Special Weather Statement, NWS Raleigh, NC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
304 PM EDT MON SEP 5 2011

.SYNOPSIS...
THE REMNANTS OF TROPICAL STORM LEE ARE EXPECTED TO MOVE NORTHEAST
ALONG A STATIONARY FRONT THROUGH THE TENNESSEE VALLEY AND THE
CENTRAL APPALACHIANS THROUGH TUESDAY NIGHT. THIS WILL BRING A THREAT
FOR HEAVY RAIN AND SEVERE WEATHER TONIGHT THROUGH TUESDAY NIGHT TO
CENTRAL NC.

&&

.NEAR TERM /THROUGH TUESDAY NIGHT/...
AS OF 255 PM MONDAY...

BASED ON LATEST 12Z MODEL GUIDANCE AND MODEL TRENDS...FORECAST FOR
CENTRAL NC THROUGH TUESDAY NIGHT APPEARS TO BE SHIFTING FOCUS FROM
HEAVY RAIN TO SEVERE T-STORMS WITH THE POSSIBILITY OF SHORT-LIVED
TORNADOES TONIGHT THROUGH TUESDAY EVENING. PLAN TO KEEP FLASH FLOOD
WATCH UP AS A PRECAUTION AS PWAT VALUES 2-2.33 INCHES SUPPORTS
TROPICAL DOWNPOURS WITH RAINFALL RATES IN EXCESS OF 2 INCHES PER
HOUR. HOWEVER...APPEARS THAT SURFACE FRONT THAT WAS TO FOCUS
WIDESPREAD SHOWERS WILL STALL NEAR OR NORTH OF THE VIRGINIA BORDER.
THIS WILL KEEP MOST IF NOT ALL OF CENTRAL NC IN THE WARM MOIST
SECTOR. MEANWHILE THIS AFTERNOON...PARTIAL SUNSHINE HAS AIDED TO
DESTABILIZE THE AIR MASS OVER THE SOUTHERN AND WESTERN PIEDMONT.
THIS DESTABILIZATION IN ASSOCIATION WITH AN INCREASING S-SW FLOW
ABOVE THE SURFACE DUE TO THE APPROACH OF AN AREA OF LOW PRESSURE
CURRENTLY LIFTING NE ACROSS WRN ALABAMA HAS INCREASED THE LOW LEVEL
HELICITY.

2 PM MESO ANALYSIS HAS BULK SHEAR OF 30-35KTS ACROSS THE REGION
(GREATEST W-SW) WHILE 0-1KM HELICITY APPROACHING THE CRITICAL VALUE
OF 150 M2/S2 IN THE FAR WEST. NEAR TERM MODEL GUIDANCE HAS THIS LOW
LEVEL HELICITY INCREASING OVER THE WESTERN HALF OF THE FORECAST AREA
TONIGHT (THOUGH GFS MAY BE BLOWING THIS OUT OF PORTION WITH 0-1
HELICITY VALUES AROUND 250 M2/S2). APPEARS THAT THERE WILL BE
SUFFICIENT MLCAPE OVERNIGHT (500-800 J/KG OVER THE SW COUNTIES) TO
WARRANT A THREAT FOR SEVERE STORMS TONIGHT AND INTO THE OVERNIGHT
HOURS WITH THE MAIN THREAT LOCALIZED DAMAGING WIND GUSTS AND
ISOLATED SHORT-LIVED TORNADOES.

FOR NOW WILL MAINTAIN FLASH FLOOD WATCH AS IS AS CONVECTIVE-ALLOWING
MODELS DEPICT A COUPLE WAVES OF SHOWERS/T-STORMS LIFTING ACROSS THE
PIEDMONT COUNTIES TONIGHT.

DID ADJUST MIN TEMPS TONIGHT A FEW DEGREES WARMER SINCE IT APPEARS
SURFACE FRONT WILL NOT PROGRESS SWD ACROSS THE TRIAD REGION AS
EARLIER FORECAST.

