000
FXUS62 KRAH 171855
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
255 PM EDT FRI MAY 17 2013

.SYNOPSIS...
A COLD FRONT OVER CENTRAL VIRGINIA WILL SAG SOUTH AND BE LOCATED
NEAR THE NORTH CAROLINA BORDER BY LATE TONIGHT. UPPER LEVEL LOW
PRESSURE WILL CONTINUE TO MOVE EAST AND HAVE A MAJOR INFLUENCE ON
THE WEATHER OF THE REGION OVER THE WEEKEND. ATLANTIC HIGH PRESSURE
GRADUALLY BUILDS INTO THE CAROLINAS EARLY NEXT WEEK.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 255 PM FRIDAY...

THE LATEST SURFACE ANALYSIS SHOWED A SURFACE FRONT ACROSS NORTHERN
INDIANA AND OHIO...THEN BENDING SOUTHEAST ACROSS SOUTHWEST
PENNSYLVANIA TOWARD FAR NORTHEAST NORTH CAROLINA. ALONG AND JUST
AHEAD OF THE FRONT OVER NORTHEAST NORTH CAROLINA...INCLUDING THE
COASTAL PLAIN...INSTABILITY WAS HIGHEST WITH MIXED-LAYER CAPE ON THE
ORDER OF 500J/KG ON THE LATEST SPC MESOANALYSIS. GIVEN THE LIGHT
WIND FIELD CONVERGENCE WAS MEAGER...AND WHILE SURFACE DEW POINTS HAD
YET TO FALL AS FAR AS THE RAP FORECAST...THEY STILL HAVE FALLEN
SOLIDLY INTO THE 50S OVER MOST PARTS OF CENTRAL NORTH CAROLINA. THIS
HAS MADE IT DIFFICULT FOR ANY DEEP CONVECTION TO DEVELOP.

GIVEN THE DIFFERENCES IN THE MOISTURE GUIDANCE AS WELL AS THE OUTPUT
OF THE CONVECTION-ALLOWING MODELS...AND CONSIDERING THE NAM APPEARS
SO OUT OF TOLERANCE WITH ITS FORECAST SURFACE DEW POINTS...HAVE
LEANED TOWARD A CONSENSUS OF THE RAP AND GFS OVERNIGHT. THIS WOULD
SUPPORT THE BETTER CHANCES OF ANY SHOWERS OR THUNDERSTORMS OVERNIGHT
ALONG AND AHEAD OF THE SURFACE FRONT...MAINLY ALONG AND NORTH OF A
LINE FROM ABOUT KTDF TO KRWI. THIS IS WHERE THE RAP SHOWS LOW-LEVEL
MOISTURE POOLING AROUND AND AFTER 00Z...SUPPORTED MOST BY THE LOCAL
RALIEGH WRF REFLECTIVITY FORECAST. IN THE RELATIVELY MORE STABLE AIR
OVER THE SOUTHWEST PIEDMONT IN PARTICULAR...WILL KEEP CONDITIONS DRY
UNTIL THE OVERNIGHT HOURS...AND THEN ONLY NOTE A SLIGHT CHANCE OF A
SHOWER THERE AND ELSEWHERE ACROSS CENTRAL NORTH CAROLINA. IT SHOULD
BE NOTED THAT SOME OF THE GUIDANCE POPS...PARTICULARLY FROM THE
NAM...ARE QUITE HIGH OVERNIGHT...BUT THE NAM ALSO FORECASTS SURFACE
DEW POINTS GETTING NEAR 70 OVER PARTS OF THE NORTHWEST PIEDMONT BY
21Z...WHICH CURRENTLY SEEMS UNLIKELY. UNDER A CONTINUING MID-LEVEL
RIDGE AHEAD OF THE TROUGH MOVING EAST...WILL ANTICIPATE A DIURNAL
REDUCTION IN CHANCES FOR PRECIPITATION FOR ONE MORE NIGHT. OVERNIGHT
LOWS IN THE LOWER TO MID 60S.

&&

.SHORT TERM /SATURDAY AND SATURDAY NIGHT/...
AS OF 255 PM FRIDAY...

