Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 290718 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 315 AM EDT MON SEP 29 2014 .SYNOPSIS...A WEAK AREA OF LOW PRESSURE WILL DEVELOP OVER THE NORTHEAST GULF TODAY... CROSS NORTHERN FLORIDA TONIGHT... AND THEN TRACK NORTHEASTWARD OFF THE SOUTHEAST COAST TUESDAY. WEAK HIGH PRESSURE WILL BUILD OVER THE AREA WEDNESDAY. && .NEAR TERM /THROUGH TONIGHT/...
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AS OF 310 AM MONDAY... AS ALLUDED TO BY PREVIOUS FORECASTERS AND SUPPORTED BY OBSERVED AND MODEL TRENDS... THIS INCOMING PRECIP EVENT IS SHAPING UP TO BE ONE CHARACTERIZED BY OVERALL LOW QPF AND MODEST COVERAGE... DECREASING FROM WEST TO EAST. THE LATEST SURFACE MAP SHOWS A STATIONARY FRONTAL ZONE FROM THE GULF COAST ACROSS NRN FL AND OFF THE SE COAST... AND WEAK HIGH PRESSURE COVERING INTERIOR NC/VA. A BAGGY POSITIVELY- TILTED MID LEVEL TROUGH APPROACHING FROM THE NW WILL PUSH EAST THROUGH CENTRAL NC TODAY... BRINGING WEAK MID LEVEL DPVA. IN FACT... THE VAST MAJORITY OF FORCING FOR ASCENT IS LIKELY TO OCCUR FROM 700 MB TO 500 MB... A RESULT OF A VERY LIGHT WIND FIELD BELOW 700 MB MINIMIZING LOW LEVEL MASS CONVERGENCE. MODELS AGREE ON THE PRIME LOW LEVEL MOISTURE TRANSPORT HOLDING SOUTH AND SE OF CENTRAL NC THROUGH TONIGHT... WITH NONEXISTENT LOW LEVEL MOIST ISENTROPIC UPGLIDE... ALL LEADING TO FAIRLY HIGH CLOUD BASES AND LOW QPF DESPITE PW VALUES SURGING UP TOWARD 1.5 INCHES. THE LATEST WRF-ARW/WRF-NMM AND HRRR RUNS SUPPORT THIS WEAKENING TREND WITH 700-500 MB TROUGH PASSAGE. WILL HOLD POPS AT GOOD CHANCE/LIKELY ACROSS THE AREA... DUE TO THE TROUGH ALOFT AND DECENT 40-50 M HEIGHT FALLS... BUT QPF SHOULD BE UNDER A QUARTER INCH (AND MAY BE JUST A FEW HUNDREDTHS IN MANY SPOTS). WITH ANY RAIN EXPECTED TO BE LIGHTER AND OF SHORTER DURATION... TEMPS SHOULD BE ALLOWED TO WARM UP A BIT MORE THAN PREVIOUSLY THOUGHT... ALTHOUGH THE MID CLOUD DECK LOOKS TO BE THICK ENOUGH TO GREATLY CURB SOLAR INSOLATION. WILL KEEP HIGHS ON THE LOW END OF STATISTICAL GUIDANCE... 70-75 UNDER MOSTLY CLOUDY SKIES... WITH THE COOLER READINGS THROUGH THE CENTRAL CWA WHERE CLOUDS AND PATCHY LIGHT RAIN SHOULD BE MOST PRONOUNCED NEAR THE TIME OF PEAK HEATING. PRECIP CHANCES WILL DROP FROM NW TO SE THROUGH THE LATE AFTERNOON AND EARLY EVENING AS THE FORCING BEGINS TO EXIT. THE MID/HIGH CLOUDS SHOULD MOSTLY CLEAR OUT TONIGHT WITH WEAK SUBSIDENCE POST-TROUGH... BUT THIS COMBINED WITH THE LIGHT SURFACE WINDS COULD SET THE STAGE FOR SHALLOW STRATUS AND FOG DEVELOPMENT OVERNIGHT. MODELS DON`T AGREE ON THIS AT ALL... SO CONFIDENCE IS NOT HIGH... BUT WILL INDICATE A TREND FROM FAIR BACK UP TO PARTLY CLOUDY LATE. LOWS 55-60.
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&& .SHORT TERM /TUESDAY AND TUESDAY NIGHT/...
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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
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&& .LONG TERM /WEDNESDAY THROUGH SUNDAY/...
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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
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&& .AVIATION /06Z MONDAY THROUGH FRIDAY/... AS OF 120 AM MONDAY... VFR CONDITIONS WILL DOMINATE FOR THE NEXT 24 HOURS AT CENTRAL NC TAF SITES... WITH A FEW BRIEF EXCEPTIONS. 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. LOOKING BEYOND 06Z EARLY TUE MORNING... 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. 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...HARTFIELD NEAR TERM...HARTFIELD SHORT TERM...HARTFIELD LONG TERM...DJF AVIATION...HARTFIELD

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