ON THE TYPICAL AFTERNOON THROUGH EARLY EVENING TIME FRAME. MUCH LOWER PW OVER THE REMAINDER OF THE CWA AND MODEST RISE IN HEIGHTS AS THE MID LEVEL TROUGH WEAKENS AND STARTS TO PULL AWAY WILL LEAD TO MAINLY DRY WEATHER ELSEWHERE... ALTHOUGH ONE LAST SHOT OF DPVA THROUGH THE EXITING TROUGH BASE WILL DELAY THE MOST SUBSTANTIAL NW-TO-SE CLEARING UNTIL LATE IN THE DAY. HIGHS 85-90... WITH THICKNESSES SLIPPING TO 10-15 M BELOW NORMAL. LOWS 65-71. -GIH SAT AND SAT NIGHT: THE REMNANT FRONT FROM EARLY IN THE PERIOD WILL BECOME INCREASINGLY DIFFUSE AS IT RETREATS NORTH ACROSS THE MIDDLE ATLANTIC STATES ON SAT...WITH A PRIMARY IMPACT FOR CENTRAL NC BEING THE DEVELOPMENT OF A WARMING SOUTHERLY FLOW - HIGH TEMPERATURES GENERALLY BETWEEN 89 AND 93 DEGREES. ALOFT...MODEL GUIDANCE IS IN GOOD AGREEMENT REGARDING THE PROGRESSION OF A MID LEVEL TROUGH FROM THE APPALACHIAN SPINE SAT MORNING TO THE COAST BY EVENING...BUT ASSOCIATED FORECAST VERTICAL MOTION WITH THIS FEATURE IS WEAK...AND NAM FORECAST BUFR SOUNDINGS ARE AN UNSTABLE AND UNCAPPED OUTLIER RELATIVE TO THE GFS AND SREF ENSEMBLE MEAN. THE NAM QPF MAXIMUM FROM SE VA THROUGH CENTRAL NC IS ALSO AN OUTLIER RELATIVE TO OTHER GUIDANCE...WHICH OTHERWISE CONFINES ANY SHOWERS/STORMS TO THE SEA BREEZE. WILL CONSEQUENTLY MAINTAIN JUST A SLIGHT CHANCE POP OVER THE SOUTHERN AND CENTRAL COASTAL PLAIN...WHERE WEAK LIFT AHEAD OF THE AFOREMENTIONED TROUGH ALOFT WILL BE DIURNALLY MAXIMIZED AND COINCIDENT WITH LOW LEVEL FORCING INVOF THE SEA-BREEZE/SUBSEQUENT OUTFLOW DURING THE MID-LATE AFTERNOON. SHORTWAVE RIDGING WILL BRIEFLY FOLLOW FOR SAT NIGHT...WITH DRY CONDITIONS AND LOWS MAINLY IN THE LOWER 70S. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... AS OF 335 AM THURSDAY... THE MEDIUM RANGE WILL BE CHARACTERIZED BY THE DEVELOPMENT OF ANOTHER UNSEASONABLY HIGH AMPLITUDE TROUGH ALOFT FROM SOUTHEASTERN CANADA INTO THE EASTERN US...WITH AN ASSOCIATED NEGATIVE HEIGHT OF NEARLY 3 STANDARD DEVIATIONS BELOW AVERAGE FROM THE LOWER GREAT LAKES/OHIO VALLEY TO THE CENTRAL APPALACHIANS. PRECIPITATION CHANCES DURING THE PERIOD WILL BE HIGHEST SUN NIGHT-MON...AS THE TROUGH AMPLIFIES SOUTHEASTWARD AND SPREADS THE ASSOCIATED MID LEVEL HEIGHT FALLS AND UNDERLYING LO LEVEL FRONTAL ZONE ACROSS NC. IN FACT...THE DAY SUNDAY SHOULD BE MAINLY DRY...AND HOT AS THE MID LEVEL RIDGE OVER THE DESERT SW AND PLAINS BUILDS BRIEFLY EAST IN ADVANCE OF THE AMPLIFYING TROUGH ALOFT. H85 TEMPERATURES ARE FORECAST TO SURGE...ALBEIT BRIEFLY...INTO THE 22-24 DEGREE RANGE OVER THE CAROLINAS...SUPPORTIVE OF HIGH