Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000
FXUS62 KRAH 241546
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
1146 AM EDT THU JUL 24 2014

.SYNOPSIS...
A COLD FRONT WILL MOVE SOUTHWARD INTO SOUTHERN VIRGINIA LATE THIS
AFTERNOON...PROGRESS INTO CENTRAL NORTH CAROLINA TONIGHT...THEN
STALL NEAR THE SOUTH CAROLINA BORDER ON FRIDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 1145 AM THURSDAY...

THOUGH MID-LEVEL LAPSE RATES REMAIN WEAK AT ~5.5 C/KM...A VERY
MOIST BOUNDARY LAYER ALONG WITH STRONG INSOLATION WILL YIELD
MODERATE DIURNAL DESTABILIZATION TODAY WITH MLCAPE VALUES
GENERALLY RANGING FROM 1000-2000 J/KG...HIGHEST IN THE EAST WHERE
LOW-LEVEL MOISTURE AND SUNSHINE ARE MOST ABUNDANT AND LOWEST WEST.
MODERATE INSTABILITY AND LITTLE OR NO CIN HAS ALREADY ALLOWED
CONVECTION TO DEVELOP IN VICINITY OF THE PRE-FRONTAL SFC TROUGH
OVER THE COASTAL PLAIN AS OF 15Z...PERHAPS AIDED BY THE RIGHT
ENTRANCE REGION OF AN H3 JET OVER NC/VA. MORNING CONVECTION HAS
TURNED THE PRE-FRONTAL TROUGH INTO AN EFFECTIVE FRONTAL ZONE OVER
CENTRAL NC...ORIENTED ROUGHLY WEST-EAST ALONG OR NEAR HWY 64...
WITH TEMPS IN THE MID/UPPER 70S IN THE NORTHERN PIEDMONT TO
MID/UPPER 80S IN THE SANDHILLS AND SE COASTAL PLAIN. THE PRESENCE
OF THE AFOREMENTIONED `EFFECTIVE FRONT` INTRODUCES ADDITIONAL
UNCERTAINTY TO THE CONVECTIVE FORECAST THROUGH THIS EVENING.

WITH THE ABOVE IN MIND...CONFIDENCE IS TOO LOW TO PROVIDE MORE
SPECIFIC DETAILS ABOUT CONVECTIVE COVERAGE AND EVOLUTION THIS
AFT/EVE. HOWEVER...FORECAST REASONING REMAINS THE SAME. GIVEN CYCLONIC
FLOW ALOFT (CYCLONIC SHEAR VORTICITY AND DPVA ASSOC/W SMALL
AMPLITUDE SHORTWAVES)...A SFC-925 MB TROUGH SINKING SOUTHWARD
INTO THE REGION NEAR PEAK HEATING...ABUNDANT MOISTURE...AND
MODERATE INSTABILITY...EXPECT ABOVE NORMAL CHANCES FOR CONVECTION
THROUGH MIDNIGHT...WITH CONVECTION POSSIBLY PERSISTING THROUGH
SUNRISE FRIDAY MORNING SOUTH AND EAST OF THE TRIANGLE.

SEVERE THREAT: DEEP LAYER SHEAR ON THE ORDER OF 25-35 KT WILL
SUPPORT MULTICELL CLUSTERS AND PERHAPS AN ISOLD SUPERCELL THIS
AFT/EVE. EXPECT A POTENTIAL FOR A FEW SEVERE STORMS WITH A PRIMARY
THREAT OF DAMAGING WINDS...PRIMARILY SOUTH OF HWY 64 WHERE DCAPE
VALUES BEGIN TO INCREASE ABOVE 500 J/KG...APPROACHING 1000 J/KG
NEAR THE SC BORDER. DIME TO QUARTER SIZE HAIL WILL ALSO BE
POSSIBLE WITH ANY ROBUST UPDRAFTS. THOUGH WEAK MID-LEVEL LAPSE
RATES WILL REDUCE THE POTENTIAL FOR HAIL > 1.00 IN...LARGE HAIL UP
TO THE SIZE OF GOLFBALLS WOULD BE POSSIBLE IN ANY ROBUST UPDRAFT
THAT ACQUIRES ROTATION...ESP SOUTH OF HWY 64 WHERE MLCAPE VALUES
WILL APPROACH 2000-2500 J/KG DURING PEAK HEATING. -VINCENT

&&

.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
AS OF 320 AM THURSDAY...

