MESOSCALE DISCUSSION 2119
   NWS STORM PREDICTION CENTER NORMAN OK
   0951 PM CDT THU OCT 18 2007
   
   AREAS AFFECTED...MIDDLE TN...CNTRL/NERN MS...NRN/CNTRL AL
   
   CONCERNING...TORNADO WATCH 722...
   
   VALID 190251Z - 190415Z
   
   THE SEVERE WEATHER THREAT FOR TORNADO WATCH 722 CONTINUES.
   
   VALID PORTIONS OF TORNADO WATCH 722 EXPIRES AT 04Z.  A NEW TORNADO
   WATCH WILL BE ISSUED SHORTLY DOWNSTREAM FROM THE LINE OF TSTMS TO
   ENCOMPASS MOST OF MIDDLE TN SWD INTO PERHAPS CNTRL PORTIONS OF AL
   AND ECNTRL MS. 
   
   SRN PORTIONS OF THE UPR TROUGH MOVING INTO THE UPR GRTLKS REGION
   CONTINUES TO EDGE EWD INTO THE MID-SOUTH WITH DARKENING NOTED IN
   WATER VAPOR IMAGERY OVER THE SRN PLAINS.  THIS WILL CONTINUE TO 
   SUPPORT A LINE OF TSTMS FROM WRN TN SWWD INTO WCNTRL MS.  DOWNSTREAM
   AIR MASS HAS MOISTENED THROUGH THE DAY AND SUFFICIENT BUOYANCY WILL
   EXIST FOR A CONTINUED THREAT FOR STORMS.  PRIMARY SLY LLJ AXIS HAS
   BEEN SHIFTING NWD INTO THE OH VLY THIS EVENING WHERE STRONGEST
   LOW-LEVEL SHEAR EXISTS.  BUT...BMX/HTX/BNA VWPS AND 00Z BNA SOUNDING
   STILL EXHIBIT SUBSTANTIAL 0-1KM SHEAR ALONG WITH MORE THAN ENOUGH
   DEEP LAYER SHEAR TO SUPPORT ORGANIZED STORMS.  NUMBER OF STORMS THAT
   DEVELOPED HAS CONTRIBUTED TO A MORE LINEAR EVOLUTION...BUT THE
   THREAT FOR EMBEDDED SUPERCELLS WILL EXIST THROUGH EARLY MORNING
   FRIDAY DOWNSTREAM INTO MIDDLE TN AND NRN/CNTRL PORTIONS OF MS/AL. 
   IN FACT...IMPROVING VERTICAL SHEAR /OWING TO APPROACH OF THE SRN
   PLAINS JET STREAK/ MAY LEAD TO CELLS BECOMING MORE DISCRETE IN
   NATURE WITH TIME.  THUS...AT LEAST AN ISOLD TORNADO THREAT WILL
   CONTINUE ALONG WITH RISKS FOR DMGG WIND GUSTS.  AS SUCH...A NEW WW
   WILL BE ISSUED DOWNSTREAM SHORTLY.
   
   ..RACY.. 10/19/2007
   
   
   ATTN...WFO...MRX...OHX...BMX...HUN...MEG...JAN...
   
   33398990 36528727 36498522 32898633 32778911