Severe Thunderstorm Warning 353, NWS Raleigh, NC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
322 PM EDT WED JUL 23 2008

.SYNOPSIS...A WARM MOIST AIR MASS AND AN AREA OF LOW PRESSURE
APPROACHING FROM THE WEST WILL PRODUCE SCATTERED THUNDERSTORMS
ACROSS THE REGION TONIGHT. A SURFACE TROUGH WILL CROSS THE REGION
EARLY THURSDAY. NORTHWEST WINDS BEHIND THE BOUNDARY WILL USHER DRIER
MORE STABLE AIR INTO THE REGION THURSDAY AND THURSDAY NIGHT.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
AS OF 130 PM WEDNESDAY...

SFC ANALYSIS THIS AFTERNOON SHOWED A WEAK LOW NEAR THE CENTRAL NC/SC
BORDER...WITH A TROUGH AXIS EXTENDING NORTH THROUGH THE WESTERN NC
PIEDMONT. A COLD FRONT WAS LOCATED WEST OF THE MOUNTAINS...
APPROACHING THE TN VALLEY ON THE SOUTHERN END AND CROSSING THROUGH
OH INTO PA FURTHER NORTH.

ALOFT...12Z RAOB ANALYSIS SHOWED WHAT APPEARS TO BE A NEGATIVELY
TILTED SHORTWAVE JUST TO OUR WEST BETWEEN BNA/FFC AND RNK/GSO AT
250MB...IN ASSOC/W A LONGWAVE TROUGH STRETCHING SOUTH FROM AN UPPER
LOW JUST NORTH OF THE GREAT LAKES. AT 500MB...30 METER 12-HR HEIGHT
FALLS WERE NOTED ACROSS THE MID-ATLANTIC AND EASTERN OHIO VALLEY (AT
GSO/RNK/PIT/ILN). THE 925-700MB TROUGH AXIS WAS LOCATED JUST TO THE
WEST OVER THE SOUTHERN APPALACHIANS.

TODAY AND TONIGHT:

PRECIP CHANCES:
EXPECTING AN ACTIVE DAY TODAY WITH VERY GOOD DEEP-LAYER LIFT (FOR
LATE JULY) COMBINED WITH THE MOISTURE/INSTABILITY OF A LATE JULY
ATMOSPHERE. A SSW-NNE ORIENTED LINE OF CONVECTION DEVELOPED THIS
MORNING ALONG THE I-85 CORRIDOR BETWEEN CHARLOTTE AND LEXINGTON...
AND IS CURRENTLY PROPAGATING EASTWARD INTO SOUTHWEST PIEDMONT AND
SANDHILLS. THIS LINE OF CONVECTION SEEMS TO HAVE BEEN FORCED BY THE
SHORTWAVE BETWEEN BNA/FFC AND GSO/RNK ALONG A ZONE OF LOW LEVEL
CONFLUENCE IN THE WESTERN PIEDMONT. ANTICIPATE THAT THIS LINE WILL
CONTINUE TO INTENSIFY THROUGH THE AFTERNOON AS IT SLOWLY PROPAGATES
INTO AN AIRMASS THAT WILL BECOME MORE AND MORE UNSTABLE WITH DIURNAL
HEATING. ALSO...EXPECT THAT THERE WILL BE SEVERAL WAVES OF ACTIVITY
THROUGHOUT THE DAY INTO TONIGHT AS ANOTHER STOUT SHORTWAVE ROUNDS
THE BASE OF THE LONGWAVE TROUGH TOWARD NC/VA BETWEEN 00-06Z
TONIGHT...WITH INCREASING DEEP LAYER LIFT THROUGHOUT THE DAY AS THE
925-700MB TROUGH AXIS MOVES INTO CENTRAL NC AND A WEAK ELONGATED SFC
LOW OVER THE AREA DEEPENS A FEW MILLIBARS IN RESPONSE TO SYNOPTIC
FORCING AND DIURNAL HEATING. AS SUCH...HAVE DECIDED TO GO WITH
CATEGORICAL (80%) POPS THROUGHOUT TODAY THROUGH MIDNIGHT
TONIGHT...WITH POPS DECREASING FROM THE WEST BETWEEN MIDNIGHT AND
SUNRISE THU MORNING AS THE LONGWAVE TROUGH AXIS BEGINS TO SWING OVER
US TOWARD THE EAST.

