Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 291419 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 1019 AM EDT MON SEP 29 2014 .SYNOPSIS...
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AN UPPER LEVEL DISTURBANCE WILL CROSS THE CAROLINAS TODAY...THEN MOVE OFFSHORE TONIGHT. WEAK HIGH PRESSURE WILL EXTEND SOUTHWARD INTO THE REGION ON TUESDAY AND WEDNESDAY.
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&& .NEAR TERM /THROUGH TONIGHT/...
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AS OF 1015 AM MONDAY... OVERVIEW: A SHORTWAVE OVER THE TN VALLEY WILL PROGRESS EAST OF THE MOUNTAINS TODAY...MOVING OFFSHORE THE CAROLINA/MID-ATLANTIC COAST LATE TONIGHT. WEAK HIGH PRESSURE WILL PERSIST OVER CENTRAL NC THROUGH TONIGHT AS A WEAK LOW PRESSURE SYSTEM (ATTENDANT THE APPROACHING SHORTWAVE) DEVELOPS ALONG A BAROCLINIC ZONE OFFSHORE THE SOUTHEAST COAST. PRECIPITATION: FORCING FOR ASCENT TODAY WILL BE CONFINED TO DPVA...H5 WARM ADVECTION...AND DIFFLUENT FLOW ALOFT ATTENDANT THE APPROACHING SHORTWAVE. IN THE ABSENCE OF ANY LOW-LEVEL FORCING OR MOISTURE ADVECTION...FORCING ASSOC/W THE SHORTWAVE WILL BE SUFFICIENT TO MAINTAIN BROKEN/OVERCAST MID-LEVEL CEILINGS THROUGHOUT THE DAY...BUT LITTLE MORE THAN SPRINKLES GIVEN A RELATIVELY DRY AIRMASS IN THE LOWEST 10 KFT PER 12Z GSO/MHX RAOB DATA. MARGINAL ELEVATED DESTABILIZATION (LESS THAN 100 J/KG) ASSOC/W MID-LEVEL WARM ADVECTION MAY BE SUFFICIENT FOR OCCASIONAL ELEVATED SHOWER ACTIVITY THIS AFTERNOON ALONG/EAST OF THE HWY 1 CORRIDOR...PER LATEST RAP/NAM FCST SOUNDINGS. WILL INDICATE A 40-60% CHANCE OF SPRINKLES OR LIGHT SHOWERS THROUGH SUNSET...DECREASING/ENDING FROM WEST-EAST BETWEEN 00- 06Z AS THE SHORTWAVE PROGRESSES TOWARD THE COAST AND SUBSIDENCE ENSUES OVER WESTERN/CENTRAL NC. TEMPERATURES: OVERCAST MID-LEVEL CEILINGS WILL REDUCE INSOLATION THIS AFTERNOON...WHILE SPRINKLES AND ELEVATED SHOWERS FOSTER THE DEVELOPMENT OF AN IN-SITU TYPE CAD WEDGE. AS A RESULT...HIGHS TODAY ARE SOMEWHAT CHALLENGING. GIVEN THE LATEST TRENDS...EXPECT HIGHS RANGING FROM THE UPPER 60S TO LOWER 70S OVER MOST OF CENTRAL NC... COOLEST IN THE N/NW PIEDMONT. -VINCENT
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&& .SHORT TERM /TUESDAY AND TUESDAY NIGHT/... AS OF 315 AM MONDAY... EXPECT A QUIET DAY WITH WEAK SUBSIDENCE BEHIND THE DEPARTING TROUGH AND WEAK HIGH PRESSURE CONTINUING TO SPAN THE REGION. DESPITE THE WEAK LOW TRACKING NE OFF THE SOUTHEAST COAST... THE NAM/GFS FORECAST SOUNDINGS DEPICT A MOSTLY DRY COLUMN... REASONABLE CONSIDERING THE LOW IS WELL OFFSHORE AND NOT DEEP ENOUGH TO STRENGTHEN THE NE SURFACE WINDS OVER CENTRAL NC. UNDER PARTLY TO MOSTLY SUNNY SKIES... TEMPS SHOULD RISE NICELY TO PEAK WITHIN A COUPLE DEGREES OF 80... STILL ON THE LOW END OF STATISTICAL GUIDANCE BUT FOLLOWING ITS TREND AS WELL AS WARMING THICKNESSES. THE POWERFUL TROUGH NOW OVER SOUTH CENTRAL ONTARIO DROPS SOUTH THEN SE ACROSS THE MIDATLANTIC REGION TUE NIGHT... WHICH SHOULD BRING A SHOT OF PARTLY CLOUDY SKIES ACROSS