MESOSCALE DISCUSSION 0667
   NWS STORM PREDICTION CENTER NORMAN OK
   0536 PM CDT THU APR 28 2011
   
   AREAS AFFECTED...ERN NC...SERN VA...AND SRN DELMARVA
   
   CONCERNING...TORNADO WATCH 255...256...
   
   VALID 282236Z - 282330Z
   
   CORRECTED FOR WATCH NUMBER REFERENCE IN LAST PARAGRAPH.
   
   THE SEVERE WEATHER THREAT FOR TORNADO WATCH 255...256...CONTINUES.
   
   AT 2215Z...REGIONAL RADAR IMAGERY AND LIGHTNING DATA INDICATED AN
   ACTIVE ZONE OF TSTMS ALONG A PRE-FRONTAL TROUGH/CONVERGENCE ZONE
   EXTENDING FROM SERN NC TO FAR SERN VA.  AIR MASS WITHIN THIS
   CORRIDOR REMAINS MODERATELY UNSTABLE AND STRONGLY SHEARED /EFFECTIVE
   SRH 200-300 M2 PER S2/ TO SUSTAIN A TORNADO THREAT EARLY THIS
   EVENING.  
   
   REGIONAL RADAR/VISIBLE SATELLITE IMAGERY ALSO SHOWED AN ISOLATED
   STORM FORMED ALONG AN APPARENT OUTFLOW BOUNDARY LOCATED TO THE WEST
   OF THE PRE-FRONTAL TROUGH...WITH THE OUTFLOW BOUNDARY EXTENDING FROM
   CUMBERLAND TO WILSON COUNTIES NC.  THE STORM ITSELF WAS TRACKING
   NEWD FROM JOHNSTON INTO WILSON COUNTIES.  THE WRN FRINGE OF MODERATE
   INSTABILITY IS OVER THIS BOUNDARY WITH STRONG EFFECTIVE BULK SHEAR
   SUPPORTIVE YET FOR ORGANIZED STORMS.  
   
   MEANWHILE...REGIONAL RADARS ARE SHOWING REFLECTIVITY INCREASING
   ALONG A COLD FRONT ADVANCING EWD THROUGH SOUTH CENTRAL VA TO FAR SRN
   MD...WITH LIGHTNING RECENTLY DETECTED INVOF THE MOUTH OF THE POTOMAC
   RIVER.  ALTHOUGH INSTABILITY IS WEAK ALONG AND JUST AHEAD OF THE
   FRONT...CONTINUED HEIGHT FALLS ACROSS THIS REGION AS THE UPSTREAM
   UPPER TROUGH SPREADS EWD SHOULD SUPPORT ADDITIONAL CONVECTION ALONG
   THE FRONT.  STRONG DEEP LAYER SHEAR MAY SUPPORT AN ISOLATED SEVERE
   THREAT WITH CONVECTION ALONG THE FRONT THIS EVENING.  THUS...THE
   COUNTIES OVER SERN VA INTO NERN NC SHOULD NOT BE CANCELLED FROM WW
   255 UNTIL SURFACE WINDS VEER TO A WLY COMPONENT.
   
   ..PETERS.. 04/28/2011
   
   
   ATTN...WFO...PHI...AKQ...MHX...LWX...RAH...ILM...
   
   LAT...LON   34707589 34057747 34157820 34647838 36147746 36617788
               38437691 38637580 38507561 38377471 37177538 36147528
               35407504 34967528 34707589