FOR FRI/FRI NIGHT: LITTLE CHANGED NEEDED TO EXISTING FORECAST. STILL
EXPECT THE FRONT TO HANG UP ACROSS THE SE CWA AND ALONG THE
CENTRAL/WRN NC/SC BORDER... AS THE 850 MB TROUGH HOLDS TO ITS NW
ACROSS FAR NRN NC. THIS WILL KEEP ENOUGH LOW LEVEL MOISTURE WHICH...
IN CONJUNCTION WITH LINGERING PW VALUES NEAR 1 INCH... NECESSITATES
A MENTION OF ISOLATED TO SCATTERED SHOWERS AND STORMS OVER THE FAR
SE CWA... FOCUSED ON THE TYPICAL AFTERNOON THROUGH EARLY EVENING
TIME FRAME. MUCH LOWER PW OVER THE REMAINDER OF THE CWA AND MODEST
RISE IN HEIGHTS AS THE MID LEVEL TROUGH WEAKENS AND STARTS TO PULL
AWAY WILL LEAD TO MAINLY DRY WEATHER ELSEWHERE... ALTHOUGH ONE LAST
SHOT OF DPVA THROUGH THE EXITING TROUGH BASE WILL DELAY THE MOST
SUBSTANTIAL NW-TO-SE CLEARING UNTIL LATE IN THE DAY. HIGHS 85-90...
WITH THICKNESSES SLIPPING TO 10-15 M BELOW NORMAL. LOWS 65-71. -GIH

SAT AND SAT NIGHT: THE REMNANT FRONT FROM EARLY IN THE PERIOD WILL
BECOME INCREASINGLY DIFFUSE AS IT RETREATS NORTH ACROSS THE MIDDLE
ATLANTIC STATES ON SAT...WITH A PRIMARY IMPACT FOR CENTRAL NC BEING
THE DEVELOPMENT OF A WARMING SOUTHERLY FLOW - HIGH TEMPERATURES
GENERALLY BETWEEN 89 AND 93 DEGREES. ALOFT...MODEL GUIDANCE IS IN
GOOD AGREEMENT REGARDING THE PROGRESSION OF A MID LEVEL TROUGH FROM
THE APPALACHIAN SPINE SAT MORNING TO THE COAST BY EVENING...BUT
ASSOCIATED FORECAST VERTICAL MOTION WITH THIS FEATURE IS WEAK...AND
NAM FORECAST BUFR SOUNDINGS ARE AN UNSTABLE AND UNCAPPED OUTLIER
RELATIVE TO THE GFS AND SREF ENSEMBLE MEAN. THE NAM QPF MAXIMUM FROM
SE VA THROUGH CENTRAL NC IS ALSO AN OUTLIER RELATIVE TO OTHER
GUIDANCE...WHICH OTHERWISE CONFINES ANY SHOWERS/STORMS TO THE SEA
BREEZE. WILL CONSEQUENTLY MAINTAIN JUST A SLIGHT CHANCE POP OVER THE
SOUTHERN AND CENTRAL COASTAL PLAIN...WHERE WEAK LIFT AHEAD OF THE
AFOREMENTIONED TROUGH ALOFT WILL BE DIURNALLY MAXIMIZED AND
COINCIDENT WITH LOW LEVEL FORCING INVOF THE SEA-BREEZE/SUBSEQUENT
OUTFLOW DURING THE MID-LATE AFTERNOON.

SHORTWAVE RIDGING WILL BRIEFLY FOLLOW FOR SAT NIGHT...WITH DRY
CONDITIONS AND LOWS MAINLY IN THE LOWER 70S.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
AS OF 335 AM THURSDAY...

THE MEDIUM RANGE WILL BE CHARACTERIZED BY THE DEVELOPMENT OF ANOTHER
UNSEASONABLY HIGH AMPLITUDE TROUGH ALOFT FROM SOUTHEASTERN CANADA
INTO THE EASTERN US...WITH AN ASSOCIATED NEGATIVE HEIGHT OF NEARLY 3
STANDARD DEVIATIONS BELOW AVERAGE FROM THE LOWER GREAT LAKES/OHIO
VALLEY TO THE CENTRAL APPALACHIANS. PRECIPITATION CHANCES DURING THE
PERIOD WILL BE HIGHEST SUN NIGHT-MON...AS THE TROUGH AMPLIFIES
SOUTHEASTWARD AND SPREADS THE ASSOCIATED MID LEVEL HEIGHT FALLS AND
UNDERLYING LO LEVEL FRONTAL ZONE ACROSS NC.