THE TROUGH ALOFT GRADUALLY MOVES INTO CENTRAL NORTH CAROLINA
SATURDAY AND SATURDAY NIGHT. WHILE DIFFERENCES CONTINUE WITH THE
DETAILS IN THE GUIDANCE...INCLUDING VARIOUS WRF OUTPUT...ON A LARGE
SCALE...MEAN RH INCREASES TO A MAXIMUM DURING THE PERIOD LATE
SATURDAY AFTERNOON IN THE NORTHWEST PIEDMONT...AND TO ABOUT U.S. 1
OR SO SATURDAY NIGHT. THE BEST LIFT IS NORTHWEST CLOSER TO THE UPPER
TROUGH...850MB THETA-E IS GREATEST IN CONJUNCTION WITH THE MEAN
RELATIVE HUMIDITY...AND BY LATE SATURDAY AFTERNOON AND SATURDAY
NIGHT...THE 850MB FLOW FROM THE SOUTHWEST INCREASES TO AROUND 25KT
FOCUSED OVER AND NEAR THE WESTERN PIEDMONT. BUFR SOUNDINGS
ESPECIALLY FROM THE GFS SHOW GREATER MOISTURE AND INSTABILITY WEST
COMPARED TO EAST IN CENTRAL NORTH CAROLINA. AS A RESULT...INSTEAD OF
TRYING TO INTERPRET THE VARIETY OF WRF TRENDS...HAVE FOLLOWED A
PRECIPITATION FORECAST LEANING MORE TOWARD LARGE-SCALE FEATURES.
THIS RESULTS IN MOSTLY CHANCE POPS FOR SATURDAY...BECOMING LIKELY IN
THE IMMEDIATE TRIAD BY LATE IN THE DAY AND SPREADING LIKELY POPS TO
AROUND U.S. 1 BY OVERNIGHT SATURDAY CLOSER TO 850MB LOW PRESSURE.
THE LEAST CHANCES WILL BE TOWARD KGSB AND KCTZ...WHERE OVERALL
MOISTURE AND LIFT ARE LEAST...CLOSER ALSO TO THE 850MB RIDGE JUST
OFFSHORE. THE GFS HAS FAIRLY ROBUST PRECIPITATION AMOUNTS IN THE
TRIAD OVERNIGHT SATURDAY AND THIS WILL HAVE TO BE WATCHED...BUT FOR
NOW WILL FORECAST AVERAGE PRECIPITATION AMOUNTS THERE AROUND 0.75
INCH FROM LATE SATURDAY AFTERNOON THROUGH SATURDAY NIGHT. HIGHS
SATURDAY NEAR 80 NORTHWEST TO THE UPPER 80S SOUTHEAST...WITH
OVERNIGHT LOWS SATURDAY NIGHT IN THE MID 60S.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
AS OF 255 PM FRIDAY...

SUNDAY THROUGH MONDAY NIGHT: CONFIDENCE IN AN UNSETTLED WEATHER
PATTERN IS HIGH THROUGH MONDAY NIGHT...HOWEVER THE TIMING...COVERAGE
AND INTENSITY OF PRECIPITATION IS STILL AN ISSUE. AN UPPER LEVEL
SHORTWAVE DISTURBANCE ALONG THE RIDGE AXIS MOVING OVER CENTRAL NC ON
SUNDAY AND MONDAY WILL AID IN THE DEVELOPMENT OF SHOWERS AND STORMS
THROUGH THE PERIOD. AT H850 A TROUGH WILL EXTEND EAST FROM THE
PARENT LOW OVER THE MIDWEST AND INTO TN AND THE CAROLINAS THROUGH
MONDAY...WITH RIDGING TO THE NORTH FROM AN OFFSHORE HIGH. ALTHOUGH
THERE ARE STILL SOME DIFFERENCES BETWEEN THE MODELS AND FROM RUN TO
RUN...THE GFS AND ECMWF HAVE COME INTO BETTER AGREEMENT WITH RESPECT
TO THE STRENGTH AND CHARACTER OF THE SHORTWAVE LOW...THUS WILL
CONTINUE A BLEND OF THE TWO. AT THE SURFACE...THE GFS CONTINUES TO
SHOW A LOW DEVELOPING IN THE LEE OF THE ROCKIES THIS WEEKEND AND
DEEPENING AS IT MIGRATES SLOWLY EAST INTO EASTERN SD...EXTENDING SSE
THROUGH CENTRAL OK/TX...AND A TROUGH AXIS EXTENDING EAST INTO THE
MID ATLANTIC STATES. WARM MOIST SW FLOW OFF THE GULF WILL KEEP PWAT
VALUES IN THE 1.5 TO 2.0 INCH RANGE THROUGH MONDAY NIGHT FUELING THE
POTENTIAL FOR SOME SIGNIFICANT STORM TOTAL RAINFALL AMOUNTS GIVEN
THE WEAK STEERING FLOW EXPECTED. MODEL GENERATED MLCAPE IS OF THE
TALL AND SKINNY VARIETY...WITH VALUES OF 1000-1500 J/KG SUNDAY AND
MONDAY. COMBINED WITH 0-6 KM BULK SHEAR VALUES OF MAINLY 10-20
KTS...EXPECT CONVECTIVE MODE TO BE MAINLY DISORGANIZED PULSE TYPE
STORMS...ALTHOUGH AN ISOLATED STRONG STORM CANNOT BE RULED OUT. THE
BIGGER THREAT CONTINUES TO BE FROM POTENTIALLY HEAVY RAINFALL...
ESPECIALLY SUNDAY NIGHT AND MONDAY. GIVEN THE UNCERTAINTY REGARDING
PRECIP AMOUNTS...HAVE DECREASED STORM TOTAL RAINFALL TOTALS FROM THE
PREVIOUS FORECAST...HOWEVER WIDESPREAD 1-2 INCHES IS STILL LIKELY
THROUGH MONDAY NIGHT. HIGH TEMPERATURES THROUGH THIS PERIOD CONTINUE
TO BE DEPENDENT ON CLOUD COVER AND PRECIPITATION. FOR NOW...EXPECT
HIGHS IN THE UPPER 70S NORTH AND LOW TO MID 80S ACROSS THE EAST AND
SOUTH. OVERNIGHT LOWS WILL STAY MAINLY IN THE MID 60S.