THE NRN AND ERN CWA LATE TUE NIGHT... CORRESPONDING TO MODELS WHICH SHOW A SUBTLE RESURGENCE IN LOW LEVEL MOISTURE FLUX. WILL BRING IN A FEW MORE CLOUDS ESPECIALLY ACROSS THE NORTH AND EAST... BUT NO PRECIP CHANCES FOR NOW... AS THE COLUMN IS STILL PROJECTED TO BE TOO DRY AND THE LIFT TOO WEAK. LOWS 56-60. -GIH && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... AS OF 300 AM MONDAY... FOR THE PERIOD WEDNESDAY AND WEDNESDAY NIGHT...THE MID-LEVEL LOW GRADUALLY MOVES OFF OF THE NORTHERN MID-ATLANTIC COAST AS HEIGHTS SLOWLY RISE OVER CENTRAL NORTH CAROLINA. AS THE SOUTHEAST COASTAL LOW MOVES FARTHER OFFSHORE...BOTH THE NAM AND GFS SUGGEST A WEAK SURFACE BOUNDARY/COLD FRONT MOVING SOUTH INTO CENTRAL NORTH CAROLINA WITH THE RIDGE WEDNESDAY. WHILE THERE IS A RELATIVE MAXIMUM OF 850MB THETA-E FORECAST BY THE GFS...AND WEAK 850MB LIFT WEDNESDAY AFTERNOON AND EVENING...THE GFS FORECASTS DECENT 850MB DIVERGENCE AND WEAK CONFLUENCE ALOFT. THE MEAN AIR MASS IS RELATIVELY DRY... WITH BETTER MOISTURE SEEMINGLY CONFINED TO AROUND THE 850MB LEVEL. MOS GUIDANCE POPS ARE LOW...AND GFS BUFR SOUNDINGS ARE CAPPED TO ANY DEEP CONVECTION. NAM BUFR SOUNDINGS ARE MORE MOIST AND UNSTABLE... WITH STEEPER LOW LEVEL LAPSE RATES LATE IN THE DAY WEDNESDAY WHICH ARE SUFFICIENT TO GET PARCELS THROUGH THE CAP. AS SUCH...THE NAM FORECASTS MINOR QPF IN CENTRAL NORTH CAROLINA MAINLY LATE WEDNESDAY AND WEDNESDAY NIGHT. CURRENTLY THIS INSTABILITY SEEMS OVERDONE. WILL KEEP THE FORECAST DRY FOR NOW AS HEIGHTS START TO RISE ALOFT...AND IN DEFERENCE TO THE DRY GFS WHICH IS SUPPORTED BY THE ECMWF. MOS GUIDANCE HIGHS WEDNESDAY ARE IN LINE WITH 1000-850MB THICKNESSES... SUPPORTIVE OF LOWER TO MID 80S. WITH THE SURFACE BOUNDARY NEARBY... ALBEIT WEAK...MAY LEAN ABOUT A CATEGORY COOLER ON AVERAGE. LOWS WEDNESDAY NIGHT 59 TO 63. FOR THE PERIOD THURSDAY THROUGH SUNDAY...ANY WEAK LOW-LEVEL BOUNDARY SHOULD CONTINUE TO WEAKEN AS HEIGHTS RISE ALOFT. BOTH THE GFS AND THE ECMWF FORECAST MINOR QPF OVER CENTRAL NORTH CAROLINA...WHICH HAS OCCURRED OFF AND ON THE LAST FEW OF THESE MODEL RUNS. COARSE MODEL SOUNDINGS FROM THE GFS AND ECMWF INDICATE LOW-LEVEL MOISTURE TO SOME DEGREE...BUT A CAPPED AIR MASS IN GENERAL WITH PARCELS POSSIBLY ABLE TO REACH BETWEEN -10C AND -15C IN THE MOST UNSTABLE SCENARIO BEFORE REACHING THE CAP. WHILE AN ISOLATED SHOWER IS POSSIBLE...GIVEN THE SURFACE HIGH PRESSURE AND THE INCREASING HEIGHTS ALOFT...FOR NOW WILL RETAIN THE DRY FORECAST FOR THURSDAY INTO EARLY FRIDAY. THE TREND OF THE GFS AND THE ECMWF HAS BEEN TO SPEED UP THE APPROACH OF AN ELONGATED MID-LEVEL SHORTWAVE FRIDAY AHEAD OF A DIGGING MID- LEVEL LOW OVER THE GREAT LAKES. WHILE THE FRONTAL ORIENTATION IS POSITIVE...A SLIGHT NEGATIVE TILT TO THE APPROACHING MID-LEVEL TROUGH SHOWN BY BOTH THE GFS AND ECMWF COULD RESULT IN SHOWERS MOVING INTO THE AREA...AND SUBSEQUENTLY THEN EAST OF THE AREA AS WELL...FASTER THAN PREVIOUSLY FORECAST. INSTABILITY STILL SEEMS LIMITED...BUT GIVEN THE STRENGTH OF THE FRONT...SLIGHT NEGATIVE