

209 

WWUS20 KWNS 311720

SEL4

SPC WW 311720

CTZ000-DCZ000-DEZ000-MDZ000-NJZ000-NYZ000-PAZ000-VAZ000-CWZ000-01010

0-



URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 644

NWS STORM PREDICTION CENTER NORMAN OK

120 PM EDT FRI JUL 31 2009



THE NWS STORM PREDICTION CENTER HAS ISSUED A

SEVERE THUNDERSTORM WATCH FOR PORTIONS OF



       WESTERN CONNECTICUT

       DISTRICT OF COLUMBIA

       DELAWARE

       MARYLAND

       NEW JERSEY

       SOUTHERN NEW YORK

       EASTERN PENNSYLVANIA

       NORTHERN VIRGINIA

       COASTAL WATERS



EFFECTIVE THIS FRIDAY AFTERNOON AND EVENING FROM 120 PM UNTIL 900

PM EDT.



HAIL TO 1.5 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 80

STATUTE MILES EAST AND WEST OF A LINE FROM 40 MILES EAST

NORTHEAST OF POUGHKEEPSIE NEW YORK TO 15 MILES WEST OF PATUXENT

RIVER MARYLAND.  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...MESSY EVOLUTION OF THUNDERSTORMS TODAY AHEAD OF BROAD

UPPER TROUGH AND ASSOCIATED SURFACE COLD FRONT.  PREFRONTAL AIR MASS

IS VERY MOIST...ALMOST TROPICAL...AND WILL BECOME MARGINAL TO

MODERATELY UNSTABLE THIS AFTERNOON.  SHEAR WILL BE FAIRLY STRONG AND

SUPPORT SUPERCELLS AND LINES OF STRONGER STORMS...WITH PRIMARY

THREATS OF DAMAGING WINDS/HAIL WITHIN NEARLY UNIDIRECTIONAL WIND

PROFILES.  HOWEVER...LOW LEVEL SHEAR WILL BE SUFFICIENT FOR AN

ISOLATED TORNADO WITH ANY PERSISTENT SUPERCELL.  HEAVY RAINFALL WILL

BE AN ADDITIONAL HAZARD THROUGH THE EVENING.



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

TO 1.5 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60

KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 450. MEAN STORM

MOTION VECTOR 25030.





...EVANS

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