Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 301505 CCA AFDRAH AREA FORECAST DISCUSSION...CORRECTED NATIONAL WEATHER SERVICE RALEIGH NC 1100 AM EDT THU JUL 20 2015 .SYNOPSIS...A COLD FRONT WILL APPROACH FROM THE WEST TODAY...THEN TRACK SOUTHEAST ACROSS THE CAROLINAS TONIGHT AND SETTLE SOUTH OF THE AREA ON FRIDAY. MEANWHILE...A WEAK LOW PRESSURE SYSTEM OFF THE SOUTHEAST COAST WILL TRACK NORTH NORTHEAST OFFSHORE THE CAROLINA COAST. && .NEAR TERM /THROUGH TONIGHT/... AS OF 1100 AM THURSDAY... THE COLD FRONT IS CONTINUING ITS PROGRESSION EASTWARD...NOW LOCATED ROUGHLY FROM W-CNTRL PA SOUTHWEST ALONG THE APPALACHIANS. THE CURRENT FORECAST IS STILL ON TRACK...WITH THE FRONT EXPECTED TO MOVE ACROSS CENTRAL NC FROM LATE THIS AFTERNOON THROUGH THE EVENING. LATEST HRRR STILL ON TRACK WITH PREV ASSESSMENTS...SUGGESTING WIDELY SCATTERED PREFRONTAL SHOWERS AND PERHAPS A FEW TSTMS ARE POSSIBLE THIS AFTERNOON...AND AGAIN THIS EVENING AS THE FRONT PUSHES ACROSS. NO MAJOR CHANGES TO THE CURRENT FORECAST GRIDS...AND PREV DISC INCLUDED BELOW FOR REFERENCE. PREV NEAR TERM AS OF 405 AM THURSDAY...A COLD FRONT...WHICH CURRENTLY STRETCHES FROM EASTERN CANADA THROUGH THE OHIO VALLEY TO THE MID-MISSISSIPPI VALLEY REGION...WILL APPROACH FROM THE NORTHWEST TODAY AS A SHORTWAVE TRAVERSES THE GREAT LAKES ON THE SOUTHERN PERIPHERY OF AN INTENSE VORTEX NEAR THE HUDSON. THE FRONT IS GENERALLY EXPECTED TO MOVE INTO THE WESTERN PIEDMONT BY LATE AFTERNOON...REACHING THE TRIANGLE AREA PRIOR TO MIDNIGHT AND THEN MOVE INTO THE COASTAL PLAIN EARLY FRIDAY MORNING. AHEAD OF THE FRONT...CENTRAL NC LIES IN A RELATIVE MIN IN PW...WITH AREAS NEAR 2 INCHES TO OUR NORTH AND SOUTH. STRONG HEATING AND 850MB TEMPS APPROACHING 20C WILL PUSH TEMPS IN TO THE LOW TO MID 90S TODAY...BUT INSTABILITY IS PROJECTED TO BE WEAK TODAY...AROUND 500-1000 J/KG WITH A SLIGHT UPTICK IMMEDIATELY AHEAD OF THE FRONT OWING TO A NARROW RIBBON OF PREFRONTAL MOISTURE CONVERGENCE. MOST OF THE CAMS AND LARGER SCALE NWP ARE IN GOOD AGREEMENT ON JUST A BAND OF SCATTERED CONVECTION REACHING THE TRIAD AROUND 20Z OR SO...THEN STRUGGLING TO MAINTAIN INTENSITY AND COVERAGE AS IT MOVES EAST IN TO A LESS SUPPORTIVE ENVIRONMENT AFTER SUNSET. SHEAR PARAMETERS WILL BE WEAK THROUGHOUT AND WIDESPREAD SEVERE WEATHER IS NOT ANTICIPATED. NOTICEABLY DRIER AIR BEHIND THE FRONT AND CLEARING WILL HELP ALLOW TEMPS TO FALL INTO THE UPPER 60S IN THE NORTHWEST PIEDMONT..BUT AREAS TO THE EAST SHOULD STILL SEE LOWS IN THE LOWER 70S AS THE FRONT SLOWLY PUSHES EAST AND PRECIP CHANCES DWINDLING TOWARD 12Z FRIDAY. NOTE...THE WEAK LOW OFF THE GEORGIA COAST ...NOTED BY NHC AS HAVING ONLY A 10 PERCENT CHANCE OF FURTHER DEVELOPMENT...WILL DRIFT NORTH TODAY AND BECOME ABSORBED BY THE FRONT OFF THE OBX LATER TONIGHT...WITH NO DIRECT SENSIBLE IMPACT INLAND. && .SHORT TERM /FRIDAY AND FRIDAY NIGHT/... AS OF 405 AM THURSDAY... THE COLD FRONT WILL PUSH TOWARD THE COAST AND THEN STALL ON FRIDAY AS A BROAD UPPER TROUGH SLOWLY BEGINS TO DEVELOP OVER THE EASTERN US. THE NAM AND GFS ARE FAR ENOUGH EAST WITH