TUESDAY THROUGH FRIDAY: AS THE SHORTWAVE MOVES NORTHEAST AWAY FROM
THE AREA ON TUESDAY...HIGH PRESSURE WILL BUILD INTO THE CAROLINAS
FROM THE ESE...WITH RETURN FLOW OFF THE ATLANTIC AND A SLIGHT
DECREASE IN FORECAST PWAT VALUES. DRY AIR ADVECTING IN AT THE MID
LEVELS SHOULD DECREASE RAINFALL CHANCES THROUGH WEDNESDAY ALTHOUGH
THE POSSIBILITY OF AN AFTERNOON SHOWER EXISTS. A MIDWEST LOW WILL
MOVE EAST TOWARD THE MID ATLANTIC DURING THIS TIME AND THE TROUGH
BECOMES ORIENTED MORE SW TO NE...WITH INCREASING PRECIPITATION
CHANCES ON THURSDAY AND FRIDAY. GIVEN THE DIFFERENCES BETWEEN THE
MODELS...THERE IS A GOOD DEAL OF UNCERTAINTY WITH RESPECT TO
PRECIPITATION MOVING AHEAD OF THE SYSTEM. FOR NOW WILL CARRY SLIGHT
CHANCES FOR SHOWERS AND STORMS DURING THE AFTERNOON HOURS THURSDAY
AND FRIDAY...WITH A DRY FORECAST FOR THE OVERNIGHT HOURS. HIGH TEMPS
TUESDAY AND WEDNESDAY IN THE MID TO UPPER 80S...DECREASING AGAIN
THROUGH FRIDAY...UPPER 70S TO LOW 80S. OVERNIGHT LOWS REMAIN STEADY
IN THE MID TO UPPER 60S.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
AS OF 255 PM FRIDAY...

MAINLY VFR CONDITIONS ARE EXPECTED TO CONTINUE THROUGH THE 18Z VALID
TAF PERIOD OUTSIDE OF ANY SHOWERS OR THUNDERSTORMS. THE BEST CHANCE
OF A SHOWER OR THUNDERSTORM OVERNIGHT APPEARS TO BE TOWARD THE TRIAD
AND KRWI...POSSIBLY AS FAR SOUTH AS KRDU...WITH LESSER POTENTIAL
TOWARD KFAY. A PERIOD OF MVFR FOG IS POSSIBLE AT ALL TAF SITES LATE
TONIGHT INTO EARLY SATURDAY IN AN AIR MASS THAT GRADUALLY MOISTENS
AGAIN OVERNIGHT. SOME OF THE GUIDANCE SUGGESTS A SMALL POTENTIAL FOR
LOW CLOUDS AT KINT...KGSO...AND KRWI...BUT THE SREF MODEL
PROBABILITIES OF CLOUDS BELOW 1000FT ARE HIGHEST JUST NORTHEAST OF
KRWI TOWARD KIXA...KASJ...AND KEDE AND WERE ONLY NOTED WITH FEW IN
THE TAFS LATE TONIGHT INTO EARLY SATURDAY IN CENTRAL NORTH CAROLINA.

BEYOND THE 18Z VALID TAF PERIOD...CHANCES FOR SHOWERS AND
THUNDERSTORMS GRADUALLY INCREASE...BECOMING LIKELY IN THE TRIAD BY
LATE SATURDAY AFTERNOON AND EXTENDING AT LEAST TO KRDU BY SATURDAY
NIGHT AS AN UPPER-LEVEL TROUGH MOVES EAST. ALONG WITH THE SUB-VFR
CONDITIONS THAT THE SHOWERS AND STORMS WOULD BRING...THERE IS A GOOD
CHANCE OF IFR CONDITIONS IN LOW CLOUDS AT LEAST TOWARD KINT AND KGSO
LATE SATURDAY NIGHT INTO SUNDAY MORNING...WITH SOME CHANCE OF THOSE
CLOUDS EXTENDING ACROSS THE REST OF CENTRAL NORTH CAROLINA. PERIODS
OF MVFR AND IFR CONDITIONS IN LOW CLOUDS...OR SHOWERS OR
THUNDERSTORMS...CONTINUE INTO EARLY NEXT WEEK...WITH THE BEST
POTENTIAL FOR SUSTAINED VFR CONDITIONS TUESDAY AFTERNOON INTO THE
OVERNIGHT HOURS OF TUESDAY...AND AGAIN AFTER MIXING DURING THE DAY
WEDNESDAY.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

$$

SYNOPSIS...DJF
NEAR TERM...DJF
SHORT TERM...DJF
LONG TERM...KMC
AVIATION...DJF