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URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 794

NWS STORM PREDICTION CENTER NORMAN OK

1255 PM EDT SUN AUG 21 2011



THE NWS STORM PREDICTION CENTER HAS ISSUED A

SEVERE THUNDERSTORM WATCH FOR PORTIONS OF



       WESTERN CONNECTICUT

       DISTRICT OF COLUMBIA

       DELAWARE

       MARYLAND

       NEW JERSEY

       EXTREME SOUTHEAST NEW YORK

       CENTRAL AND EASTERN PENNSYLVANIA

       NORTHERN VIRGINIA

       THE EASTERN WEST VIRGINIA PANHANDLE

       COASTAL WATERS



EFFECTIVE THIS SUNDAY AFTERNOON AND EVENING FROM 1255 PM UNTIL

900 PM EDT.



HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70

MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.



THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 90

STATUTE MILES EAST AND WEST OF A LINE FROM 45 MILES NORTH

NORTHEAST OF ALLENTOWN PENNSYLVANIA TO 55 MILES SOUTH SOUTHWEST

OF WASHINGTON DISTRICT OF COLUM.  FOR A COMPLETE DEPICTION OF THE

WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU4).



REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE

FAVORABLE FOR 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. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY

DO PRODUCE TORNADOES.



DISCUSSION...THUNDERSTORMS WILL INCREASE IN COVERAGE/INTENSITY THIS

AFTERNOON IN CONJUNCTION WITH CONTINUED DAYTIME HEATING...AND AS A

MIDLEVEL TROUGH AND COLD FRONT MOVE EWD/SEWD ACROSS THE REGION.

ADDITIONALLY...A SURFACE LEE TROUGH FROM NRN VA TO ERN PA/SE NY WILL

HELP FOCUS THUNDERSTORM DEVELOPMENT.  MODERATE-STRONG INSTABILITY

/MLCAPE AOA 2000 J PER KG/ AND INCREASING VERTICAL SHEAR THROUGH THE

AFTERNOON WILL BE FAVORABLE FOR STORMS TO PRODUCE DAMAGING WINDS AND

LARGE HAIL.  A COUPLE OF SUPERCELLS MAY ALSO DEVELOP THIS AFTERNOON

NEAR THE LEE TROUGH...AND AN ISOLATED TORNADO WILL BE POSSIBLE WHERE

LOW-LEVEL SHEAR WILL BE SOMEWHAT STRONGER.



AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT

TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60

KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500. MEAN STORM

MOTION VECTOR 27035.





...THOMPSON

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