TUESDAY...POTENTIAL FOR ANOTHER ROUND OF SEVERE STORMS AS REMNANT
CIRCULATION OF T.S. LEE LIFTS FROM NORTH GA INTO THE MOUNTAINS OF
NC. WHILE MODEL SEVERE WEATHER PARAMETERS NOT AS ROBUST AS THIS
AFTERNOON/TONIGHT BULK SHEAR OF 25-35KTS WILL EXIST AND 0-1KM SRH
100-180 M2/S2 EXPECTED BY AFTERNOON MAINLY ALONG AND EAST OF HIGHWAY
1. DOES NOT APPEAR THAT WE WILL BE LACKING IN INSTABILITY AS MLCAPE
1400-2000 J/KG. SPC WRF SUGGEST TWO BATCHES OF STORMS TRAVERSING
SW-NE ACROSS THE REGION WITH THE FIRST CLUSTER DEVELOPING OVER THE
SW PIEDMONT BY LATE MORNING WITH ANOTHER AFFECTING THE EAST HALF
LATE AFTERNOON-EARLY EVENING.

DUE TO SURFACE FRONT REMAINING NORTH OF THE FORECAST AREA...HAVE
RAISED MAX TEMPS SEVERAL DEGREES FROM EARLIER FORECAST WITH MAX
TEMPS NEAR 80 NW TO NEAR 90 SE.

TUESDAY NIGHT...BEST PARAMETERS FOR CONVECTION SHIFT INTO THE
COASTAL PLAIN AFTER 8 PM TUESDAY WHERE BETTER LIFT/LOW LEVEL
CONVERGENCE EXPECTED TO RESIDE THOUGH A FEW SHOWERS/T-STORMS STILL
PROBABLE IN THE WEST. RAISED MIN TEMPS IN THE NW A FEW DEGREES BUT
ADJUSTMENT MAY NOT STILL BE ENOUGH.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY/...
AS OF 130 PM MONDAY...

FORECAST CONFIDENCE IS REDUCED BY MODEL DIFFERENCES WITH RESPECT TO
THEIR HANDLING OF THE MID LEVEL VORTEX TO OUR WEST... ALTHOUGH THEY
APPEAR TO BE IN BETTER AGREEMENT THAN YESTERDAY'S RUNS WERE. THE
LATEST NAM/GFS ARE FAIRLY SIMILAR (WITH THE GFS A LITTLE FASTER) IN
EASING THE CLOSED LOW SLOWLY NORTHWARD THROUGH KY BEFORE HOLDING IT
(WITH A SLIGHT WESTWARD DRIFT) OVER THE OH VALLEY THROUGH
THURSDAY... WHILE THE ECMWF IS FURTHER SOUTH OVER TN/KY WHILE ALSO
EVENTUALLY DRIFTING IT WEST. TOUGH CALL ON WHICH SOLUTION IS MORE
CORRECT... ALTHOUGH THE OP ECMWF DOES AGREE VERY WELL WITH THE ECMWF
ENSEMBLE MEAN. THIS SYSTEM SHOULD HAVE BECOME WELL OCCLUDED BY THIS
POINT... SO WHILE WE SHOULD SEE UNSETTLED SKIES BOTH DAYS... THE
WIND FIELD THROUGH THE COLUMN IS LIKELY TO BE FAIRLY LIGHT AND
BENIGN... WITH PRECIP WATER LOWERING WEST TO EAST ON WEDNESDAY...
AND DRYING/STABILIZING MID LEVELS ACCORDING TO FORECAST SOUNDINGS.
HURRICANE KATIA IS ALSO A WILD CARD HERE AS IT IS FORECAST TO BE
TRACKING TO THE N OR NNE OFF THE NC COAST (ROUGHLY ALONG 70W) LATE
THURSDAY (PLEASE SEE THE LATEST NHC FORECASTS FOR DETAILS ON KATIA).
THE NAM/ECMWF MOST CLOSELY FOLLOW THE NHC OFFICIAL FORECAST. WE
SHOULD SEE ENOUGH SUBSIDENCE BETWEEN KATIA AND THE VORTEX TO OUR
WEST TO KEEP THE CHANCE OF SHOWERS ON THE LOW SIDE... SO HAVE
REDUCED POPS TO LOW CHANCE... HIGHER WEDNESDAY THAN THURSDAY...
FOCUSED DURING DAYTIME HEATING... AND HIGHEST IN THE NORTHEAST CWA
WHERE E/SE LOW LEVEL FLOW WILL PERSIST ALONG AND NORTH OF A SUBTLE
BOUNDARY WITH ENHANCED LOW LEVEL MASS CONVERGENCE. MODEL LOW LEVEL
THICKNESSES ARE VERY CLOSE TO SEASONAL NORMALS... SO EXPECT TEMPS TO
FOLLOW SUIT WITH LOWS IN THE LOWER-MID 60S AND HIGHS IN THE LOWER TO
MID 80S BOTH DAYS. -GIH

