-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KRAH 180202
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
1000 PM EDT FRI MAY 17 2013

.SYNOPSIS...
A STATIONARY FRONT OVER NORTHERN NC TONIGHT AND SATURDAY WILL
RETREAT NORTHWARD THROUGH VA ON SUNDAY...BENEATH A TROUGH ALOFT THAT
WILL SLOWLY APPROACH AND CROSS CENTRAL NC THROUGH MONDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
AS OF 1000 PM FRIDAY...

THE COLD FRONT THAT APPROACHED NORTHEASTERN NC EARLIER TODAY HAS
ESSENTIALLY STALLED...AND CENTRAL NC REMAINS IN THE WEAKLY UNSTABLE
AIRMASS SOUTH OF THE FRONT.  CONVECTION HAS BEEN PRIMARILY FOCUSED
NEAR A SURFACE TROUGH OVER THE WESTERN PIEDMONT THIS EVENING...AND
HAS DRIFTED NORTH TOWARD THE NC/VA BORDER OVER THE PAST COUPLE OF
HOURS.  ELSEWHERE...CONVECTION HAS BEEN VERY ISOLATED...WITH THE
STRONGEST CELL OVER EASTERN WAYNE NOW DRIFTING EAST SINCE BEING
UNDERCUT BY THE SEABREEZE.  DESPITE BETTER DEEP LAYER MOISTURE THIS
EVENING (PW UP TO 1.25")...THE BOUNDARY LAYER REMAINS FAIRLY
DRY...SO IT WILL TAKE QUITE A BIT OF LIFT/MOISTENING TO TAP INTO THE
WEAK ELEVATED INSTABILITY THAT SHOULD RESIDE ABOVE THE NOCTURNAL
INVERSION.  THE REMNANT VORTEX FROM CONVECTION OVER THE SOUTHERN US
THE PAST COUPLE OF DAYS WILL DRIFT EAST TOWARD NC OVERNIGHT...BUT
SHOULD FAR ENOUGH TO OUR WEST NOT TO HAVE SIGNIFICANT INFLUENCE
UNTIL SATURDAY.  AHEAD OF THE WAVE...LOW LEVEL FLOW WILL INCREASE
SLIGHTLY ACROSS WESTERN NC TONIGHT AND MAY PROVIDE ENOUGH WARM
ADVECTION TO AID ADDITIONAL ISOLATED SHOWERS/STORMS.  THIS IS
SUPPORTED BY A FEW CONVECTION ALLOWING MODELS...PRIMARILY TOWARD
09-12Z.  WILL KEEP A SLIGHT CHANCE IN OVERNIGHT WITH VARIOUS
OUTFLOW/SEABREEZE BOUNDARIES LINGERING...AND BRING IN A LOW END
CHANCE OVER THE WESTERN PIEDMONT BY 12Z.  SKIES WILL REMAIN MOSTLY
CLOUDY...THOUGH IF PARTIAL CLEARING OCCURS...IT WOULD MOST LIKELY
OVER EASTERN AREAS.  LOWS SHOULD SETTLE INTO THE MID 60S IN MOST
LOCATIONS..THOUGH SOME LOWER 60S MAY BE POSSIBLE IN THE EAST IF
CLEARING OCCURS.
-- End Changed Discussion --

&&

.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 /00Z SATURDAY THROUGH WEDNESDAY/...
AS OF 815 PM FRIDAY...

VFR CONDITIONS ARE EXPECTED FOR THE MAJORITY OF THE 00Z TAF
PERIOD...WITH THE FOLLOWING THREE RELATIVELY SHORT-DURATION
EXCEPTIONS: 1) VICINITY SHOWERS AT TRIAD TERMINALS FOR THE NEXT HOUR
OR SO... 2) MAINLY MVFR VISIBILITY RESTRICTIONS SAT MORNING...
PARTICULARLY WHERE RAIN HAS FALLEN IN THE VICINITY OF TRIAD
TERMINALS THIS AFTERNOON-EVENING...AND 3) A 30 TO 50 PERCENT CHANCE
OF SHOWERS AND STORMS SAT AFTERNOON-EVENING...WITH THE RELATIVE
HIGHEST CHANCE/COVERAGE NORTH OF KFAY.  THOUGH THIS CONVECTION WILL
BE SCATTERED ABOUT SAT AFTERNOON...CONFIDENCE OF OCCURRENCE AT ANY
PARTICULAR TERMINAL IS TOO LOW TO INCLUDE A MULTI-HOUR PREVAILING
CONDITION OF SUCH AT THIS TIME RANGE.  -MWS

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.  -DJF

&&

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

&&

$$

SYNOPSIS...MWS
NEAR TERM...SMITH
SHORT TERM...DJF
LONG TERM...KMC
AVIATION...MWS/DJF