IN FACT...THE DAY SUNDAY SHOULD BE MAINLY DRY...AND HOT AS THE MID
LEVEL RIDGE OVER THE DESERT SW AND PLAINS BUILDS BRIEFLY EAST IN
ADVANCE OF THE AMPLIFYING TROUGH ALOFT. H85 TEMPERATURES ARE
FORECAST TO SURGE...ALBEIT BRIEFLY...INTO THE 22-24 DEGREE RANGE
OVER THE CAROLINAS...SUPPORTIVE OF HIGH TEMPERATURES SOLIDLY IN THE
MIDDLE 90S. UPSTREAM PRE-FRONTAL CONVECTION AND ASSOCIATED OUTFLOW
WILL THEN LIKELY DRIFT...IN A GRADUALLY WEAKENING STATE OWING TO
NOCTURNAL COOLING...INTO THE NORTHERN AND WESTERN PIEDMONT DURING
THE EVENING AND EARLY NIGHTTIME HOURS.

THE PATTERN ON THE SYNOPTIC SCALE...CHARACTERIZED BY THE APPROACHING
STRONG TROUGH AND DOWNSTREAM JET ENTRANCE DIVERGENCE ALOFT...AND LOW
LEVEL FRONTAL FORCING...FAVORS CONSIDERABLE THUNDERSTORM COVERAGE
MON...INCLUDING SOME SEVERE OWING TO UNSEASONABLY STRONG MID LEVEL
FLOW ON THE ORDER OF 40-50 KTS. HOWEVER...PREDICTABILITY ASSOCIATED
WITH MESOSCALE INFLUENCES (IE. OUTFLOW AND CLOUD COVER) FROM EARLIER
CONVECTION SUN NIGHT COMPLICATE THE FORECAST SO PROBABILITY OF
PRECIPITATION WILL BE KEPT IN THE HIGH CHANCE RANGE FOR NOW...WITH
THE EXPECTATION THAT THESE PROBABILITIES WILL BE INCREASED OVER AT
LEAST A PORTION OF THE FORECAST AREA (LIKELY THE SOUTHEAST HALF).
THE MIDDLE OF THE WEEK SHOULD BE DRY...AND COOLER...AS POST-FRONTAL
HIGH PRESSURE BUILDS INTO THE CENTRAL APPALACHIANS...AND
PRECIPITABLE WATER VALUES FALL BELOW AN INCH OVER CENTRAL NC. HIGHS
AND LOWS ONLY IN THE LOWER TO MIDDLE 80S AND LOWER TO MIDDLE
60S...RESPECTIVELY...TUE-WED.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
AS OF 1145 AM THURSDAY...

AREAS OF MVFR CEILINGS ASSOCIATED WITH A VERY MOIST LOW-LEVEL
AIRMASS WILL PERSIST INTO EARLY/MID AFTERNOON OVER CENTRAL NC...
ESPECIALLY IN OR NEAR LOCATIONS AFFECTED BY CONVECTION. SCATTERED
TO NUMEROUS SHOWERS AND THUNDERSTORMS WILL LIKELY EFFECT ALL
TERMINALS FOR A PERIOD OF AT LEAST SEVERAL HOURS THIS AFTERNOON
THROUGH TONIGHT AS A COLD FRONT MOVES SOUTHWARD INTO THE AREA.
WINDS WILL BECOME VARIABLE AS THE FRONT APPROACHES FROM THE NORTH
THIS AFT/EVE AND WILL REMAIN LIGHT AT 5-10 KT (OUTSIDE OF
CONVECTION).

LOOKING AHEAD: AT THIS TIME...VFR CONDITIONS ARE EXPECTED ON
FRIDAY AS THE FRONT STALLS OUT JUST S/SE OF CENTRAL NC...THOUGH
SOME OF THE LATEST MODEL GUIDANCE SUGGESTS THAT THE FRONT MAY
REMAIN CLOSE ENOUGH FOR SUB-VFR CEILINGS AND/OR A POTENTIAL FOR
SHOWERS AND THUNDERSTORMS TO PERSIST. A PERSISTENT PATTERN OF
TROUGHING OVER THE EASTERN CONUS WILL RESULT IN AN ABOVE NORMAL
CHANCE FOR CONVECTION THROUGH EARLY NEXT WEEK...WITH NUMEROUS
THUNDERSTORMS POSSIBLE DURING THE AFTERNOON AND EVENING HOURS EACH
DAY. -VINCENT

&&

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

&&

$$

SYNOPSIS...VINCENT
NEAR TERM...VINCENT
SHORT TERM...HARTFIELD/MWS
LONG TERM...MWS
AVIATION...VINCENT


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