MESOSCALE DISCUSSION 1730
   NWS STORM PREDICTION CENTER NORMAN OK
   0742 PM CDT THU JUL 30 2009
   
   AREAS AFFECTED...PORTIONS OF TN/AL
   
   CONCERNING...TORNADO WATCH 640...642...
   
   VALID 310042Z - 310145Z
   
   THE SEVERE WEATHER THREAT FOR TORNADO WATCH 640...642...CONTINUES.
   
   THREAT FOR DMGG WINDS AND HEAVY RAINFALL WILL PERSIST ACROSS MUCH OF
   TN/AL...WITH A TORNADO THREAT PRIMARILY CONFINED TO WRN/MIDDLE TN.
   
   AN UPPER VORTICITY MAXIMUM NOTED ON WV IMAGERY NEAR THE AR/MS/TN
   BORDERS AND AN ATTENDANT SURFACE LOW ROUGHLY NEAR JACKSON TN
   CONTINUE TO PROVIDE FAVORABLE FORCING FOR ASCENT ACROSS THE WATCH
   AREA. 00Z SOUNDINGS REFLECT MODERATE INSTABILITY IS
   LINGERING...WHICH COMBINED WITH 40 TO 50 KT EFFECTIVE SHEAR WILL
   CONTINUE TO SUPPORT OCCASIONAL ROTATION...ALTHOUGH RECENT ROTATION
   NOTED ON RADAR HAS PRIMARILY BEEN CONFINED TO NRN PORTIONS OF BOWING
   SEGMENTS. OCCURRENCE OF SUPERCELLULAR STRUCTURES APPEARS TO BE
   WANING...AND MAY BE PARTLY DUE TO AN EXTENSIVE REGION OF LEADING
   EDGE STRATIFORM PRECIPITATION THAT HAS LIKELY COOLED MUCH OF THE
   AIRMASS AHEAD OF THE STRONGER CONVECTION...WITH A DECREASING NUMBER
   OF LIGHTNING STRIKES SUGGESTING UPDRAFTS MAY NOT BE AS ROBUST AS A
   FEW HOURS AGO. CONVECTION HAS DECREASED CONSIDERABLY ACROSS
   CNTRL/NRN MS AND CNTRL AL AND WW MAY BE CANCELLED EARLY IN THESE
   AREAS. HOWEVER...AS PREVIOUSLY MENTIONED...GIVEN THE UNSEASONABLY
   STRONG UPPER WAVE/FORCING...A THREAT WILL REMAIN IN AREAS CLOSEST TO
   THE SURFACE LOW. ABUNDANT MOISTURE/ABOVE NORMAL PWAT WILL ALSO
   SUPPORT A HEAVY RAIN THREAT AS WELL.
   
   ..HURLBUT.. 07/31/2009
   
   
   ATTN...WFO...OHX...BMX...HUN...MEG...
   
   LAT...LON   34598593 34238776 35098938 35838903 35898765 35148639
               34598593