Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 270720 RRA AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 319 AM EDT WED AUG 27 2014 .SYNOPSIS...HIGH PRESSURE WILL EXTEND SOUTHWARD INTO THE REGION TODAY. A BACK DOOR COLD FRONT WILL SETTLE SOUTH INTO CENTRAL NC THU AND THU NIGHT. && .NEAR TERM /THROUGH THURSDAY/... AS OF 310 AM WEDNESDAY... THIS MORNING: CLAM WINDS AND CLEAR SKIES COUPLED WITH LINGERING MOISTURE FROM NORTHEASTERLY LOW LEVEL FLOW HAS LEAD TO SOME PATCHY FOG DEVELOPING ACROSS NORTHEAST PORTIONS OF THE AREA THIS MORNING. WHILE WE DO NOT EXPECT WIDESPREAD FOG THIS MORNING... A FEW OF THE FOG PRONE AREAS NORTH AND EAST OF THE TRIANGLE MAY SEE SOME VERY PATCHY... BUT DENSE FOG. TODAY AND TONIGHT: SURFACE HIGH PRESSURE EXTENDING INTO OUR AREA FROM THE NORTHEAST THIS MORNING... WILL BE PINCHED OFF TODAY AS HURRICANE CRISTOBAL LIFTS NORTHWARD AND PASS WELL OFFSHORE. A PINCHED OFF PIECE OF THE SURFACE HIGH WILL REMAIN ACROSS WESTERN PORTIONS OF NC TODAY. PW`S WILL REMAIN BELOW AND INCH TODAY... AS THE HURRICANE PASSES TO OUR EAST LEAVING US ON THE DRY SUBSIDENCE SIDE OF THE OVERALL SYSTEM. THIS WILL RESULT IN CONTINUED DRY CONDITIONS... WITH UNSEASONABLY LOW AFTERNOON RH VALUES. HOWEVER... AFTERNOON LOW LEVEL THICKNESS VALUES ARE EXPECTED TO INCREASE SOME 15 TO 20 METERS TODAY. LOW LEVEL FLOW WILL BACK TO MORE OF A NORTHERLY DIRECTION TODAY... HELPING TO LIMIT LOW LEVEL MOISTURE FURTHER... AND RESULTANT CLOUD COVER. HOWEVER... WE WILL STILL LIKELY SEE A PERIOD OF SCATTERED TO AT TIMES BROKEN STRATOCUMULUS THIS MORNING WITH THE ONSET OF SURFACE HEATING. AS THE MIXED LAYER DEEPENS THIS CLOUD COVER SHOULD LIFT/SCATTER/DISSIPATE. THIS SHOULD YIELD HIGH TEMPS A GOOD 2 TO 5 DEGREES WARMER TODAY WITH MOSTLY SUNNY SKIES DURING THE AFTERNOON. THIS YIELDS HIGH TEMPS NEAR STATISTICAL GUIDANCE IN THE MID TO UPPER 80S... A BIT LOWER THAN PREVIOUSLY FORECAST AS GUIDANCES WITH THE NORTHERLY TO NORTHEASTERLY LOW LEVEL FLOW THE PAST FEW DAYS HAS OVER FORECAST HIGH TEMPS. SURFACE HIGH PRESSURE AND THE CORE OF THE LOWER PW`S ARE FORECAST TO DRIFT TO THE SOUTH OF THE AREA OVERNIGHT AS A BACKDOOR COLD FRONT BEGINS TO APPROACH THE AREA FROM THE NORTH... ASSOCIATED WITH THE S/W TROUGH THAT IS FORECAST TO ABSORB HURRICANE CRISTOBAL AND SWEEP HIM QUICKLY OFF TO THE NORTHEAST (AWAY FROM ANY COASTAL AREAS IN THE U.S.). LOWS TONIGHT WILL AGAIN BE COMFORTABLE FOR LATE AUGUST IN CENTRAL NC... BELOW NORMAL IN THE LOWER TO MID 60S... MAYBE EVEN A FEW LOCATIONS TOUCHING THE UPPER 50S. THURSDAY: THE PREVIOUSLY MENTIONED BACKDOOR COLD FRONT IS EXPECTED TO DROP SOUTHWARD INTO CENTRAL NC THURSDAY AFTERNOON. MOISTURE WILL INCREASE ACROSS THE AREA IN ADVANCE OF THIS FEATURE. HOWEVER... WITH LOW LEVEL WESTERLY TO WEST-SOUTHWESTERLY FLOW IN ADVANCE OF THE SYSTEM WE SHOULD SEE DEWPOINTS IN THE LOWER TO MAYBE MID 60S (AS THE FRONT APPROACHES... POOLING