AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 644 AM EDT MON SEP 9 2013 .SYNOPSIS... HIGH PRESSURE WILL BUILD INTO THE CAROLINAS FROM THE NORTH TODAY AND PERSIST OVER THE REGION THROUGH WEDNESDAY. && .NEAR TERM /THROUGH TUESDAY/... AS OF 240 AM MONDAY... TODAY: A COLD FRONT SETTLING SOUTHWARD THROUGH THE AREA THIS MORNING WILL STALL AND DISSIPATE JUST TO THE SOUTH OF CENTRAL NC TODAY. SURFACE HIGH PRESSURE WILL BUILD INTO THE AREA BEHIND THE FRONT. HOWEVER... ANY SIGNIFICANT AIRMASS CHANGE WILL REMAIN TO THE NORTH OF CENTRAL NC TODAY... AS THE SURFACE HIGH CENTERED OVER NY/PA THIS MORNING IS FORECAST TO SHIFT OFF THE NORTHEAST U.S. COAST TODAY... RESULTING IN ONLY A BRIEF PERIOD OF NORTHERLY/NORTHEASTERLY SURFACE WINDS. SURFACE WINDS/LOW LEVEL FLOW IS EXPECTED TO BECOME EASTERLY/EAST-SOUTHEASTERLY THIS AFTERNOON RESULTING IN A MOIST LOW LEVEL AIRMASS. HOWEVER... ALOFT THE MID/UPPER LEVEL RIDGE OVER THE CENTRAL U.S. WILL CONTINUE TO SLOWLY SHIFT EASTWARD TODAY AND EXTEND INTO OUR AREA. THIS WILL HELP SUPPRESS CONVECTION TODAY... WITH A PRONOUNCED SUBSIDENCE INVERSION AT AROUND 700 MB AND LOTS OF MID LEVEL DRY AIR. HOWEVER... WITH THE EASTERLY LOW LEVEL FLOW DEVELOPING WE COULD SEE SOME VERY LIGHT SHOWERS DEVELOP ACROSS OUR COASTAL PLAIN CLOUD ALONG THE INLAND MOVING TROUGH/SEA BREEZE... AS NAM/GFS BUFR SOUNDING ARE SHOWING SOME VERY WEAK INSTABILITY BELOW THE PROMINENT SUBSIDENCE INVERSION. AFTERNOON LOW LEVEL THICKNESS VALUES ARE EXPECTED TO BE SIMILAR TO YESTERDAY VALUES DESPITE THE PASSAGE OF THE WEAK FRONT. THUS... WILL GO WITH HIGH TEMPS NEAR YESTERDAYS VALUES AND CLOSE TO STATISTICAL GUIDANCE. THIS YIELDS HIGH TEMPS IN THE MID TO UPPER 80S NORTH TO AROUND 90/LOWER 90S SOUTH. TONIGHT AND TUESDAY: ANY SHOWERS THAT DEVELOP TODAY ARE EXPECTED TO DISSIPATE WITH THE LOSS OF DAYTIME HEATING THIS EVENING. QUITE WEATHER IS THEN EXPECTED TO PREVAIL TONIGHT INTO TUESDAY... AS THE MID LEVEL RIDGE BUILDS OVER THE AREA... WHILE HIGH PRESSURE AT THE SURFACE CONTINUES TO EXTEND INTO THE REGION FROM WELL OFF THE NORTHEAST U.S. COAST. THIS SHOULD RESULT IN DRY WEATHER AND ONLY SOME HIGH LEVEL CLOUDS TONIGHT AND SOME FLAT CUMULUS ON TUESDAY. LOW TEMPS TONIGHT ARE EXPECTED TO BE IN THE MID TO UPPER 60S. HIGHS TUESDAY WILL BE A FEW DEGREES WARMER THAN TODAY AS AFTERNOON LOW LEVEL THICKNESS VALUES ARE FORECAST TO INCREASE ABOUT 5 METERS. THIS YIELDS HIGH TEMPS IN THE UPPER 80S TO LOWER 90S... WHICH IS A CATEGORY OR TWO ABOVE NORMAL FOR THIS TIME OF YEAR. && .SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY NIGHT/... AS OF 300 AM MONDAY... TUESDAY NIGHT AND WEDNESDAY... A RIDGE OF HIGH PRESSURE AT THE SURFACE AND ALOFT ACROSS CENTRAL NC WILL SLOWLY WEAKEN THIS PERIOD. STILL...PRESENCE OF SUBSIDENCE INVERSION ALOFT AND LAYER OF DRY AIR SHOULD INHIBIT CONVECTIVE DEVELOPMENT ACROSS MUCH OF THE