SPC MESOSCALE DISCUSSION 1246 FOR SRN PA...MUCH OF MD AND
   NRN/CENTRAL VA
   CONCERNING...SEVERE THUNDERSTORM POTENTIAL...
   
   VALID 261748Z - 262100Z
   
   ISOLATED SEVERE THUNDERSTORMS WILL BE POSSIBLE ACROSS THE AREA
   THROUGH THE AFTERNOON. WE WILL CONTINUE TO MONITOR THE AREA FOR A
   POSSIBLE WW.
   
   CONVECTION DEVELOPING OVER THE HIGHER TERRAIN FROM SCENTRAL PA SWD
   INTO ERN WV WILL CONTINUE TO MOVE EWD AROUND 20 KTS AND LIKELY
   BEGIN TO POSE AN ISOLATED SEVERE THREAT OVER THE NEXT FEW HOURS.
   ADDITIONAL CONVECTION DEVELOPING ALONG THE SHENANDOAH VALLEY MAY
   ALSO BECOME SEVERE IN THE NEXT 2-3 HOURS. THE 17Z SURFACE ANALYSIS
   INDICATED TEMPERATURES HAVE WARMED INTO THE LOWER 90S...WITH
   DEWPOINTS IN THE LOWER TO MID 70S. THIS HAS CONTRIBUTED TO MODERATE
   INSTABILITY WITH BETWEEN 2500-3000 J/KG OF MUCAPE PER LATEST RUC
   OBJECTIVE ANALYSIS. MID LEVEL FLOW WILL REMAIN RATHER WEAK...LESS
   THAN 25 KTS WAS NOTED ON RECENT VWP DATA. EXTENT OF MID LEVEL DRY
   AIR NOTED ON 12Z IAD SOUNDING ALONG WITH STEEP LOW LEVEL LAPSE
   RATES AND THETA-E DIFFERENCE VALUES AROUND 30 DEG C WILL SUPPORT
   SOME POTENTIAL FOR DAMAGING DOWNBURST WINDS WITH THE STRONGEST
   CONVECTION. IF NUMEROUS CELL MERGERS CAN AID IN THE GENESIS OF A
   SUFFICIENTLY STRONG COLD POOL THEN A MORE ORGANIZED SEVERE THREAT
   WOULD BE POSSIBLE LATER THIS AFTERNOON OVER PORTIONS OF NERN VA AND
   MD. THE OVERALL LARGE HAIL THREAT WILL BE MITIGATED BY WEAK DEEP
   LAYER SHEAR LESS THAN 30 KTS AND HIGH WBZ HEIGHTS /AROUND 12 KFT/.
   
   ..CROSBIE.. 06/26/02