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
AS OF 205 PM MONDAY...

WILL FOLLOW FAIRLY CLOSE TO THE ECMWF FOR THE MEDIUM RANGE... GIVEN
ITS FAVORED HANDLING OF KATIA EARLY ON AND ITS GOOD AGREEMENT WITH
ITS ENSEMBLE MEAN. THE UPPER LOW IS PROJECTED TO MEANDER OVER THE
WESTERN OHIO VALLEY AND MID MISS VALLEY FRIDAY... AND THEN SLOWLY
FILL AND WEAKEN TO A POSITIVELY-TILTED OPEN WAVE AS IT DRIFTS BACK
TO THE ENE THROUGH THE OHIO VALLEY AND INTERIOR MID ATLANTIC OVER
THE WEEKEND... BECOMING INCREASINGLY CAUGHT UP IN THE PREVAILING
WESTERLIES TO ITS NORTH. THIS RESULTS IN A PRIMARILY WESTERLY MID
LEVEL FLOW OVER NC WITH WEST TO WNW 925-850 MB FLOW AND A WEAK
SURFACE TROUGH SITTING OVER OR JUST SOUTHEAST OF THE COASTAL PLAIN
THROUGH THE PERIOD. MOISTURE DOES NOT APPEAR PLENTIFUL ENOUGH IN
THIS SORT OF PATTERN FOR MANY SHOWERS/STORMS... ALTHOUGH THE CHANCES
ARE A BIT HIGHER OVER THE NW AND NORTHERN CWA CLOSER TO THE CORE OF
COOLER MID LEVELS AS THE WEAKENED LOW PASSES TO OUR NORTH
SUNDAY/MONDAY. WILL HOLD JUST A SMALL CHANCE OF SHOWERS/STORMS
MAINLY IN THE NORTHERN CWA EACH AFTERNOON/EVENING. WITH EXPECTATIONS
OF DAILY SCATTERED/BROKEN DIURNAL CLOUDINESS AND THICKNESSES HOLDING
CLOSE TO SEASONAL NORMALS... WILL HAVE LOWS IN THE 60-65 RANGE AND
HIGHS IN THE UPPER 70S TO LOWER 80S. -GIH

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
AS OF 130 PM MONDAY...

VARIABLE CEILING BASES ACROSS THE REGION RESULTING IN VFR/MVFR
CONDITIONS. THIS SITUATION WILL REMAIN STATUS QUO THROUGH THE
AFTERNOON INTO THE EARLY EVENING HOURS. THROUGH 8 PM...MAJORITY OF
THE SHOWERS WILL OCCUR NEAR OR WEST OF THE TRIAD TERMINALS THOUGH
SCATTERED SHOWERS AND A FEW T-STORMS WILL ALSO OCCUR IN THE EASTERN
PIEDMONT...COASTAL PLAIN AND SANDHILLS. EXPECT MVFR/IFR VISIBILITY
IN VICINITY OF THE HEAVIER SHOWERS AND STORMS AS WELL AS WIND GUSTS
20-30KTS. WHILE CONVECTION WILL DISSIPATE IN THE EAST WITH LOOS OF
HEATING AFTER 8 PM...THE APPROACH OF A STRONG SURFACE COLD FRONT
FROM THE NW INTERACTING WITH THE TROPICAL MOISTURE IN PLACE WILL
RESULT IN NUMEROUS SHOWERS AND A FEW T-STORMS OVER THE NW PIEDMONT
INCLUDING TH TRIAD REGION LATER THIS EVENING INTO THE OVERNIGHT
HOURS. VERY HEAVY RAINFALL IS EXPECTED WITH THE SHOWERS AND STORMS.
IN ADDITION...WINDS JUST ABOVE THE SURFACE WILL BE INCREASING TO
NEAR 30 MPH AFTER MIDNIGHT. AT THIS TIME...CONDITIONS DO NOT WARRANT
MENTION OF LLWS IN THE TERMINAL FORECAST BUT WILL CONTINUE TO
MONITOR.