REGION WEDNESDAY AFTERNOON. EXCEPTION WILL BE THE NW PIEDMONT/TRIAD VICINITY WHERE AN ISOLATED SHOWER OR THUNDERSTORM MAY DEVELOP AS MARGINAL MOISTURE AND MLCAPE CLOSE TO 1000 J/KG MAY BE ENOUGH TO TRIGGER ISOLATED CONVECTION. THICKNESSES WEDNESDAY AFTERNOON WILL AVERAGE 10-15M ABOVE NORMAL. THIS SUPPORTS MAX TEMPS WELL INTO THE 80S TO AROUND 90 IN THE PIEDMONT AND COASTAL PLAIN...AND THE LOWER 90S IN THE SANDHILLS. OVERNIGHT TEMPS TUESDAY NIGHT IN THE MID-UPPER 60S. THURSDAY...MID-UPPER LEVEL HEIGHTS WILL LOWER ACROSS OUR REGION IN RESPONSE TO A FAIRLY STOUT S/W IN THE NORTHERN STREAM CROSSING THE GREAT LAKES. THE FALLING HEIGHTS WILL AID TO SHARPEN THE PIEDMONT SFC TROUGH BY THU AFTERNOON. WHILE RH CROSS SECTIONS SUGGEST THAT MOISTURE REMAINS FAIRLY LIMITED THROUGH THE DEPTH OF THE ATMOSPHERE...A SLIGHT TO MODERATELY UNSTABLE AIR MASS OVER THE NW PIEDMONT (MLCAPE OFF THE 00Z GFS 1000-1500 J/KG INT HE AFTERNOON) MAY PROVIDE ENOUGH FUEL TO TRIGGER A THUNDERSHOWER OR TWO ACROSS THE NW PIEDMONT LATE IN THE DAY. ANOTHER VERY WARM/HOT DAY IN STORE AS THICKNESSES CONTINUE TO AVERAGE 10-15M ABOVE NORMAL. EXPECT HIGH TEMPS THU VERY COMPARABLE TO WEDNESDAY...UPPER 80S NW TO LOWER 90S SE. THURSDAY NIGHT...SURFACE COLD FRONT ASSOCIATED WITH THE NORTHERN STREAM S/W WILL APPROACH CENTRAL NC FROM THE NW AND LIKELY ENTER OUR NORTHERN AFTER MIDNIGHT. DUE TO TIMING (LOWER LAYERS OF THE ATMOSPHERE UNDERGOING NOCTURNAL STABILIZATION) AND BEST UPPER LEVEL SUPPORT WELL REMOVED FROM OUR REGION...CURRENTLY JUST EXPECT A SMATTERING OF SHOWERS/ISOLATED THUNDER WITH THE FRONT ACROSS THE NORTHERN COUNTIES DURING THE EVENING...AND ACROSS THE BULK OF CENTRAL NC OVERNIGHT. PLAN TO MAINTAIN SMALL CHANCE POP AT THIS TIME AS WLY FLOW PRECEDING THE FRONT IN THE SFC-500MB LAYER HAS A TENDENCY TO LIMIT MOISTURE TRANSPORT. WITH LESS THAN IDEAL MOISTURE AVAILABILITY AND NOCTURNAL TIMING...BELIEVE SHOWER DEVELOPMENT WILL BE INHIBITED. MIN TEMPS WILL BE MILD THU NIGHT DUE TO PRESENCE OF CLOUD COVERAGE AND LIGHT MIXING DUE TO PROXIMITY OF THE SFC FRONT. MIN TEMPS IN THE UPPER 60S TO LOWER 70S. && .LONG TERM /FRIDAY THROUGH SUNDAY/... AS OF 300 AM MONDAY... MEDIUM RANGE GUIDANCE COMING INTO BETTER CONSENSUS FOR FRIDAY...LEANING TOWARD A SLIGHTLY FASTER SOLUTION COMPARED TO 24-36 HOURS AGO. SFC FRONT SHOULD BISECT THE FORECAST AREA AT THE START OF THE DAY WITH THE BULK OF ISOLATED-SCATTERED SHOWERS OCCURRING ALONG AND SOUTH OF HIGHWAY 64. S/W PROJECTED TO LIFT NEWD ACROSS NEW ENGLAND WHICH SHOULD CONTINUE TO PROPEL THE SFC FRONT SEWD. THIS SURFACE FEATURE SHOULD EXIT OUR SOUTHERN COUNTIES BY FRIDAY AFTERNOON. MODEL GUIDANCE SUGGEST THAT DRIER AIR WILL OVERWHELM THE MID-UPPER LEVELS OF THE ATMOSPHERE...ESSENTIALLY SQUELCHING CHANCES FOR SHOWERS AND T-STORMS AS THE AFTERNOON PROGRESSES. THUS HAVE PLACED GREATER EMPHASIS ON POP CHANCES