THE FRONT AND MORE UNSTABLE AIRMASS TO BE COMPLETELY DRY ON FRIDAY AFTERNOON. THE ECMWF HANGS THE FRONT UP JUST A TOUCH FURTHER WEST AND DEVELOPS SOME QPF EAST OF I-95. WILL KEEP A SLIGHT CHANCE OF THE COASTAL PLAIN...BUT MOST OF THE AREA WILL BE DRY AND MUCH LESS HUMID AS DEWPOINTS HAVE THE POTENTIAL TO MIX OUT INTO THE UPPER 50S AND LOWER 60S BEHIND THE FRONT. ONLY A SMALL DROP IN THICKNESSES IS FORECAST...SO HIGHS WILL STILL BE AROUND NORMAL...89-94. LOWS FRIDAY NIGHT IN THE MID 60S TO AROUND 70. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... AS OF 405 AM THURSDAY... THE MODELS GENERALLY AGREE THAT A RELATIVELY AMPLIFIED FLOW PATTERN...ONE CHARACTERIZED BY A STRONG RIDGE CENTERED OVER THE SOUTHWESTERN US AND DOWNSTREAM A BROAD TROUGH EAST OF THE ROCKIES...WILL PERSIST THROUGH THE MEDIUM RANGE...THOUGH WITH INDICATION FROM THE ENSEMBLE MEANS OF SOME RELAXATION OF THE FLOW/TOWARD LESS AMPLIFIED-MORE ZONAL LATE NEXT WEEK. DESPITE THE PRESENCE OF A TROUGH AND CYCLONIC FLOW ALOFT OVER THE EASTERN US...UNSEASONABLY DRY AIR WILL HAVE SPREAD IN THE LEE OF THE APPALACHIANS...BEHIND A LEAD FRONTAL ZONE FORECAST TO CROSS THE RAH FORECAST AREA LATER TONIGHT. THIS BOUNDARY WILL EFFECTIVELY PIN RICH MOISTURE ALONG COASTAL AREAS...WITH MUCH DRIER AIR...PARTICULARLY WITH DIURNAL HEATING/BL MIXING WESTWARD ACROSS CENTRAL NC. THIS DRIER AIR WILL SIGNIFICANTLY HINDER THE DEVELOPMENT OF ANY APPRECIABLE CAPE AND PRECIPITATION CHANCES THROUGHOUT THE FORECAST PERIOD...WITH THE RELATIVELY HIGHER CHANCES/~25 PERCENT ALONG AND EAST OF A PIEDMONT TROUGH EACH LATE AFTERNOON-EARLY EVENING. ANY EVEN WEAK STORM WRT REFLECTIVITY...HOWEVER...WOULD POSE A RISK OF STRONG...EVAPORATIVELY-COOLED DOWNBURST WINDS IN AN ENVIRONMENT CHARACTERIZED BY STEEP/NEAR DRY ADIABATIC LOW LEVEL LAPSE RATES AND LARGE/~30-35 DEGREE SURFACE DEWPOINT DEPRESSIONS...DRIVEN BY A COMBINATION OF THE AFOREMENTIONED MIXING OUT OF AN ALREADY CONTINENTAL AIR MASS...AND HOT SURFACE TEMPERATURES IN THE LOWER TO MIDDLE 90S. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... AS OF 645 AM THURSDAY... A COLD FRONT WILL APPROACH FROM THE NORTHWEST TODAY AND CROSS THE AREA LATE THIS EVENING AND TONIGHT. INSTABILITY IS EXPECTED TO BE LIMITED WHEN THE FRONT ARRIVES...ESPECIALLY TONIGHT AS THE FRONT MOVES EAST ACROSS CENTRAL NC. SCATTERED SHOWERS AND A FEW ISOLATED STORMS WILL BE POSSIBLE ALONG THE FRONT...FIRST AROUND KINT/KGSO AFTER 21Z AND THEN KRDU/KRWI/KFAY IN THE 00Z TO 06Z TIME FRAME. WHILE WIDESPREAD ADVERSE AVIATION CONDITIONS ARE NOT EXPECTED...A HEAVY DOWNPOUR AND BRIEF PERIOD OF SUB-VFR CONDITIONS IS POSSIBLE AT AT TAF SITE. LIGHT WINDS WILL SHIFT TO NORTHWESTERLY BEHIND THE FRONT AS IT SETTLES SOUTH AND EAST OF THE AREA. LOOKING AHEAD: THE COLD FRONT WILL STALL NEAR THE COAST ON FRIDAY...WITH PREDOMINATELY VFR CONDITIONS EXPECTED THIS WEEKEND. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ CORRECTED ISSUE TIME SYNOPSIS...22 NEAR TERM...RAH/22 SHORT TERM...22 LONG TERM...26 AVIATION...22

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