Severe Thunderstorm Warning 255, NWS Raleigh, NC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
400 PM EDT FRI JUL 31 2009

.SYNOPSIS...
A SURFACE FRONT WILL APPROACH FROM THE NORTHWEST THIS EVENING...THEN
PUSH INTO CENTRAL NC TONIGHT. THE FRONT WILL BECOME NEARLY
STATIONARY OVER OUR REGION ON SATURDAY. ANOTHER SFC FRONT WILL
MOVE INTO THE AREA FROM THE WEST ON SUNDAY.

&&

.NEAR TERM /THROUGH TONIGHT/...
AS OF 1030 AM FRIDAY...

SURFACE ANALYSIS THIS MORNING SHOWS THE COLD FRONT STILL WELL TO OUR
NORTHWEST...WITH A FAIRLY UNIFORM SURFACE FLOW AND NO REAL TROUGH
EAST OF THE MOUNTAINS.  SHOWERS AND ISOLATED THUNDERSTORMS CONTINUE
TO MOVE ALONG THE THE SPINE OF THE APPALACHIANS..EXTENDING BACK INTO
THE DEEP SOUTH THROUGH MISSISSIPPI.  THIS ACTIVITY IS OCCURRING
ALONG AN AXIS OF H85 TO H7 CONVERGENCE...AND BEING SUPPORTED BY AN
H25 JET THAT EXTENDS DOWN THE WESTERN SIDE OF THE APPALACHIANS.  A
COUPLE VORT MAXES WERE NOTED ACROSS THE DEEP SOUTH IN WATER VAPOR
IMAGERY EARLIER THIS MORNING...BUT THESE APPEAR THE BE SHEARING OUT
A BIT....AND WE MAY BE LEFT WITH JUST AN ELONGATED SHEAR AXIS.

FORECAST PROBLEM TODAY CONTINUES TO BE CONVECTIVE INITIATION AND
TIMING OF UPPER FORCING...AND WHETHER THESE WILL COINCIDE ENOUGH FOR
AN ENHANCED SEVERE WEATHER THREAT.  THE NORTHERN 2/3 OF THE CWA
REMAINS IN A SLIGHT RISK FOR TODAY.  UPPER SUPPORT STILL LOOKS TO
ARRIVE LATER THIS EVENING...AS THE UPPER JET (WHICH WAS ANALYZED A
BIT STRONGER THAN FORECAST IN THIS MORNINGS RAOB ANALYSIS) SHIFTS
EASTWARD. UPPER DIVERGENCE WILL BE INCREASING THROUGH OUT THE
DAY...BUT THE COLD FRONT IS EXPECTED TO REMAIN WEST OF THE
MOUNTAINS...AND EVEN A WELL DEFINED SURFACE TROUGH IS NOT EXPECTED
TO DEVELOP EAST OF THE MOUNTAINS UNTIL MIDNIGHT OR LATER.  THUS...IT
LOOKS LIKE WE WILL BE DEPENDING ON LOW LEVEL CONVERGENCE IN THE FORM
OF A 35 KT LOW LEVEL JET AND A WEAK H85 TROUGH.  ANOTHER FACTOR THAT
MAY HELP INITIATION IS A POTENTIAL DIFFERENTIAL HEATING
BOUNDARY...AS CLOUD COVER IS RATHER EXTENSIVE FROM CHARLOTTE TO THE
TRIAD AND POINTS WEST.  MLCAPES UNDER STRONG INSOLATION TO THE EAST
COULD REACH 2000 J/KG...BUT LAPSE RATES LOOK WEAK WITH PROFILES NEAR
MOIST ADIABATIC AND THE CLOSEST MID LEVEL DRY AIR BACK TO THE WEST
OF NASHVILLE.

