Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 201419 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1015 AM EDT MON APR 20 2015 .SYNOPSIS...A COLD FRONT WILL APPROACH THE MOUNTAINS FROM THE WEST THIS AFTERNOON...THEN TRACK EAST ACROSS THE CAROLINAS THIS EVENING. HIGH PRESSURE WILL BUILD INTO THE REGION FROM THE SOUTHWEST LATE TONIGHT THROUGH TUESDAY. && .NEAR TERM /TODAY AND TONIGHT/... AS OF 1015 AM MONDAY... UPDATE: MOST IMMEDIATE CONCERN IS THE CONVECTIVE COMPLEX WHICH CROSSED TN OVERNIGHT AND MAINTAINED STRENGTH CROSSING THE FAR SRN APPALACHIANS. BAND OF STRONG TO SEVERE STORMS WITH BOWING SEGMENTS IS PROGRESSING QUICKLY EASTWARD AND WILL ARRIVE IN OUR SW COUNTIES WITHIN THE NEXT 30 MIN. CLT TDWR INDICATES WINDS OVER 45 MPH ARE LIKELY... AND THE OB SITE AT SHELBY REPORTED 46 MPH JUST UNDER AN HOUR AGO. UNFORTUNATELY NONE OF THE CONVECTION-ALLOWING MODELS PICKED UP ON THIS LONG-LIVED SYSTEM VERY WELL... INCLUDING THE HRRR WHICH CONTINUES TO SHOW THE BAND DYING WITH LITTLE TO NOTHING ON ITS SOUTHERN FLANK. THIS LINE SHOULD CONTINUE TO PRESS TO THE ENE FOR THE FORESEEABLE FUTURE WITH DYNAMIC SUPPORT VIA DPVA ALOFT AND GOOD UPPER DIVERGENCE IN THE LEFT FRONT QUAD OF THE STJ. CLOUDS EAST OF THIS LINE HAVE LARGELY DISSIPATED... ALLOWING EARLY-DAY HEATING TO FURTHER DESTABILIZE A COLUMN THAT HAS ALREADY (ACCORDING TO SPC MESOANALYSIS) ATTAINED AN MUCAPE NEAR 500 J/KG (ALBEIT WITH SOME REMAINING CINH)... AND INCOMING EFFECTIVE SHEAR OF 40-45 KTS IS SUFFICIENT TO MAINTAIN THIS CONVECTION. IF THIS INDEED HOLDS TOGETHER AND FEEDS ON THE EXISTING INSTABILITY... ANY TRAILING SUBSIDENCE WOULD LIMIT ADDITIONAL CONVECTION THIS AFTERNOON. HOWEVER... WE`RE ALREADY SEEING INSTABILITY REBOUND OVER CENTRAL AL INTO NRN GA... AND THE 00Z WRF NMM/ARW DEPICTED A BROKEN BAND OF CONVECTION FORMING JUST EAST OF HERE AND PROGRESSING ENE THROUGH CENTRAL NC LATE AFTERNOON AND EARLY EVENING (ALTHOUGH THEY DID NOT DEPICT THE CURRENT COMPLEX SURVIVING THIS LONG AT ALL). SO MANY QUESTIONS REMAIN REGARDING TIMING. WILL STAY WITH THE SCENARIO OF THIS CURRENT CONVECTION HOLDING TOGETHER AS A WAVY BAND WITH STRAIGHT LINE WIND GUSTS... THEN STILL EXPECT ANOTHER ROUND OF STORMS LATE THIS AFTERNOON THROUGH MID EVENING... AND GUSTY WINDS AND LARGE HAIL WILL STILL BE POSSIBLE WITH THESE IF WE CAN ACHIEVE ENOUGH REBOUND INSTABILITY AHEAD OF THE COLD FRONT. -GIH EARLIER DISCUSSION FROM 345 AM: OVERVIEW: A LOW-LEVEL TROUGH /WEAK WARM FRONT/...ASSOCIATED WITH A DEEPENING SFC CYCLONE ATTENDANT A POTENT UPPER TROUGH DIGGING SE INTO THE UPPER GREAT LAKES...WILL LIFT NORTHWARD THROUGH THE DELMARVA THIS MORNING AND NORTHEAST ALONG THE MID-ATLANTIC COAST THIS AFT/EVE INTO TONIGHT. MEANWHILE...A COLD FRONT (ATTENDANT THE AFOREMENTIONED CYCLONE) TRACKING