238 FXUS62 KRAH 200538 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 137 AM EDT FRI SEP 20 2013 .SYNOPSIS... SURFACE HIGH PRESSURE WILL LINGER OVER OUR REGION THROUGH TONIGHT...THEN DRIFT OFFSHORE ON FRIDAY. A COLD FRONT WILL APPROACH THE REGION FROM THE NORTHWEST ON SATURDAY AND THEN MOVE THROUGH THE REGION ON SUNDAY. && .NEAR TERM /THROUGH TONIGHT/... AS OF 10 PM THURSDAY... WEAK S/W ENERGY ALOFT WILL CONTINUE TO CROSS THE AREA OVERNIGHT IN THE NORTH-NORTHWEST TO NORTHWEST FLOW ALOFT. AT THE SURFACE HIGH PRESSURE WILL LINGER OVER OUR REGION... WHILE BEGINNING TO DRIFT AWAY FROM THE MID-ATLANTIC COAST. THIS WILL LEAD TO A WARMER NIGHT TONIGHT... WITH PARTLY TO MOSTLY CLOUDY SKIES FROM TIME TO TIME ALONG WITH LOW LEVEL WAA INCREASING FIRST IN THE WEST (AS ODD AS IT MAY SOUND... WITH A NORTHWESTERLY FLOW AT 850 MB). WE COULD SEE SOME LIGHT SHOWERS AND OR SPRINKLES LATER TONIGHT... GENERALLY AFTER 07Z... AND MAINLY ACROSS THE NORTHERN AND WESTERN PIEDMONT. THUS... WILL ADD A SLIGHT CHANCE OF SHOWERS THERE. TEMPERATURES WILL BE DEPENDENT ON CLOUD COVER AND THUS WE COULD SEE A WIDE RANGE IN OVERNIGHT LOWS RANGING FROM MID 50S TO LOWER/MID 60S... WARMEST WHERE THE CLOUD COVER IS EXPECTED TO BE THE GREATEST. THUS... EXPECT THE COOLEST TEMPS ACROSS OUR FAR EAST/NORTHEAST. && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... AS OF 256 PM THURSDAY... SHORTWAVE RIDGE CENTERED OVER THE TN AND OHIO VALLEY THIS AFTERNOON WILL SHIFT EASTWARD OVER THE AREA ON FRIDAY...AND THEN SHIFT EAST AND OFFSHORE FRIDAY EVENING WITH THE APPROACH OF AN UPPER TROUGH INTO THE CENTRAL AND SOUTHERN APPALACHIANS BY DAYBREAK SATURDAY. FRIDAY: WEAK LOW TO MID LEVEL WARM MOIST AIR ADVECTION WILL CONTINUE TO SUPPORT CONSIDERABLE CLOUDINESS IN THE 850-700MB LAYER. HOWEVER...WITH UPPER RIDGE AXIS OVERHEAD....SUPPORT ALOFT WILL REMAIN NEGLIGIBLE WITH VERY MODEST DESTABILIZATION OF LESS THAN 500J/KG FORECAST TO DEVELOP ACROSS WESTERN NC IN A ZONE OF LOW-LEVEL MOISTURE RETURN (INCREASING BL DEWPOINTS)ALONG THE WESTERN PERIPHERY OF THE RETREATING SURFACE HIGH. WILL ADD ISOLATED SHOWERS/SPRINKLES ACROSS THE FAR WESTERN ZONES FOR FRIDAY...BUT EXPECT GENERALLY DRY CONDITIONS FRIDAY AFTERNOON/EVENING. HIGHS ON FRIDAY WILL RANGE FROM NEAR 80 NW WHERE CLOUD COVER WILL BE THICKER TO MID 80S EAST. FRIDAY NIGHT: MID AND UPPER LEVEL FLOW WILL BACK TO SWLY WITH THE APPROACH OF THE UPPER TROUGH AND ATTENDANT SFC COLD FRONT INTO THE MOUNTAINS. ISOLATED TO SLIGHT CHANCE SHOWERS WILL BE POSSIBLE ACROSS THE WESTERN AND POSSIBLY CENTRAL PIEDMONT AS PWATS INCREASE TO .75" AS WEAK DPVA AND UPPER LEVEL DIFFLUENCE SPREADS EAST OF THE MOUNTAINS. LOWS TONIGHT IN THE LOWER TO MID 60S. && .LONG TERM /SATURDAY THROUGH THURSDAY/... AS OF 305 PM THURSDAY... SATURDAY THROUGH SUNDAY NIGHT... MODEL GUIDANCE CONTINUES TO WAVER WITH THE 12Z GFS AS MUCH AS 6-12 HOURS FASTER COMPARED TO YESTERDAY'S 12Z RUN. IN ADDITION LAST NIGHT'S ECMWF CAME IN WETTER FOR EARLY NEXT WEEK COMPARED TO THE DRY AND MILD GFS. ONE THING THE MODELS APPEAR TO HAVE IN COMMON IS A S/W CROSSING THE GREAT LAKES INTO SOUTHERN NEW ENGLAND LATE SATURDAY INTO EARLY SUNDAY. A SURFACE COLD FRONT ASSOCIATED WITH THIS FEATURE PROJECTED TO ENTER OUR REGION SATURDAY NIGHT. FROM HERE THE MODELS DIVERGE WITH THE GFS PROPELLING THE FRONT OFFSHORE WHILE THE ECMWF HANGS THE