TILT TO THE APPROACHING MID-LEVEL WAVE...AND INDEED A LITTLE INSTABILITY ON COARSE MODEL SOUNDINGS...WILL CONTINUE TO MENTION THUNDER. COOL HIGH PRESSURE THEN BUILDS OVER CENTRAL NORTH CAROLINA FOR SATURDAY INTO SUNDAY WITH DRY CONDITIONS FORECAST. THE ECMWF DIGS THE MID- LEVEL LOW MUCH FARTHER SOUTH THAN THE GFS WITH AMPLE VORTICITY ADVECTION JUST TO OUR NORTH OVER VIRGINIA LATE SATURDAY AND SATURDAY NIGHT. IF THIS TREND CONTINUES...MORE CLOUDS AND EVEN A SMALL CHANCE OF LIGHT PRECIPITATION WOULD BE POSSIBLE WITH SUCH A VIGOROUS WAVE IN THE NORTHWEST FLOW ALOFT...BUT GIVEN THE UNCERTAINTY OUT TO THE END OF THE LONG-TERM PERIOD WILL KEEP THE FORECAST DRY FOR NOW. 1000- 850MB THICKNESSES ARE QUITE CHILLY BY SUNDAY AT 12Z AND HAVE TRENDED LOWER...SO IT CURRENTLY SEEMS LIKELY THERE WILL BE SOME 40S FOR LOWS SATURDAY NIGHT ESPECIALLY FROM THE TRIANGLE WEST AND NORTH...AND MAINLY NORTHEAST OF THE TRIANGLE SUNDAY NIGHT. -DJF && .AVIATION /12Z MONDAY THROUGH FRIDAY/... AS OF 655 AM MONDAY... VFR CONDITIONS WILL DOMINATE FOR THE NEXT 24 HOURS AT CENTRAL NC TAF SITES... WITH A FEW BRIEF EXCEPTIONS. RWI IS SEEING ISOLATED POCKETS OF SHALLOW DENSE FOG DRIFT NEAR THE AIRFIELD OVER THE LAST SEVERAL HOURS... CAUSING WIDELY VARYING CONDITIONS FROM VFR DOWN TO LIFR (OVERNIGHT OBSERVATIONS HAVE DIPPED AS LOW AS 1/2 MI AND 200 FT CIGS BEFORE RETURNING TO MVFR OR VFR) AT THAT SITE. THIS ADVERSE AVIATION WEATHER SHOULD END BEFORE 14Z. OTHERWISE... A THICK DECK OF MID CLOUDS WILL CONTINUE TO OVERSPREAD THE REGION THIS MORNING... WITH CIGS EXPECTED TO LOWER BELOW 5 000 FT AGL BY MID MORNING (GSO/INT) OR THE NOON HOUR (RDU/RWI/FAY) ASSOCIATED WITH APPROACH AND PASSAGE OF A WEAK LOW-MID LEVEL TROUGH. THESE LOWERING CIGS WILL COINCIDE WITH AREAS OF LIGHT RAIN THAT WILL EXPAND OVER THE AREA FROM SW TO NE... SHIFTING EASTWARD OVER CENTRAL NC THROUGH TODAY WHILE DECREASING IN COVERAGE TO BECOME VERY LIGHT/PATCHY. BRIEF MVFR VSBYS ARE POSSIBLE IN THIS LIGHT RAIN... MAINLY NEAR GSO/INT... BUT OVERALL VFR VSBYS SHOULD STILL PREVAIL TODAY INTO TONIGHT. WINDS WILL BE LIGHT... MAINLY CALM THROUGH DAYBREAK THEN LIGHT/VARIABLE UNDER 6 KTS TODAY THROUGH TONIGHT. ONCE THE VEIL OF MID/HIGH CLOUDS EXITS TO THE EAST THIS EVENING LEAVING MOSTLY CLEAR SKIES... WITH WINDS STAYING VERY LIGHT... CONDITIONS ARE RIPE FOR POSSIBLE SHALLOW LIFR STRATUS/FOG BETWEEN 07Z AND 12Z TUE MORNING... HOWEVER UNCERTAINTY IS HIGH BASED ON DIFFERING COMPUTER MODEL SOLUTIONS. IF IT OCCURS... VFR CONDITIONS SHOULD RETURN BY MID MORNING TUE. LOOKING BEYOND 12Z TUE MORNING... MVFR FOG IS POSSIBLE JUST BEFORE AND AROUND SUNRISE WED MORNING UNDER MOSTLY CLEAR SKIES WITH LIGHT WINDS. A STRONG STORM SYSTEM WILL MOVE IN FROM THE WEST FRI... BRINGING A CHANCE OF SUB-VFR CONDITIONS AND RISING SHOWER CHANCES STARTING FRI. -GIH && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...VINCENT NEAR TERM...VINCENT SHORT TERM...HARTFIELD LONG TERM...DJF AVIATION...HARTFIELD

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