Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000 FXUS62 KRAH 291900 AFDRAH AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE RALEIGH NC 300 PM EDT SAT AUG 29 2015 .SYNOPSIS...SURFACE HIGH PRESSURE REMAINS OVER MUCH OF THE REGION FOR THE REST OF THE WEEKEND. AN UPPER LEVEL DISTURBANCE MOVES FROM THE NORTHERN GULF OF MEXICO ACROSS THE WESTERN CAROLINAS SUNDAY THROUGH EARLY MONDAY. && .NEAR TERM /THROUGH TONIGHT/...
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AS OF 300 PM SATURDAY... WATER VAPOR IMAGERY SHOWS MID-LEVEL MOISTURE HAS INCREASED THROUGHOUT CENTRAL NORTH CAROLINA DURING THE DAY...AND AS EARLIER NOTED BY MODEL GUIDANCE...THE SPC MESOANALYSIS SHOWS A GRADUAL INCREASE IN PRECIPITABLE WATER OVER THE AREA FROM THE SOUTHWEST. BUFR SOUNDINGS FROM BOTH THE NAM AND THE GFS...THOUGH...SHOW A FAIRLY DRY AIR MASS CONTINUING BELOW ABOUT 700MB. WITH LOW K INDICES OVERNIGHT AND THE GFS AND NAM FORECAST OF A PERSISTENT 850MB THETA-E TROUGH...DESPITE THE MID- AND UPPER-LEVEL MOISTURE WILL HAVE ANY PRECIPITATION CHANCES BELOW SLIGHT. REGIONAL RADAR IMAGES SHOW AN AREA OF LIGHT RAIN OVER EASTERN SOUTH CAROLINA GRADUALLY MOVING NORTH THAT IS LIKELY TO DIMINISH AS IT APPROACHES THE DRIER MEAN AIR MASS TOWARD CENTRAL NORTH CAROLINA...BUT WHICH RECENT RUNS OF THE HRRR WRF WOULD SUGGEST HOLDING TOGETHER JUST ENOUGH TO PRODUCE A FEW SPRINKLES TOWARD KFAY AND KCTZ THIS EVENING. WILL NOTE THE SLIGHT CHANCE OF THAT OCCURRENCE IN THE FORECAST...HOWEVER...ANY PRECIPITATION MAY BE BARELY PERCEPTIBLE DUE TO THE DRY AIR FORECAST ON THE GFS BUFR SOUNDING FOR KFAY THIS EVENING DESPITE MOISTENING ALOFT. THE FORECAST OF OVERNIGHT LOWS ARE OF SOMEWHAT LOWER CONFIDENCE BASED ON VISIBLE IMAGES OF AREAS OF BREAKS IN THE CLOUDS...IN COMBINATION WITH RELATIVELY LOW SURFACE DEW POINTS AND A PARTLY TO MOSTLY CLOUDY SKY EXPECTED ON AVERAGE OVERNIGHT. WITH AREAS OF CLOUDS THE TEMPTATION IS TO FORECAST LOWS AT LEAST ON THE HIGH SIDE OF GUIDANCE...BUT GIVEN THE BREAKS ON THE VISIBLE IMAGERY AND THE CURRENT DEW POINTS WILL COMPROMISE WITH A BLEND OF THE HIGHER MAV AND LOWER MET MOS. OVERNIGHT LOWS AS A RESULT...LOWER TO MID 60S.
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&& .SHORT TERM /SUNDAY AND SUNDAY NIGHT/...