FRONT ACROSS OUR REGION SUNDAY THEN DRIFTS IT SWD BEFORE STALLING BY MONDAY. TRYING TO BLEND THESE TWO DISTINCT SOLUTIONS PROBLEMATIC. SINCE APPEARS BEST RAIN CHANCES ACROSS CENTRAL NC APPEAR TO OCCUR SATURDAY AFTERNOON (MAINLY WEST) INTO SATURDAY NIGHT...WILL PLACE HIGH CHANCE/LOW END LIKELY POPS ACROSS THE REGION DURING THIS PERIOD. PRECIP AMOUNTS DO NOT LOOK AS ROBUST AS 48 HOURS AGO BUT COULD STILL SEE WIDESPREAD QUARTER TO POSSIBLY A THIRD OF AN INCH AMOUNTS WITH POCKETS OF HALF OF AN INCH. FOR SUNDAY...PLAN TO DEPICT A SLOWER THAN ADVERTISE GFS SOLUTION WITH THE FRONT DRIFTING EWD...EXITING THE EASTERN PERIPHERY BY MID-LATE AFTERNOON. THIS LEADS TO POPS DIMINISHING FROM THE NW TO SE DURING THE COURSE OF THE DAY. IF THE CURRENT MODEL RUN OF THE GFS VERIFIES...POPS WOULD NEED TO BE LOWERED AT LEAST 20-25 PERCENT SUNDAY. TEMPERATURES SATURDAY AND SUNDAY HIGHLY DEPENDENT ON SHOWER COVERAGE. STILL EXPECT THE BULK OF SHOWERS AFTER 1-2 PM IN THE WEST...AND NOT UNTIL LATE AFTERNOON-EARLY EVENING IN THE TRIANGLE AREA. HOWEVER...CONSIDERABLE CLOUDINESS SHOULD PREVAIL BY MORNING WHICH WILL CUT BACK ON INSOLATION. THUS HAVE TRIMMED A DEGREE OR TWO OFF SATURDAY'S MAX TEMPS FROM PREVIOUS FORECAST. FOR SUNDAY...COUNTING ON CONSIDERABLE CLOUDINESS THROUGH A GOOD PART OF THE DAY. THIS SHOULD RESULT IN MAX TEMPS IN THE MID-UPPER 70S. IF PARTIAL CLEARING OCCURS SOONER...MAX TEMPS WILL NEED TO BE RAISED 3-4 DEGREES...ESPECIALLY ACROSS THE PIEDMONT. THE MONDAY-WEDNESDAY PERIOD STILL HOLDS MUCH UNCERTAINTY. WHILE GFS FAVORS A DRY AND MILD WEATHER PATTERN WITH THE SFC FRONT CONTINUING SWD INTO THE FL PENINSULA...ECMWF STALLS FRONT NEAR OUR SC BORDER AND AWAITS A SFC WAVE TO RIDE UP ALONG THE FRONT MONDAY INTO TUESDAY. DECIDED TO MAINTAIN A SLIGHT-CHANCE OF RAIN ACROSS CENTRAL NC DURING THIS PERIOD TO GIVE SOME RECOGNITION TO THE ECMWF SOLUTION. IF THE OTHER MODEL GUIDANCE BEGINS TO TREND TO THE ECMWF SOLUTION...LIKELY POPS WOULD BE NEEDED SOUTH OF HIGHWAY 64 LATE MONDAY INTO EARLY TUESDAY. CONVERSELY..IF THE GFS VERIFIES...POPS LESS THAN 20 PERCENT PREFERRED WITH MAX TEMPS 5-10 DEGREES WARMER THAN FORECAST TUESDAY INTO WEDNESDAY. EVEN ON THURSDAY...DIFFERENCES NOTED WITH THE GFS MUCH WARMER THAN THE ECMWF AS THICKNESSES VARY AS MUCH AS 30-40M WEDNESDAY INTO THURSDAY. FAVOR TEMPS CLOSE TO CLIMO FOR NOW. && .AVIATION /06Z FRIDAY THROUGH MONDAY/... AS OF 137 AM FRIDAY... EXTENSIVE CLOUD COVER HAS EVOLVED OVER MUCH OF CENTRAL NC DURING THE PAST FEW HOURS...ALONG WITH A FEW LIGHT RAIN SHOWERS MOVING TO THE SOUTHEAST ACROSS THE CENTRAL PIEDMONT...ASSOC WITH A MID-LEVEL S/W MOVING ACROSS THE AREA ATTM. DESPITE THIS CLOUD COVERAGE...FLT CONDITIONS SHOULD REMAIN MOSTLY VFR WITH CIGS REMAINING ABOVE 3K FT. THESE CONDITIONS EXPECTED TO PERSIST THROUGH MUCH OF THE 24 HOUR TAF PERIOD. LOOKING FARTHER AHEAD...A COLD FRONT WILL CROSS THE AREA SATURDAY NIGHT INTO EARLY SUNDAY. SCATTERED SHOWERS AND TSTMS WILL BE POSSIBLE ALONG AND AHEAD OF THIS FRONT SATURDAY AND SATURDAY NIGHT...WHICH MAY RESULT IN BRIEF PERIODS OF SUB-VFR FLIGHT CONDITIONS. FLIGHT CONDITIONS WILL IMPROVE LATE SUNDAY INTO EARLY NEXT WEEK AS HIGH PRESSURE AND DRIER AIR BUILDS INTO THE REGION BEHIND THE FRONT. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...CBL NEAR TERM...BSD/CBL SHORT TERM...CBL LONG TERM...WSS AVIATION...NP