EASTWARD THROUGH THE TN VALLEY THIS MORNING WILL PROGRESS ACROSS THE SOUTHERN APPALACHIANS LATE THIS AFTERNOON...THEN TRACK EAST ACROSS THE CAROLINAS THIS EVENING INTO TONIGHT. PRECIPITATION: THE RECENT INCREASE IN CONVECTIVE INTENSITY OVER THE COASTAL PLAIN IS ASSOCIATED WITH ENHANCED LOW-LEVEL CONVERGENCE /BACKED LOW-LEVEL FLOW/ AND THE ADVECTION OF A HIGHER THETA-E AIRMASS INTO EASTERN NC...APPARENTLY IN RESPONSE TO DPVA ATTENDANT THE NEGATIVELY TILTED SHORTWAVE CURRENTLY TRACKING NE ACROSS THE REGION. NOW THAT THIS ACTIVITY HAS EXITED CENTRAL NC...EXPECT DRY CONDITIONS TO PREVAIL THROUGH SUNRISE IN ASSOC/W SUBSIDENCE IN THE WAKE OF THE AFOREMENTIONED SHORTWAVE AND...FOR THE REMAINDER OF THE MORNING...A LACK OF FORCING DRYING ALOFT COMBINED WITH DRYING ALOFT ASSOC/W THE ADVECTION OF A MODIFIED ELEVATED MIXED LAYER INTO THE REGION FROM THE SOUTHWEST. UNCERTAINTY INCREASES BY EARLY AFTERNOON REGARD TO WHETHER OR NOT THE ONGOING TN VALLEY MCS WILL CROSS THE SOUTHERN APPALACHIANS AND RESULT IN A POTENTIAL FOR CONVECTION IN THE FAR WESTERN PIEDMONT BETWEEN 15-18Z. THE LATEST HIGH-RES CAMS SUGGEST THAT THE MCS WILL DECAY ON APPROACH TO THE MOUNTAINS ~12Z AS THE THERMODYNAMIC ENVIRONMENT BECOMES LESS FAVORABLE (I.E. INCREASING CINH). EVEN IF THE MCS DECAYS PRIOR TO CROSSING THE MOUNTAINS...THE PRESENCE OF AN MCV AND/OR LINGERING CONVECTIVE DEBRIS/OUTFLOW WOULD INTRODUCE CONSIDERABLE UNCERTAINTY TO THE CONVECTIVE FORECAST THIS AFTERNOON. EVEN IF ONE ASSUMES THE AFOREMENTIONED FEATURES ARE ABSENT...THE CONVECTIVE FORECAST REMAINS UNCERTAIN GIVEN A TEMPORAL/SPATIAL DISCONNECT BETWEEN THE BEST SYNOPTIC FORCING (DPVA IN SW FLOW ALOFT BETWEEN 00-06Z)...THE BEST LOW-LEVEL FOCUS (PRE-FRONTAL TROUGH IN THE WESTERN PIEDMONT BY ~00Z)...AND PEAK HEATING. WITH THE ABOVE IN MIND...WILL MAKE LITTLE CHANGE TO THE PRIOR FORECAST AND INDICATE PRECIP CHANCES OF 40-50% DURING THE LATE AFTERNOON AND EVENING...ENDING FROM WEST-EAST BY OR SHORTLY AFTER MIDNIGHT. SEVERE THREAT: MODERATE DESTABILIZATION (MLCAPE 1000-2000 J/KG) IS EXPECTED TODAY IN ASSOC/W CLEARING SKIES...A MODIFIED ELEVATED MIXED LAYER (CHARACTERIZED BY H7-H5 LAPSE RATES ON THE ORDER OF 6.5-7 C/KM)...AND SEASONABLY RICH LOW-LEVEL MOISTURE (DWPTS UPPER 50S/LOWER 60S) WITHIN AN INCREASINGLY WELL-MIXED WARM SECTOR DOWNSTREAM OF THE APPROACHING COLD FRONT...WITH LITTLE IN THE WAY OF A CAP /CINH/ PER LATEST FORECAST SOUNDINGS. ANY UPDRAFTS THAT DEVELOP PRIOR TO ~21Z THIS AFTERNOON ARE UNLIKELY TO SURVIVE ENTRAINMENT GIVEN A DRY AIRMASS ALOFT AND A GENERAL LACK OF FORCING THROUGHOUT THE COLUMN...ASSUMING NO CONTRIBUTION FROM UPSTREAM CONVECTION. BY LATE AFTERNOON...MODERATE/UNTAPPED INSTABILITY AND AN ABSENCE OF CONVECTIVE INHIBITION ARE EXPECTED TO ALLOW CONVECTIVE DEVELOPMENT ALONG THE