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WWUS20 KWNS 181455

SEL2

SPC WW 181455

DCZ000-MDZ000-NCZ000-VAZ000-WVZ000-CWZ000-182300-



URGENT - IMMEDIATE BROADCAST REQUESTED

TORNADO WATCH NUMBER 642

NWS STORM PREDICTION CENTER NORMAN OK

1055 AM EDT TUE SEP 18 2012



THE NWS STORM PREDICTION CENTER HAS ISSUED A

TORNADO WATCH FOR PORTIONS OF



       DISTRICT OF COLUMBIA

       MARYLAND

       NORTHERN NORTH CAROLINA

       CENTRAL AND EASTERN VIRGINIA

       EASTERN WEST VIRGINIA PANHANDLE

       COASTAL WATERS



EFFECTIVE THIS TUESDAY MORNING AND EVENING FROM 1055 AM UNTIL 700

PM EDT.



TORNADOES...HAIL TO 0.5 INCH IN DIAMETER...THUNDERSTORM WIND

GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE

AREAS.



THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 90 STATUTE

MILES EAST AND WEST OF A LINE FROM 35 MILES EAST SOUTHEAST OF

SOUTH HILL VIRGINIA TO 40 MILES NORTH NORTHEAST OF BALTIMORE

MARYLAND.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE

ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU2).



REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR

TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH

AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR

THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS

AND POSSIBLE WARNINGS.



OTHER WATCH INFORMATION...CONTINUE...WW 641...



DISCUSSION...A GRADUAL INCREASE IN STORM COVERAGE AND INTENSITY IS

EXPECTED TODAY AS LARGE-SCALE FORCING FOR ASCENT INCREASES ACROSS A

VERY MOIST AND MARGINALLY UNSTABLE WARM SECTOR.  CURRENT VAD AND

MODEL FORECAST DATA INDICATE A CONTINUED STRENGTHENING OF THE

DEEP-LAYER WIND FIELD THROUGH THE PEAK OF THE DIURNAL HEATING CYCLE

WITH THE ENVIRONMENT BECOMING SUPPORTIVE OF EMBEDDED BOW/LEWP AND

SUPERCELL STRUCTURES CAPABLE OF TORNADOES AND DAMAGING WINDS.



AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL

SURFACE AND ALOFT TO 0.5 INCH. EXTREME TURBULENCE AND SURFACE

WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO

500. MEAN STORM MOTION VECTOR 23040.





...MEAD

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