TEMPERATURES SOLIDLY IN THE MIDDLE 90S. UPSTREAM PRE-FRONTAL CONVECTION AND ASSOCIATED OUTFLOW WILL THEN LIKELY DRIFT...IN A GRADUALLY WEAKENING STATE OWING TO NOCTURNAL COOLING...INTO THE NORTHERN AND WESTERN PIEDMONT DURING THE EVENING AND EARLY NIGHTTIME HOURS. THE PATTERN ON THE SYNOPTIC SCALE...CHARACTERIZED BY THE APPROACHING STRONG TROUGH AND DOWNSTREAM JET ENTRANCE DIVERGENCE ALOFT...AND LOW LEVEL FRONTAL FORCING...FAVORS CONSIDERABLE THUNDERSTORM COVERAGE MON...INCLUDING SOME SEVERE OWING TO UNSEASONABLY STRONG MID LEVEL FLOW ON THE ORDER OF 40-50 KTS. HOWEVER...PREDICTABILITY ASSOCIATED WITH MESOSCALE INFLUENCES (IE. OUTFLOW AND CLOUD COVER) FROM EARLIER CONVECTION SUN NIGHT COMPLICATE THE FORECAST SO PROBABILITY OF PRECIPITATION WILL BE KEPT IN THE HIGH CHANCE RANGE FOR NOW...WITH THE EXPECTATION THAT THESE PROBABILITIES WILL BE INCREASED OVER AT LEAST A PORTION OF THE FORECAST AREA (LIKELY THE SOUTHEAST HALF). THE MIDDLE OF THE WEEK SHOULD BE DRY...AND COOLER...AS POST-FRONTAL HIGH PRESSURE BUILDS INTO THE CENTRAL APPALACHIANS...AND PRECIPITABLE WATER VALUES FALL BELOW AN INCH OVER CENTRAL NC. HIGHS AND LOWS ONLY IN THE LOWER TO MIDDLE 80S AND LOWER TO MIDDLE 60S...RESPECTIVELY...TUE-WED. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... AS OF 1145 AM THURSDAY... AREAS OF MVFR CEILINGS ASSOCIATED WITH A VERY MOIST LOW-LEVEL AIRMASS WILL PERSIST INTO EARLY/MID AFTERNOON OVER CENTRAL NC... ESPECIALLY IN OR NEAR LOCATIONS AFFECTED BY CONVECTION. SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS WILL LIKELY EFFECT ALL TERMINALS FOR A PERIOD OF AT LEAST SEVERAL HOURS THIS AFTERNOON THROUGH TONIGHT AS A COLD FRONT MOVES SOUTHWARD INTO THE AREA. WINDS WILL BECOME VARIABLE AS THE FRONT APPROACHES FROM THE NORTH THIS AFT/EVE AND WILL REMAIN LIGHT AT 5-10 KT (OUTSIDE OF CONVECTION). LOOKING AHEAD: AT THIS TIME...VFR CONDITIONS ARE EXPECTED ON FRIDAY AS THE FRONT STALLS OUT JUST S/SE OF CENTRAL NC...THOUGH SOME OF THE LATEST MODEL GUIDANCE SUGGESTS THAT THE FRONT MAY REMAIN CLOSE ENOUGH FOR SUB-VFR CEILINGS AND/OR A POTENTIAL FOR SHOWERS AND THUNDERSTORMS TO PERSIST. A PERSISTENT PATTERN OF TROUGHING OVER THE EASTERN CONUS WILL RESULT IN AN ABOVE NORMAL CHANCE FOR CONVECTION THROUGH EARLY NEXT WEEK...WITH NUMEROUS THUNDERSTORMS POSSIBLE DURING THE AFTERNOON AND EVENING HOURS EACH DAY. -VINCENT && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...VINCENT NEAR TERM...VINCENT SHORT TERM...HARTFIELD/MWS LONG TERM...MWS AVIATION...VINCENT

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