FROM ALL OF THIS...NOT STRAYING TOO FAR FROM THE CURRENT FORECAST.
EXPECT THERE COULD BE SOME ISOLATED STORMS ACROSS THE EASTERN HALF
OF THE CWA DURING THE EARLY AFTERNOON HOURS....BUT EXPECT A
SCATTERED CONVECTION TO DEVELOP OVER THE WESTERN AND SOUTHWESTERN
CWA...IN THE 20-22Z TIME FRAME...AS UPPER DIVERGENCE INCREASES...AND
THEN SPREAD EASTWARD FEEDING OFF REMAINING INSTABILITY THROUGH THE
REST OF THE EVENING.  UNIDIRECTIONAL WIND PROFILES WITH BULK SHEAR
VALUES OF 25-30 KT STILL SUPPORTS SOMEWHAT ORGANIZED MULTICELL
CLUSTERS WITH A THREAT OF DAMAGING STRAIGHT LINE WINDS.  THE
FREEZING LEVEL IN THE KGSO RAOB THIS MORNING AT WELL ABOVE 14K
FT...AND THE WETBULB ZERO LEVEL WAS ABOVE 13KT FT...SO HAIL
POTENTIAL IS LIMITED.

HIGHS IN THE WEST THIS AFTERNOON WILL LIKELY BE A BIT LOWER THAN
FORECAST DUE TO CLOUD COVER. WILL LOWER COUPLE DEGREES....STILL
85-91 FROM NW TO SE.

UPPER JET DIVERGENCE IN THE ENTRANCE REGION OF A 70KT UPPER JET
TRAVERSING THE NORTHEAST AND MID-ATLANTIC REGIONS COMBINED WITH
LOW-LEVEL CONVERGENCE ALONG THE SURFACE TROUGH WILL SUPPORT
A CONTINUATION OF SHOWERS WITH ISOLATED THUNDER OVERNIGHT. LOWS
70 TO 75.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
AS OF 330 PM FRIDAY...
SATURDAY AND SATURDAY NIGHT...
UPPER DIVERGENCE ASSOCIATED WITH THE RIGHT ENTRANCE REGION OF UPPER
JET EXTENDING SOUTH ACROSS THE NORTHEAST AND MID-ATLANTIC STATES
WILL WANE AS SHIFTS EAST OF THE MID ATLANTIC COAST BETWEEN 18 TO 21Z.
MOISTURE POOLING ALONG THE STALLED SURFACE BOUNDARY/LOW-LEVEL S/W
TROUGH COMBINED WITH THE UPPER JET SHOULD SUPPORT SOME LIKELY SHOWERS
ACROSS EASTERN AREAS THROUGH THE LATE MORNING HOURS. CONVECTIVE
COVERAGE MAY DIMINISH BRIEFLY BEFORE STRONG DIURNAL HEATING WILL
RESULT IN MODERATELY INSTABILITY WITH CONVECTION RE-FIRING ALONG THE
SURFACE BOUNDARY. EXPECT BEST COVERAGE EAST OF THE BOUNDARY...IN
LOW-LEVEL THETA-E RIDGE AXIS. WILL TREND POPS FROM HIGH NW TO LIKELY
EAST. COVERAGE WILL GREATLY DIMINISH WITH LOSS OF DIURNAL HEATING
SATURDAY EVENING. LINGERING LOW-LEVEL TROUGH ALONG WITH A
SUFFICIENTLY MOIST AIRMASS AS LOW-LEVEL SWLY WARM AND MOIST AIR
ADVECTION BECOMES RE-ESTABLISHED AS WINDS BACK WITH THE APPROACH OF
THE MID-LEVEL TROUGH UPSTREAM. WILL CUT POPS TO SLIGHT CHANCE
OVERNIGHT. HIGHS ON SATURDAY IN THE MID 80S NW TO AROUND 90 SE AS
WIDESPREAD CLOUD COVER WILL LIMIT FURTHER HEATING.