SEVERE THREAT:
LOOKING AT THE 12Z GSO SOUNDING...IT IS MORE MOIST THAN AT THIS TIME
YESTERDAY...WITH A PWAT OF 1.54 (VERSUS 1.31 YESTERDAY AT THIS
TIME). ALTHOUGH THE LOW/MID LEVEL LAPSE RATES WERE NOT AS STEEP AT
12Z THIS MORNING AS THEY WERE YESTERDAY AT THIS TIME...WITH STRONG
DIURNAL HEATING...THEY WILL STEEPEN THIS AFTERNOON. ALTHOUGH THERE
IS NOT AS MUCH DRY AIR IN THE LOW LEVELS TO HELP ENHANCE DOWNDRAFTS
VIA EVAPORATIVE COOLING...THE WATER LOADING POTENTIAL IS A BIT
HIGHER...AND ANY STRONG CONVECTION THIS AFTERNOON WILL CERTAINLY
HAVE THE POTENTIAL TO PRODUCE DAMAGING WINDS. LARGE HAIL WILL ALSO
BE POSSIBLE. ALTHOUGH THE DEEP LAYER SHEAR IS GENERALLY WEAK AND
THERE SHOULD NOT BE MUCH CONVECTIVE ORGANIZATION...DEPENDING ON
WHERE THE SFC LOW SETS UP AND SLIGHTLY DEEPENS THIS AFTERNOON...
THERE COULD BE A SMALL THREAT OF AN ISOLATED TORNADO IN EAST/NE
PORTIONS OF THE FCST AREA (COASTAL PLAIN). THIS IS BASED ON FCST
SOUNDINGS THAT SHOW THE POTENTIAL FOR SOME DECENT VEERING IN THE
LOWER LEVELS AT FAY/RWI. HOWEVER...THE SPEED SHEAR IS VERY WEAK...
AT ALL LAYERS/LEVELS...AND IF IT WERE TO HAPPEN...IT WOULD NOT BE
SUPERCELLULAR AND WOULD BE DUE TO A FREAK INTERACTION BETWEEN A
STRONG UPDRAFT AND SOME WEAK LOW-LEVEL ENVIRONMENTAL HORIZONTAL
VORTICITY. THE THREAT FOR SEVERE WEATHER WILL BE HIGHEST BETWEEN 1PM
THIS AFTERNOON AND 10PM THIS EVENING. LOCALLY TORRENTIAL RAINFALL
CAN BE EXPECTED IN ANY THUNDERSTORMS AS WELL...WITH RAINFALL RATES
AS HIGH AS 2 AN HOUR.

TEMPERATURES:
HIGHS TODAY WILL BE DIFFICULT DUE TO EXPECTED SCATTERED TO
WIDESPREAD CONVECTION. GIVEN CLOUD COVER AND PRECIP...WILL ADVERTISE
HIGH TEMPS IN THE UPPER 80S TO LOWER 90S...AROUND NORMAL FOR THIS
TIME OF YEAR. LOW TEMPS TONIGHT WILL DEPEND ON THE TIMING OF THE
DRIER AIR COMING IN FROM THE WEST AS THE LONGWAVE TROUGH AXIS PASSES
OVER US AND TOWARD THE EAST. LOWEST TEMPS SHOULD BE ALONG/WEST OF
HIGHWAY 1...WHERE MID/UPPER 60S SEEM REASONABLE...WITH UPPER 60S TO
LOWER 70S FURTHER EAST WHERE THERE IS MORE LIKELY TO BE CLOUD COVER
AND PRECIP ONGOING IN SOME FORM THROUGH SUNRISE THU MORNING. -VINCENT

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH SATURDAY/...
AS OF 315 PM WEDNESDAY...

MORE TRANQUIL WEATHER REGIME ANTICIPATED FOR THURSDAY INTO FRIDAY.
MID LEVEL TROUGH CROSSES THE REGION EARLY THU WITH NW FLOW IN ITS
WAKE ADVECTING DRIER MORE STABLE AIR INTO THE CENTRAL NC THU. THIS
STABLE AIR MASS WILL LINGER ACROSS REGION THU NIGHT INTO FRI.
THICKNESSES SUPPORT MAX TEMPS CLOSE TO NORMAL VALUES.

FRIDAY NIGHT INTO SATURDAY...AIR MASS WILL BEGIN TO RECOVER AS LOW
LEVEL FLOW BEGINS TO ADVECT MOISTURE FROM THE GULF/ATLANTIC AND
UPPER LEVEL WINDS ADVECT MOISTURE FROM CENTRAL U.S. CONVECTION.
BY SATURDAY AFTERNOON....PRECIP WATER VALUES 1.5 TO 1.8 INCHES
EXPECTED (IF GFS VERIFIES CLOSER TO 2 INCHES). MODEL GUIDANCE
DEPICT WEAK LOW/MID LEVEL TROUGH APPROACHING THE NORTH AND WEST
SECTIONS BY 00Z SUNDAY WITH MID LEVEL LAPSE RATES CLOSE TO 6 DEG
C/KM BY 00Z SUN. WITH AFTERNOON HEATING AND APPROACH OF
TROUGH...BEST CHANCE OF SCATTERED CONVECTION WILL BE LATE IN THE
DAY OVER THE NW THIRD OF CWA. THICKNESSES SUPPORT MAX TEMPS A
SHADE ABOVE NORMAL VALUES.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
AS OF 315 PM WEDNESDAY...