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AS OF 300 PM SATURDAY... THE GFS AND NAM ARE FAIRLY SIMILAR ON THE OVERALL EVOLUTION OF SYNOPTIC FEATURES...WITH A WEAKENING UPPER-LEVEL LOW MOVING NORTH FROM THE GULF INTO AND THROUGH THE WESTERN CAROLINAS...WHILE HIGH PRESSURE REMAINS IN PLACE AT THE SURFACE. OVERALL MOISTENING OF THE AIR MASS CONTINUES...AND BY 00Z MONDAY PRECIPITABLE WATER VALUES SHOULD RANGE FROM 1.5 INCHES IN THE FAR WESTERN PIEDMONT TO JUST OVER 1.75 INCHES TOWARD INTERSTATE 95. THE 850MB THETA-E TROUGH OVER CENTRAL NORTH CAROLINA OVERNIGHT SLOWLY RETREATS NORTH AND NORTHEAST...WITH K INDICES FINALLY ABOVE 30 THROUGHOUT CENTRAL NORTH CAROLINA BY SUNDAY NIGHT WHEN A 55 TO 60KT 300MB JET MAXIMUM MOVES OVER THE AREA COUPLED WITH THE LEFT EXIT REGION OF A 35KT 500MB JETLET. DESPITE THE AIR MASS MOISTENING AND THE APPROACH AND PASSAGE OF MID- LEVEL VORTICITY ALONG AND AHEAD OF THE WEAKENING UPPER LOW...THE PRESENCE OF SURFACE HIGH PRESSURE RESULTS IN LIMITED LIFT...AND THE BROAD...RELATIVELY WEAK LIFT WILL HAVE TO OVERCOME THE INITIAL LOW- LEVEL DRY AIR TO GENERATE ANY PRECIPIATATION. THROUGH 18Z SUNDAY BOTH THE NAM AND THE GFS HAVE ESSENTIALLY NO QPF IN CENTRAL NORTH CAROLINA. THERE COULD BE PATCHY SPRINKLES BEFORE THEN...BUT BUFR SOUNDINGS AND PLAN VIEW 700MB MOISTURE FIELDS SHOW THE 700MB LAYER MOISTENING GRADUALLY SUNDAY MORNING AND SUNDAY AFTERNOON. BY 00Z MONDAY THE NAM HAS MEASURABLE PRECIPITATION...WITH A SLIGHTLY FASTER APPROACH OF THE UPPER LOW...ALONG AND SOUTH OF A LINE FROM ABOUT KAFP TO KRWI...WHILE THE GFS IS DRY. IN GENERAL THINK THERE MAY BE JUST ENOUGH LIFT SUNDAY AFTERNOON IN THE MID- AND UPPER-LEVELS AS THOSE LEVELS MOISTEN WITH THE APPROACH OF THE WEAKENING UPPER LOW TO PROVIDE FOR A SMALL CHANCE OF A SHOWER MAINLY OVER THE SANDHILLS AND SOUTHERN COASTAL PLAIN WITH A CHANCE OF SPRINKLES FARTHER NORTH. FOR SUNDAY NIGHT...WEAK DIFFLUENCE AND LIFT ARE FORECAST OVER CENTRAL NORTH CAROLINA WITH MOST MOS GUIDANCE POPS IN THE CHANCE CATEGORY. FOR THIS FORECAST...WILL BLEND THE GFS AND NAM OUTPUT IN THE GRIDDED FORECAST WITH HIGHER CHANCE POPS ALONG AND SOUTH OF A LINE FROM ABOUT KVUJ TO KRWI...LOW CHANCES NORTH OF THERE. PROS FOR SOME MEASURABLE PRECIPITATION ARE THE MID- AND UPPER-LEVEL SUPPORT AND THE AVAILABLE MOISTURE. CONS ARE THE SURFACE HIGH PRESSURE AND CONCERNS OVER HOW FAST THE LOW-LEVELS ARE ABLE TO MOISTEN. SOME CONVECTIVE ALLOWING MODELS SHOW HARDLY ANY PRECIPITATION AT ALL... AND WHERE THERE IS QPF ON THE GFS AND NAM IT IS MOSTLY PRETTY LIGHT. WHERE RAIN IS ABLE TO OCCUR...IF IT IS ABLE...AMOUNTS SHOULD BE LIGHT. BUFR SOUNDINGS SEEM TO INDICATE AN AIR MASS CAPPED FOR THUNDER SO WILL JUST NOTE SHOWERS FOR NOW...ALTHOUGH THE GFS DOES FORECAST ABOUT 100-200J/KG OF MARGINAL CAPE OVERNIGHT SUNDAY. HIGHS SUNDAY 84 TO 88. OVERNIGHT LOWS SUNDAY NIGHT IN THE MID TO UPPER 60S.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/...
