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/...
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.

&&

.SHORT TERM /SUNDAY AND SUNDAY NIGHT/...
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.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
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.

&&

.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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