ALONG IT). THIS SHOULD LEAD TO ONLY ISOLATED CONVECTION DEVELOPING IN THE AFTERNOON ACROSS NORTHERN PORTIONS OF THE AREA. HOWEVER... WE SHOULD WARM UP NICELY ON THURSDAY... WITH THE WSW TO W`ERLY FLOW. THUS... EXPECT HIGH TEMPS TO BE IN THE LOWER TO MID 90S. WRT A SEVERE THREAT... SEVERE WEATHER POTENTIAL LOOKS TO BE LOW... GIVEN THE RATHER WEAK INSTABILITY... THANKS TO THE RELATIVELY DRY LOW LEVELS. THE NAM APPEARS TO BE TOO MOIST AND CONSEQUENTLY GENERATES TOO MUCH INSTABILITY AS NOTED IN THE PREVIOUS AFD FOR THE PATTERN. IN ADDITION... DEEP LAYER SHEAR IS FORECAST TO REMAIN WEAK... ~20KTS OR SO. THUS... WE ARE NOT OUTLOOKED IN THE SPC DAY 2 CONVECTIVE OUTLOOK IN ANYWAY. IF WE GET A DECENT STORM DEVELOP THOUGH IT WOULD HAVE A DAMAGING WIND THREAT THANKS TO THE DRY DEEP SUB-CLOUD LAYER. && .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... AS OF 3 AM WEDNESDAY... BACKDOOR FRONT PROJECTED TO STALL IN VICINITY OF THE NORTHERN PIEDMONT-NORTHERN COASTAL PLAIN THURSDAY NIGHT THEN SLOWLY DISSOLVE ON FRIDAY. ENOUGH MOISTURE POOLING ALONG THIS FEATURE MAY AID TO TRIGGER AN ISOLATED SHOWER OVERNIGHT THURSDAY NIGHT TO THE NORTH-EAST OF RDU...WITH AN ISOLATED SHOWER OR T-STORM POSSIBLE OVER THE SAME REGION FRIDAY AFTERNOON. BUILDING MID-UPPER LEVEL HEIGHTS AND ASSOCIATED MID LEVEL SUBSIDENCE WILL INHIBIT CONVECTIVE DEVELOPMENT ELSEWHERE. AFTERNOON THICKNESSES STILL AVERAGING 10M ABOVE NORMAL...SUPPORTIVE OF MAX TEMPS ABOUT 3-4 DEGREES ABOVE NORMAL. THIS YIELDS HIGH TEMPS FRIDAY IN THE 90-95 DEGREE RANGE. MIN TEMPS WILL BE IN THE MID-UPPER 60S THURSDAY NIGHT AND UPPER 60S-AROUND 70 FRIDAY NIGHT. && .LONG TERM /SATURDAY THROUGH TUESDAY/... AS OF 3 AM WEDNESDAY... SATURDAY...PRESENCE OF MID-UPPER LEVEL RIDGE OVER THE FORECAST AREA SHOULD INHIBIT CONVECTIVE DEVELOPMENT DUE TO SUBSIDENCE INVERSION ALOFT. STILL EXPECT AN EXTENSIVE CUMULUS FIELD AS LOW LEVEL MOISTURE WILL BE IN THE INCREASE...WITH SOME GROWTH TO THE CU UNTIL IT HITS THE INVERSION. MODEL GUIDANCE PLACES THE 700MB ANTI-CYCLONE DIRECTLY OVERHEAD. THIS FAVORABLE PLACEMENT SUGGEST MAX TEMPS BACK IN THE 90- 95 DEGREE RANGE. SATURDAY NIGHT SHOULD SEE PARTLY CLOUDY SKIES WITH PATCHES OF FOG DEVELOPING LATE. MIN TEMPS IN THE UPPER 60S-LOWER 70S. MID/UPPER LEVEL RIDGE WILL GRADUALLY DRIFT EAST AND WEAKEN WITH TIME. INCREASING SW FLOW ALOFT WILL INCREASE MOISTURE IN THE UPPER LEVELS OF THE ATMOSPHERE. WHILE MODEL GUIDANCE SUGGEST THAT THE BULK OF BEST UPPER LEVEL SUPPORT FOR CONVECTION WILL REMAIN WEST-NW OF OUR REGION...WEAK PERTURBATIONS IN THE FLOW ALOFT INTERACTING WITH AVAILABLE MOISTURE AND WEAK-MODERATE LOW LEVEL INSTABILITY WILL SUPPORT THE DEVELOPMENT OF ISOLATED TO SCATTERED CONVECTION SUNDAY AFTERNOON-EVENING...AND SCATTERED CONVECTION MONDAY AFTERNOON- EVENING. MAY SEE CONVECTION BECOME A LITTLE MORE CONCENTRATED/ORGANIZED MONDAY AND MORE SO TUESDAY AS