MESOSCALE DISCUSSION 1968
   NWS STORM PREDICTION CENTER NORMAN OK
   0252 PM CDT TUE SEP 18 2012
   
   AREAS AFFECTED...PARTS OF CNTRL/ERN MD...WASHINGTON DC...ERN
   VA...NERN NC
   
   CONCERNING...TORNADO WATCH 642...
   
   VALID 181952Z - 182145Z
   
   THE SEVERE WEATHER THREAT FOR TORNADO WATCH 642 CONTINUES.
   
   SUMMARY...THE THREAT FOR SVR WINDS AND TORNADOES CONTINUES ACROSS WW
   642.
   
   DISCUSSION...A WELL-DEVELOPED SQUALL LINE WITH EMBEDDED LEWP
   STRUCTURES EXTENDING FROM THE BALTIMORE AREA SSWWD INTO S-CNTRL VA
   AND ADJACENT NRN NC WILL CONTINUE ADVANCING EWD. THE SQUALL LINE
   WILL LIKELY BE MAINTAINED AS IT APPROACHES THE COASTAL PLAIN AS DEEP
   SHEAR VECTORS REMAIN ORIENTED NEARLY PARALLEL TO THE LEADING EDGE OF
   THE COLD POOL WHILE INFLOW AIR IS CHARACTERIZED BY 500-1000 J/KG OF
   MLCAPE PER MODIFIED 12Z RAOBS. LOCALIZED POCKETS OF
   INSOLATION/DIABATIC HEATING...ESPECIALLY OVER SERN VA AND NERN
   NC...ARE BOOSTING BOUNDARY-LAYER INSTABILITY. AND...WITH A STRONG
   LOW-LEVEL JET STREAM SUPPORTING 35-45 KT OF 0-1-KM BULK SHEAR...SVR
   WIND GUSTS WILL BE LIKELY...AND THE TORNADO THREAT WILL BE ENHANCED
   IN CONJUNCTION WITH EMBEDDED MESOVORTICES. IN THE WAKE OF THE
   CONVECTIVE LINE...VEERED LOW-LEVEL FLOW AND STABLE THERMODYNAMIC
   PROFILES WILL MITIGATE THE SVR THREAT...AND REMAINING VALID PORTIONS
   OF WW 642 MAY BE CLEARED FOLLOWING THE PASSAGE OF THE LINE.
   
   ..COHEN.. 09/18/2012
   
   
   ATTN...WFO...PHI...AKQ...LWX...
   
   LAT...LON   36047625 35927695 36387751 36767810 37367766 38617717
               39477689 39647663 39577633 39057610 38717593 38357526
               37547540 37067560 36047625