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AS OF 300 PM SATURDAY... A SHORTWAVE TROUGH THAT WILL HAVE ORIGINATED FROM THE NORTHERN GULF OF MEXICO IS PROGGED TO CONTINUE ITS TREK INTO THE MID ATLANTIC ON MONDAY. SCATTERED SHOWERS AND STORMS WILL BE POSSIBLE (BEST CHANCES SHIFTING EAST THROUGHOUT THE DAY) AS THIS FEATURE GRADUALLY GETS PICKED UP AND MOVED OUT OF THE AREA AS A STRONGER MID LEVEL TROUGH TRACKS OVER THE NORTHEAST US/SOUTHEAST CANADA. THEN FOR EARLY/MID NEXT WEEK...THE SUBTROPICAL RIDGE IS PROGGED TO EXPAND WESTWARD TOWARDS THE SOUTHEAST COAST. IN ADDITION ANOTHER UPPER LEVEL RIDGE WILL SHIFT OVER THE UPPER MIDWEST/GREAT LAKES REGION. THE COMBINATION OF THESE TWO FEATURES WILL PROVIDE LITTLE IN THE WAY OF FORCING FOR PRECIP. IN ADDITION...ERIKA HAS WEAKENED INTO A TROUGH OF LOW PRESSURE AND ONLY HAS A SMALL CHANCE OF REDEVELOPING INTO A TROPICAL CYCLONE AS IT REEMERGES AND MOVES OVER THE EASTERN GULF SUNDAY INTO MONDAY. THEREFORE...IT NOW APPEARS THAT ERIKA IS LOOKING TO BEST LESS AND LESS A FACTOR IN OUR WEATHER...ALTHOUGH RICH TROPICAL MOISTURE MAY STILL BE DRAWN UP TOWARDS OUR REGION (ESPECIALLY IN THE SOUTHERN AND EASTERN PORTIONS OF THE FORECAST AREA)...PROVIDING A CHANCE FOR SCATTERED TO WIDELY SCATTERED (MOSTLY DIURNAL) SHOWERS AND STORMS EACH DAY. THE PATTERN WILL BEGIN TO SHIFT SOME TOWARDS THE END OF THE WEEK AS THE RIDGE GETS SUPPRESSED WHILE A SHORTWAVE TROUGH TRACKS ACROSS THE NORTHEAST...PUTTING CENTRAL NC UNDER NORTHWESTERLY FLOW ALOFT. REGARDLESS...PRECIP CHANCES ARE EXPECTED TO BE NEAR TO SLIGHTLY ABOVE CLIMATOLOGY THROUGH THE REMAINDER OF THE EXTENDED AS IT IS HARD TO FAVOR ONE DAY OVER ANOTHER. TEMPS WILL ALSO BE NEAR CLIMO.
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&& .AVIATION /18Z SATURDAY THROUGH THURSDAY/... AS OF 200 PM SATURDAY... VFR CONDITIONS ARE EXPECTED THROUGH THE 18Z VALID TAF PERIOD. SURFACE WINDS SUSTAINED UNDER 10KT SHOULD GRADUALLY VEER. SOUTHERLY SURFACE WINDS SHOULD BE LIKELY BY MID-MORNING SUNDAY. THE GUIDANCE CONSENSUS SUGGESTS A MORE VARIABLE SURFACE WIND TOWARD KFAY EARLY SUNDAY MORNING...POSSIBLY EVEN A PERIOD OF A LIGHT NORTHWEST WIND... BEFORE MIXING THERE RESULTS IN A SOUTHERLY WIND BY LATE SUNDAY MORNING. AN UPPER-LEVEL LOW OVER THE NORTHERN GULF OF MEXICO IS FORECAST BY BOTH THE NAM AND GFS MODELS TO WEAKEN SOME WHILE LIFTING SLOWLY NORTH...MOVING OVER THE WESTERN CAROLINAS LATE SUNDAY THROUGH EARLY MONDAY. A DRY AIR MASS UNDER SURFACE HIGH PRESSURE AHEAD OF THE UPPER LOW SHOULD KEEP ANY PRECIPITATION THAT OCCURS LIGHT WITH CONDITIONS VFR. SHOULD MID- AND UPPER-LEVEL DRYING OCCUR LATE SUNDAY NIGHT AS INDICATED BY THE NAM AND GFS MODEL SOUNDINGS...AREAS OF SUB- VFR CONDITIONS IN LOW CLOUDS AND FOG WOULD OCCUR PRIOR TO DAWN MONDAY IN MOIST LOW-LEVEL AIR. FROM MONDAY AFTERNOON INTO THURSDAY...IN A MOIST AIR MASS AND MODEST INSTABILITY...AVIATION INTERESTS CAN CURRENTLY ANTICIPATE WIDELY SCATTERED TO SCATTERED SHOWERS AND THUNDERSTORMS WITH THEIR ASSOCIATED SUB-VFR CONDITIONS...ALONG WITH THE POTENTIAL OF LATE NIGHT AND EARLY MORNING AREAS OF LOW CLOUDS AND FOG. && .RAH WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SYNOPSIS...DJF NEAR TERM...DJF SHORT TERM...DJF LONG TERM...KRD AVIATION...DJF

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