MID LEVEL TROUGH APPROACHES AND CROSSES THE REGION. (GFS AS MUCH AS 12=18 HOURS FASTER COMPARED TO THE ECMWF. PER INITIALIZATION TONIGHT OFF TEH PACIFIC NW...SLOWER ECMWF PREFERRED AT THIS TIME.) RAINFALL ASSOCIATED WITH THE HEAVIER SHOWERS AND STORMS MAY BE LOCALLY HEAVY AS AIR MASS WILL BECOME TROPICAL-LIKE. PRECIPITABLE WATER VALUES PREDICTED TO RETURN TO NEAR 2 INCHES BY LATE SUNDAY OR MONDAY. OUR STRETCH OF ABOVE NORMAL TEMPS EXPECTED TO PERSIST ITO EARLY NEXT WEEK WITH MAX TEMPS IN THE UPPER 80S NW-LOWER 90S SE. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... AS OF 135 AM WEDNESDAY... 24 HOUR TAF PERIOD: CANADIAN HIGH PRESSURE OVERHEAD...BETWEEN A COLD FRONT APPROACHING FROM THE MIDWEST AND HURRICANE CRISTOBAL OFFSHORE...WILL RESULT IN MAINLY VFR CONDITIONS THROUGH THE TAF PERIOD. WINDS HAVE GENERALLY WENT CALM AT MOST SITES ACROSS CENTRAL NC THIS MORNING. WHILE WIDESPREAD FOG IS NOT EXPECTED... THINK WE WILL SEE AT LEAST SOME PATCHY SUB-VFR VISBYS THIS MORNING ACROSS THE EASTERN HALF OF THE AREA. CONFIDENCE IS INCREASING THAT KRWI MAY HAVE SOME MVFR VISBYS IN THE PRE-DAWN HOURS OF THIS MORNING. THUS... HAVE INCLUDED A TEMPO FOR KRWI FOR MVFR VISBYS IN THE 07-11Z TIME FRAME. IF SUB-VFR VISBYS DO INDEED DEVELOP AT KRWI... THEN WE COULD SEE THE FOG PRONE SITE VARY FROM MVFR TO IFR/LIFR UNTIL 11/12Z. CONFIDENCE AT KFAY AND KRDU IS STILL NOT HIGH ENOUGH TO INCLUDE ANY SUB-VFR PRE-DAWN VISBYS IN THEIR TAFS YET. AFTER SUNRISE THIS MORNING WE MAY AGAIN SEE A BRIEF PERIOD OF MVFR CIGS DEVELOP... THANKS TO THE CONTINUED MOIST NORTHEASTERLY LOW LEVEL FLOW... BEFORE LIFTING AND SCATTERING. ALTHOUGH... EXPECT IT TO BE VERY BRIEF AND LIMITED TO ONLY THE EASTERN SITES (KRDU/KFAY/KRWI)... AS LOW LEVEL FLOW WILL BE BACKING TO MORE OF A NORTHERLY DIRECTION AS HURRICANE CRISTOBAL LIFTS NORTHWARD AND PASS WELL OFFSHORE TODAY. OTHERWISE... A GENERALLY LIGHT NORTHERLY SFC WIND IS EXPECTED TODAY (5-10 KTS). HOWEVER... THERE WILL LIKELY BE A SHORT PERIOD OF SOMEWHAT STRONGER WINDS (8-12 KTS WITH GUSTS INTO THE MID-UPPER TEENS KTS... MAINLY AT KFAY/KRWI) AT EASTERN SITES MID-MORNING TO MIDDAY TODAY...WHERE THE MSLP PRESSURE GRADIENT WILL BE RESIDUALLY TIGHT EARLY IN THE DAY...BETWEEN THE OVERHEAD SFC HIGH AND THE OFFSHORE HURRICANE. OUTLOOK: THE ARRIVAL OF THE COLD FRONT APPROACHING FROM THE UPPER MIDWEST WILL RESULT IN A SMALL CHANCE OF AN AFTERNOON SHOWER OR STORM THU AND FRI...AS WELL AS A CHANCE OF MVFR CEILINGS IN BRIEF NE FLOW THU NIGHT FROM KRWI AND ESPECIALLY POINTS NORTHEASTWARD... INTO NE NC AND TIDEWATER VA. THE PROBABILITY OF PATCHY FOG WILL ALSO INCREASE BY THE WEEKEND...AS THE LOW LEVEL FLOW ASSUMES A MORE SOUTHERLY (MOIST) COMPONENT. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...BSD/MWS NEAR TERM...BSD SHORT TERM...WSS LONG TERM...WSS AVIATION...BSD/MWS

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