COLD FRONT /PRE-FRONTAL TROUGH/ PROGRESSING INTO THE FOOTHILLS...WITH UNIDIRECTIONAL DEEP-LAYER SHEAR SUFFICIENT FOR SPLITTING SUPERCELLS AND UPSCALE GROWTH INTO A LINEAR MCS THEREAFTER...THOUGH THE LATEST CONVECTION ALLOWING MODELS SUGGEST MULTIPLE LINE SEGMENTS MAY DEVELOP RATHER THAN A SOLID LINE...WHICH SEEMS REASONABLE GIVEN THE UNCERTAINTIES MENTIONED ABOVE. SHOULD SPLITTING SUPERCELLS DEVELOP IN THE WESTERN PIEDMONT...LARGE HAIL (UP TO GOLFBALL SIZE) AND DAMAGING WINDS WOULD BE POSSIBLE GIVEN SPEEDY MID-LEVEL FLOW...MDT INSTABILITY AND STEEP MID-LEVEL LAPSE RATES. FURTHER EAST...DAMAGING WINDS WOULD BE THE PRIMARY THREAT... WITH SEVERE HAIL CONFINED TO ROTATING UPDRAFTS EMBEDDED WITHIN LINE SEGMENTS. -VINCENT && .SHORT TERM /TUESDAY AND TUESDAY NIGHT/... AS OF 345 AM MONDAY... DRY CONDITIONS WILL PREVAIL IN THE WAKE OF THE COLD FRONT TUE/TUE NIGHT...WITH HIGHS IN THE UPPER 60S TO LOWER 70S AND LOWS MID/UPPER 40S TO PERHAPS 50F FAR SE COASTAL PLAIN WED MORNING. -VINCENT && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... AS OF 120 AM MONDAY... DRY WEATHER EXPECTED WEDNESDAY AND THURSDAY. HIGH CONFIDENCE FORECAST FOR MID WEEK AS HIGH PRESSURE WILL DOMINATE AND BRING DRY WEATHER WITH SEASONABLY COOL TEMPERATURES WEDNESDAY AND THURSDAY. CONFIDENCE RAPIDLY DECREASES IN THE FORECAST BY FRIDAY THROUGH THE WEEKEND AS MODELS SHOW SPREAD WITH HANDLING THE PROGRESSIVELY MOVING SOUTHERN STREAM SYSTEM THAT MAY AFFECT OUR REGION AS EARLY AS FRIDAY. JUST HOW THIS SYSTEM INTERACTS WITH THE POLAR JET AND THE ASSOCIATED LOW LEVEL BOUNDARY WILL DETERMINE HOW QUICKLY THIS SYSTEM MOVES AND HOW FAR NORTH THE SIGNIFICANT RAINFALL REACHES INTO THE MID- ATLANTIC STATES. FOR NOW... WE WILL CARRY A CHANCE OF SHOWERS FOR THE WEEKEND (CENTERED ON SATURDAY INTO SATURDAY NIGHT). && .AVIATION /12Z MONDAY THROUGH THURSDAY/... AS OF 700 AM MONDAY... CONDITIONS WILL BECOME VFR BY MID/LATE MORNING. VSBYS SHOULD BECOME VFR IN ALL AREAS THROUGH 09Z WITH THE LINE OF SHOWERS AND ISOLATED STORMS FINALLY MOVING EAST OF KRWI SHORTLY. A SOUTH FLOW AT 10 MPH WILL CONTINUE THROUGH DAYBREAK... THE GRADUALLY VEER TO THE SW AT 15- 20 MPH LATE MORNING AND AFTERNOON WITH GUSTS TO 30 MPH. LOOKING FURTHER AHEAD...ISOLATED TO SCATTERED SHOWERS AND THUNDERSTORMS ARE EXPECTED TO FORM ALONG A COLD FRONT MONDAY AFTERNOON OVER THE WESTERN SECTIONS OF THE AREA. THE CHANCE WILL PUSH EAST WITH THE FRONT DURING THE LATE AFTERNOON AND EVENING. OTHERWISE... VFR CONDITIONS WILL PREVAIL TONIGHT THROUGH THE END OF THE WEEK DOMINATED BY HIGH PRESSURE BEGINNING TONIGHT. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...VINCENT NEAR TERM...HARTFIELD/VINCENT SHORT TERM...VINCENT LONG TERM...BADGETT AVIATION...BADGETT

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