Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000
FXUS62 KRAH 271038
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
638 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 615 AM WEDNESDAY...

THIS MORNING: AN AREA OF LOW STRATUS AND FOG IS EXPANDING ACROSS THE
COASTAL PLAIN AND FAR EASTERN PIEDMONT THIS MORNING... AND SHOWS UP
QUITE WELL ON THE 11U-3.9U SATELLITE CHANNEL. MODELS ARE NOT
HANDLING THIS WELL... BUT CURRENT THINKING IS THIS IT WILL CONTINUE
TO OOZE SOUTH AND WESTWARD OVER THE COURSE OF THE NEXT COUPLE OF
HOURS... THEN SLOWLY LIFT AND SCATTER THROUGH MID MORNING. AS THE
STRATUS DECK MOVES INTO A GIVEN LOCATION IT IS LOWERING AND AT SOME
SITES IS MAKING IT ALL THE WAY TO THE GROUND... CAUSING SOME LOCALLY
DENSE FOG. WE HAVE ISSUED AN SPS TO COVER THIS THREAT AND WILL
MONITOR TO SEE IF A DFA IS NEEDED.

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 /11Z WEDNESDAY THROUGH SUNDAY/...
AS OF 635 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. HOWEVER... AN AREA OF LOW STRATUS AND FOG HAS
DEVELOPED/MOVED INTO EASTERN PORTIONS OF THE AREA THIS MORNING...
CREATING LIFR CONDITIONS AT KRWI. THIS AREA OF LOW CLOUDS AND FOG IS
EXPANDING WEST AND SOUTHWARD THIS MORNING. HOWEVER... CONFIDENCE
THAT IT WILL REACH KFAY AND/OR KRDU BEFORE IT BEGINS TO LIFT IS LOW
TO MODERATE. THINK WE WILL STILL SEE A BRIEF PERIOD OF STRATOCUMULUS
THIS MORNING FROM THE MOISTURE IN THE LOW LEVEL NE TO NNE LOW LEVEL
FLOW COMBINED WITH DAYTIME HEATING AT KRDU AND KFAY THOUGH. EXPECT
THIS CLOUD COVER WILL LIFT AND SCATTER BY MID TO LATE MORNING...
WITH VFR EXPECTED BY AFTERNOON EVERYWHERE AT THE LATEST.

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.

THE LOW LEVEL FLOW IS EXPECTED TO BECOME MORE NORTHWEST TO WESTERLY
OVERNIGHT. THUS... DO NOT EXPECT A REPEAT PERFORMANCE OF IFR/LIFR
MORNING FOG AND/OR LOW STRATUS ON THURSDAY MORNING. HOWEVER... CAN
NOT COMPLETELY RULE OUT SOME MVFR VISBYS AT KRWI IN THE PRE-DAWN
HOURS OF THURSDAY... THOUGH PROBABILITIES ARE WAY TOO LOW TO INCLUDE
IN THE TAF AT THIS TIME.

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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