SUNDAY AND SUNDAY NIGHT...
A NEGATIVELY TILTED MID-LEVEL TROUGH WILL SWING EAST ACROSS THE
GREAT LAKES REGION ON SUNDAY. SHORTWAVE ENERGY ON THE SOUTHERN
PERIPHERY OF THE MEAN TROUGH WILL MOVE ACROSS THE AREA SUNDAY
AFTERNOON BRINGING ANOTHER SURFACE FRONT INTO THE AREA. WHILE THE
LOW-LEVEL WIND FIELD/KINEMATICS WILL BE MUCH WEAKER COMPARED TO
TODAY...THE LOW-LEVEL CONVERGENT WIND FLOW COMBINED WITH
UPPER DIVERGENCE ASSOCIATED WITH THE ENTRANCE REGION OF 50 TO 60 KT
250 MB JET WILL SUPPLY GOOD VERTICAL ASCENT.  +2.0 OF PWAT COMBINED
WITH MODERATE INSTABILITY OF 1500 TO 2000 J/KG OF MLCAPE WILL SUPPORT
LIKELY POPS...DIMINISHING TO CHANCE OVERNIGHT. HIGHS IN THE LOWER TO
MID 80S IN THE WEST TO UPPER 80S TO AROUND 90 IN CENTRAL AND EASTERN
SECTIONS. LOWS 70 TO 75.
&&

LONG TERM/MONDAY THROUGH FRIDAY/...
AS OF 330 PM THURSDAY...
IT STILL REMAINS UNCLEAR IF WHETHER WE WILL SEE A CHANGE IN THE
UPPER UPPER AIR PATTERN NEXT WEEK. THE GFS HAS BEEN MORE READILY
TO SHIFT THE LONG WAVE TROUGH AXIS OVER THE EASTERN US...AS IT
BUILDS THE AMPLIFIED UPPER RIDGE OVER THE SOUTHWEST EAST INTO THE
ROCKIES AND PLAINS. WHILE THE EC MAINTAINS THE LONG WAVE TROUGH AXIS
OVER ITS CURRENT LOCATION ACROSS THE EAST CENTRAL US.

BOTH SYNOPTIC SCALE PATTERNS WILL SUPPORT A CONTINUATION OF
A CONVECTIVELY ACTIVE PERIOD. WHILE THE MORE EASTERN SOLUTION OF THE
GFS WOULD POTENTIALLY FAVOR LESSER COVERAGE (ESPECIALLY IN THE WEST)
DUE TO DRIER AIR IN MID AND UPPER LEVELS VIA WESTERLY ALOFT...IT
COULD ALSO RESULT IN A MORE CONDUCIVE ENVIRONMENT FOR STRONGER/SEVERE
STORMS DUE TO STEEPER LAPSE RATES ALOFT...AND ENHANCED EVAP COOLING
DUE TO DRY AIR ENTRAINMENT.

WILL CONTINUE WITH THE DIURNAL TRENDS OF 30 TO 50 PERCENT FROM WEST
TO EAST MAXIMIZED DURING PEAK HEATING WITH PERSISTENT PIEDMONT TROUGH
...DIFFERENTIAL HEATING AND SEA BREEZE PROVIDING THE LOW-LEVEL
FOCUS. TEMPERATURES WILL REMAIN NEAR CLIMATOLOGY THROUGH THE PERIOD.
HIGHS IN THE UPPER 80S TO LOWER 90S. LOWS IN THE UPPER 60S TO LOWER
70S.

&&

.AVIATION /18Z FRIDAY THROUGH TUESDAY/...
AS OF 130 PM FRIDAY...

SCATTERED CONVECTION AND STRONG WIND GUSTS THE MAIN AVIATION CONCERN
THIS AFTERNOON.   THUNDERSTORMS HAVE DEVELOPED ACROSS THE SOUTHERN
PIEDMONT AND WILL CONTINUE TO MOVE AND DEVELOP TO THE NORTH AND
EAST...AFFECTING ANY TERMINAL ACROSS CENTRAL NC BY 20-21Z.  KRDU
WILL BE LIKELY BE THE FIRST OF THE TAF SITES TO EXPERIENCE
TSRA...WITH STORMS ALREADY NEARING THE AIRPORT.  IN ADDITION TO THE
AFTERNOON AND EVENING CONVECTION...WIND GUSTS OF
20-25KT....SOMETIMES NEAR 30KT...WILL CONTINUE UNTIL RIGHT AROUND
00Z.  EXPECT THE GUSTS AND THUNDERSTORM ACTIVITY TO DIMINISH OR WEAK
AFTER SUNSET...BUT AN APPROACHING UPPER LEVEL JET COMBINED WITH A
COLD FRONT CROSS THE APPALACHIANS WILL CONTINUE THE CHANCE OF
SHOWERS AND POSSIBLY AN ISOLATED THUNDERSTORM THROUGH THE OVERNIGHT
HOURS.  SHOWERS WILL END FROM WEST TO EAST TOWARD DAYBREAK SATURDAY
MORNING....WITH CEILINGS LOWERING TO IFR TO MVFR.  -SMITH