FRIDAY MORNING WITH DECREASING POPS NORTH-TO-SOUTH FRIDAY AFTERNOON. COOLER TEMPS EXPECTED FRIDAY AFTERNOON DUE TO MORNING CLOUDS INHIBITING INSOLATION...AND LOW LEVEL CAA COMMENCING BY MID-LATE AFTERNOON. MAX TEMPS FRIDAY AROUND 80-LOWER 80S NORTH TO MID 80S SOUTH. FRIDAY NIGHT THROUGH SUNDAY...MODEL GUIDANCE CONTINUES TO SUGGEST A SEASONABLY COOL AND DRY WEATHER REGIME FOR CENTRAL NC WITH STRONG CANADIAN HIGH PRESSURE EXTENDING SWD INTO THE CAROLINAS BEHIND THE EXITING FRONTAL SYSTEM. LOW LEVEL THICKNESSES SATURDAY AVERAGE CLOSE TO 25M BELOW NORMAL WITH SOME MODERATION EXPECTED SUNDAY. THICKNESSES SUPPORT AFTERNOON TEMPS MAINLY IN THE MID-UPR 70S SATURDAY AND 75-80 SUNDAY. MIN TEMPS IN THE UPPER 50S NORTH TO LOWER 60S SOUTH SATURDAY MORNING AS OVERNIGHT MIXING/LOW LEVEL CAA WILL AID TO KEEP TEMPS UP A BIT. WITH A COOL DRY AIR MASS IN PLACE AND 850MB THERMAL OVERHEAD...OVERNIGHT TEMPS WILL LIKELY TUMBLE INTO THE LOW-MID 50S MOST LOCALES SUNDAY MORNING. OF COURSE IF ANY CLOUDINESS OCCURS SATURDAY NIGHT...SOME MODERATION IN MIN TEMPS WILL BE NECESSARY. && .AVIATION /12Z MONDAY THROUGH FRIDAY/... AS OF 640 AM MONDAY... A WEAKENING COLD FRONT IS CURRENTLY LOCATED NEAR THE NC/VA BORDER THIS MORNING. THE COLD FRONT IS NOW EXPECTED TO MOVE THROUGH CENTRAL NC IN A VERY WEAK STATE... MOST LIKELY AIDED BY MIXING DRIER AIR ALOFT DOWN TO THE SURFACE AFTER SUNRISE THROUGH DIURNAL MIXING. THUS... EXPECT WE WILL SEE GENERALLY MVFR/IFR VISBYS ACROSS THE NORTHERN TAF SITES NOW UNTIL SUNRISE. AT KFAY... VISBYS HAVE REMAINED VFR. WE COULD SEE VISBYS AT KFAY FALL INTO THE MVFR CATEGORY NEAR SUNRISE.... HOWEVER... CONFIDENCE IS NOT HIGH ENOUGH TO INCLUDE IN THE TAFS AT THIS TIME. ANY NORTH TO NORTHEASTERLY WIND BEHIND THE FRONT WILL BE SHORT LIVED... AS SURFACE HIGH PRESSURE OVER THE NORTHERN MID-ATLANTIC THIS MORNING WILL QUICKLY SHIFT OFFSHORE THIS AFTERNOON... RESULTING IN A LIGHT EAST TO SOUTHEASTERLY LOW LEVEL FLOW ACROSS CENTRAL NC BY AFTERNOON. VFR CONDITIONS ARE EXPECTED TO PREVAIL AFTER SUNRISE TODAY CONTINUING THROUGH THIS EVENING... WITH A VERY SMALL CHANCE OF A SHOWER OR TWO AT KRWI/KFAY THIS AFTERNOON... WITH SOME SCATTERED TO AT TIMES BROKEN CLOUD COVER AT AROUND 5-7 KFT. HAVE ADDED TEMPO GROUP FOR SOME MVFR VISBYS FOR TUESDAY MORNING. AS STATISTICAL GUIDANCE AND GFS BUFR SOUNDING ARE SHOWING THE POSSIBILITY FOR SOME PRE-DAWN SUB-VFR VISBYS TUESDAY MORNING... WITH LIGHT AND VARIABLE TO CALM WINDS AND MOSTLY CLEAR SKIES. LONG TERM: PRIMARILY VFR CONDITIONS SHOULD DOMINATE THE REGION THROUGH THE MAJORITY OF THE LONG TERM...WITH A CHANCE FOR SHOWERS ON THURSDAY NIGHT THROUGH FRIDAY ALONG WITH POSSIBLY SOME BRIEF PRE-DAWN SUB-VFR CONDITIONS EACH MORNING. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. &&
Posted on: Mon, 09 Sep 2013 11:41:51 +0000
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