TUESDAY...ADVERSE AVIATION CONDITIONS EXPECTED WITH SCATTERED TO
NUMEROUS SHOWERS AND STORMS AND PROLONGED STRETCHES OF MVFR-IFR
CONDITIONS.

IMPROVING AVIATION WEATHER CONDITIONS EXPECTED WEDNESDAY INTO
THURSDAY WITH VFR CONDITIONS RETURNING TO MOST OF IF NOT ALL OF
CENTRAL NC BY THURSDAY. VFR CONDITIONS EXPECTED FRIDAY INTO
SATURDAY.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH FROM THIS EVENING THROUGH LATE TUESDAY NIGHT
FOR NCZ007>010-021>026-038>041-073>076-083-084.

&&

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

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE RALEIGH NC
353 AM EDT MON SEP 5 2011

NCZ007>010-021>026-038>041-073>076-083-084-060800-
PERSON-GRANVILLE-VANCE-WARREN-FORSYTH-GUILFORD-ALAMANCE-ORANGE-
DURHAM-FRANKLIN-DAVIDSON-RANDOLPH-CHATHAM-WAKE-STANLY-MONTGOMERY-
MOORE-LEE-ANSON-RICHMOND-
353 AM EDT MON SEP 5 2011

...FLASH FLOOD WATCH IN EFFECT FROM THIS EVENING THROUGH LATE
TUESDAY NIGHT...

THIS HAZARDOUS WEATHER OUTLOOK IS FOR CENTRAL NORTH CAROLINA.

.DAY ONE...TODAY AND TONIGHT.

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

   FLASH FLOOD WATCH.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY.

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

   FLASH FLOOD WATCH.

.SPOTTER INFORMATION STATEMENT...

SKYWARN SPOTTER ACTIVATION WILL LIKELY BE NEEDED TONIGHT AND
TUESDAY. SPOTTERS SHOULD REPORT SEVERE WEATHER INCLUDING FLOODING
AND RAINFALL AMOUNTS AS SOON AS IT IS SAFE TO DO SO. PLEASE STAY
TUNED FOR LATER WATCHES AND WARNINGS.

NCZ011-027-028-042-043-077-078-085-086-088-089-060800-
HALIFAX-NASH-EDGECOMBE-JOHNSTON-WILSON-HARNETT-WAYNE-SCOTLAND-HOKE-
CUMBERLAND-SAMPSON-
353 AM EDT MON SEP 5 2011

THIS HAZARDOUS WEATHER OUTLOOK IS FOR CENTRAL NORTH CAROLINA.

.DAY ONE...TODAY AND TONIGHT.

HEAVY RAIN AND ISOLATED SEVERE WEATHER REMAIN A POSSIBILITY LATER
TODAY AND TONIGHT. HOWEVER... IT APPEARS THAT THE BEST CHANCE OF
HEAVY RAIN AND ANY SEVERE THUNDERSTORMS WILL BE OVER THE PIEDMONT
THROUGH TONIGHT. HOWEVER... LOCALLY HEAVY RAIN MAY DEVELOP IN
SHOWERS AND THUNDERSTORMS LATER TODAY AND TONIGHT WITH LOCALLY UP
TO 2 INCHES OF RAIN POSSIBLE OVER THE SANDHILLS AND COASTAL PLAIN.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY.

SHOWERS AND THUNDERSTORMS ARE EXPECTED AT TIMES TUESDAY THROUGH
THURSDAY. LOCALLY HEAVY RAIN AND THE POTENTIAL FOR FLASH FLOODING
MAY OCCUR. THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WITH
DAMAGING WINDS OR ISOLATED TORNADOES TUESDAY THROUGH WEDNESDAY.

.SPOTTER INFORMATION STATEMENT...

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

BADGETT

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