LOOKING AHEAD THROUGH THE WEEKEND INTO EARLY NEXT WEEK...EXPECT
LITTLE CHANGE IN THE WEATHER PATTERN OVER CENTRAL NORTH CAROLINA...
WITH A CONTINUATION OF HUMID CONDITIONS THAT WILL SUPPORT THE
POTENTIAL FOR PRE-DAWN STRATUS AND FOG...PARTICULARLY AT EASTERN
TERMINALS AND/OR WHERE RAIN OCCURS...AND PRIMARILY AFTERNOON AND
EVENING THUNDERSTORMS.

&&

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

&&

SYNOPSIS...CBL
NEAR TERM...SMITH
SHORT TERM...CBL
LONG TERM...CBL
AVIATION...SMITH

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE RALEIGH NC
256 PM EDT FRI JUL 31 2009

NCZ007>011-021>028-038>043-073>078-083>086-088-089-011900-
PERSON-GRANVILLE-VANCE-WARREN-HALIFAX-FORSYTH-GUILFORD-ALAMANCE-
ORANGE-DURHAM-FRANKLIN-NASH-EDGECOMBE-DAVIDSON-RANDOLPH-CHATHAM-WAKE-
JOHNSTON-WILSON-STANLY-MONTGOMERY-MOORE-LEE-HARNETT-WAYNE-ANSON-
RICHMOND-SCOTLAND-HOKE-CUMBERLAND-SAMPSON-
256 PM EDT FRI JUL 31 2009

...SEVERE THUNDERSTORM WATCH 645 IN EFFECT UNTIL 9 PM EDT THIS
EVENING...

THIS HAZARDOUS WEATHER OUTLOOK IS FOR CENTRAL NORTH CAROLINA.

.DAY ONE...THIS AFTERNOON AND TONIGHT.

PLEASE LISTEN TO NOAA WEATHER RADIO OR GO TO WEATHER.GOV ON THE
INTERNET FOR MORE INFORMATION ABOUT THE FOLLOWING HAZARDS.

   SEVERE THUNDERSTORM WATCH.

SCATTERED SHOWERS AND THUNDERSTORMS WILL DEVELOP ACROSS CENTRAL
NORTH CAROLINA THIS AFTERNOON AND EVENING. ANY STORMS THAT DEVELOP WILL
HAVE THE POTENTIAL TO BECOME SEVERE...WITH A PRIMARY THREAT OF
DAMAGING STRAIGHT LINE WINDS. THE CHANCE OF SEVERE WEATHER WILL
CONTINUE THROUGH AT LEAST 9PM THIS EVENING.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY.

THERE WILL BE A CHANCE OF SCATTERED SHOWERS AND THUNDERSTORMS
ACROSS MOST OF CENTRAL NORTH CAROLINA AGAIN ON SATURDAY. THE MAIN
THREAT OF SEVERE WEATHER WILL BE FROM LARGE HAIL AND DAMAGING WINDS.

.SPOTTER INFORMATION STATEMENT...

SKYWARN SPOTTER ACTIVATION WILL LIKELY BE NEEDED BETWEEN 3PM AND
9PM THIS EVENING. SPOTTERS SHOULD REPORT SEVERE WEATHER AS SOON AS IT IS
SAFE TO DO SO. PLEASE STAY TUNED FOR LATER WATCHES AND WARNINGS.

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