RESPECTABLE L/W TROUGH (FOR THIS TIME OF YEAR) WILL MAINTAIN A
POSITION OVER THE EASTERN SEABOARD THROUGH THE CAROLINAS THROUGH MID
WEEK. MEANWHILE A STALLED...WAVY FRONTAL SYSTEM WILL STRETCH ACROSS
THE AREA. THE COMBINATION OF MOISTURE POOLING ALONG THE SURFACE
BOUNDARY AND PERTURBATION ROUNDING BASE OF UPPER TROUGH WILL PROVIDE
AN ENVIRONMENT FOR THE DEVELOPMENT OF SCATTERED CONVECTION.
CONVECTION SHOULD BE MORE PROMINENT DURING THE AFTERNOON/EVENING
HOURS WHEN AIR MASS MOST UNSTABLE. WILL NEED TO MONITOR UPSTREAM FOR
POSSIBLE MCS DEVELOPMENT. ANY REMNANTS OR MCS-INDUCED VORTICES WOULD
ENHANCE CONVECTION COVERAGE. MAX TEMPS DEPENDENT UPON DEGREE OF
CLOUDINESS. FOR NOW...WILL STICK CLOSE TO NEAR NORMAL TEMPS.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
AS OF 130 PM WEDNESDAY...

A LEE SIDE TROUGH IN THE FOOTHILLS AND WESTERN PIEDMONT SHOULD SERVE
AS THE PRIMARY FOCUS FOR SHOWER AND THUNDERSTORM DEVELOPMENT THIS
AFTERNOON. IN ADDITION...A COLD FRONT OVER THE OHIO AND TENNESSEE
VALLEYS THIS AFTERNOON IS FORECAST TO CROSS THE APPALACHIANS LATER
THIS AFTERNOON AND OVERTAKE THE LEE TROUGH THIS EVENING. THE LOW
LEVEL FRONTAL CIRCULATION...IN CONJUNCTION WITH AN APPROACHING UPPER
TROUGH AND ATTENDANT UPPER LEVEL JET...WILL PROVIDE FOR ADEQUATE
CONVERGENCE AND LIFT TO SUSTAIN SHOWERS AND A FEW THUNDERSTORMS WELL
INTO TONIGHT...ESPECIALLY FROM KRDU AND KFAY EASTWARD TO KRWI. HAVE
ACCORDINGLY INCLUDED A CB CLOUD GROUP AND VICINITY SHOWERS AT ALL
TERMINALS FOR THIS AFTERNOON INTO TONIGHT. MVFR CEILINGS MAY DEVELOP
WITHIN THE FRONTAL ZONE FROM KRDU EASTWARD AS THE FRONT PROGRESSES
EASTWARD ACROSS THE FORECAST AREA TONIGHT. THEN CLEARING IN THE WAKE
OF THE FRONT COMBINED WITH LINGERING LOW LEVEL MOISTURE MAY PROMOTE
MVFR/IFR FOG BY DAYBREAK THURSDAY...MAINLY FROM KRDU EASTWARD. SKIES
SHOULD CLEAR FROM WEST TO EAST AROUND OR SHORTLY AFTER SUNRISE
THURSDAY MORNING AS DRIER AIR FILTERS IN BEHIND THE SFC FRONT AND
UPPER LEVEL TROUGH.

LOOKING BEYOND THURSDAY AFTERNOON...RESIDUAL LOW LEVEL MOISTURE IN
THE COASTAL PLAIN MAY PROMOTE MVFR FOG DEVELOPMENT AT KRWI AND KFAY
EACH MORNING FRIDAY THROUGH THE WEEKEND. -VINCENT/MWS

&&

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

&&

SYNOPSIS...WSS
NEAR TERM...VINCENT
SHORT TERM...WSS
LONG TERM...WSS
AVIATION...VINCENT

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE RALEIGH NC
301 PM EDT WED JUL 23 2008

NCZ011-232000-
HALIFAX-
301 PM EDT WED JUL 23 2008

...STRONG SHOWERS AND THUNDERSTORMS WILL MOVE ACROSS HALIFAX COUNTY
THROUGH 400 PM EDT...

AT 301 PM EDT...DOPPLER RADAR INDICATED SHOWERS AND THUNDERSTORMS
WITH VERY HEAVY RAIN 5 MILES WEST OF HALIFAX...MOVING NORTHEAST AT 23
MPH.

THE STRONG THUNDERSTORMS WILL BE NEAR SOUTH WELDON AROUND 315 PM
EDT...WELDON AROUND 320 PM EDT.

GUSTY WINDS OF 30 TO 40 MPH...SMALL HAIL AND DANGEROUS
CLOUD-TO-GROUND LIGHTNING CAN BE EXPECTED WITH THESE THUNDERSTORMS.

BLAES

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