MESOSCALE DISCUSSION 0537
   NWS STORM PREDICTION CENTER NORMAN OK
   0109 PM CDT MON APR 20 2009
   
   AREAS AFFECTED...VA/NORTHERN NC
   
   CONCERNING...SEVERE POTENTIAL...WATCH LIKELY 
   
   VALID 201809Z - 202015Z
   
   SEVERE THREAT IS EXPECTED TO GRADUALLY INCREASE ACROSS PORTIONS OF
   VA/NORTHERN NC BY MID/LATE AFTERNOON. A TORNADO WATCH WILL LIKELY BE
   NEEDED WITHIN THE NEXT 1-2 HOURS ACROSS PORTIONS OF VA/NORTHERN NC.
   
   LATEST SURFACE ANALYSIS FEATURES A SECONDARY SUB-SYNOPTIC LOW ACROSS
   FAR WESTERN VA EARLY THIS AFTERNOON...WITH A GENERAL WEST-EAST
   ORIENTED WAVY WARM FRONT CONTINUING TO SLOWLY PROGRESS NORTHWARD
   ACROSS SOUTHERN VA. IN THE WAKE OF EARLY DAY PRECIPITATION AND CLOUD
   COVER...70+ F SURFACE TEMPERATURES AND UPPER 50S/LOWER 60S F
   DEWPOINTS ARE RESULTING IN A WEAKLY CAPPED/INCREASINGLY UNSTABLE
   BOUNDARY LAYER THIS AFTERNOON...WITH AROUND 500-900 J/KG MLCAPE.
   WITH INCREASING/DEEPENING CU FIELD NOTED PER VISIBLE IMAGERY...TSTMS
   SHOULD STEADILY INCREASE THROUGH MID/LATE AFTERNOON...AIDED BY A MID
   LEVEL IMPULSE CURRENTLY CROSS KY/TN PER WATER VAPOR IMAGERY. WITH
   30-35 KT SOUTHWESTERLY FLOW AROUND 1 KM...SOMEWHAT BACKED LOW LEVEL
   FLOW AHEAD OF THE SECONDARY SURFACE LOW WILL CONTRIBUTE TO
   SUFFICIENT LOW LEVEL SRH IN THE WARM SECTOR ACROSS SOUTHERN
   VA/NORTHERN NC FOR SUPERCELLS/POSSIBLE TORNADOES...ALTHOUGH THE
   PRIMARY TORNADO THREAT MAY BE TIED TO ANY STORM INTERACTING
   WITH/TRACKING PARALLEL TO THE WARM FRONT ACROSS VA.
   OTHERWISE...STEEP LAPSE RATES/AMPLE VERTICAL SHEAR WILL SUPPORT A
   RISK FOR LARGE HAIL/DAMAGING WINDS.
   
   ..GUYER.. 04/20/2009
   
   
   ATTN...WFO...AKQ...MHX...RAH...RNK...
   
   LAT...LON   37447902 38087736 37527650 35917703 35387805 36057973
               37447902