MESOSCALE DISCUSSION 1838
   NWS STORM PREDICTION CENTER NORMAN OK
   0617 PM CDT TUE AUG 21 2007
   
   AREAS AFFECTED...NERN NEB...PARTS OF SE SD...NRN IA...EXTREME SRN MN
   
   CONCERNING...TORNADO WATCH 631...
   
   VALID 212317Z - 220045Z
   
   THE SEVERE WEATHER THREAT FOR TORNADO WATCH 631 CONTINUES.
   
   TRENDS WILL BE MONITORED FOR AN ADDITIONAL WW TO THE EAST OF WW 631.
   
   VIGOROUS CONVECTIVE DEVELOPMENT IS ONGOING WEST OF THE YANKTON SD/
   NORFOLK NEB AREAS.  THIS IS BEING AIDED BY FORCING FOR LARGE-SCALE
   ASCENT NEAR A WEAK FRONTAL WAVE...IN AN ENVIRONMENT WITH EXTREME
   INSTABILITY AND STRONG DEEP LAYER SHEAR BENEATH 50 KT WESTERLY 500
   MB FLOW ON THE SOUTHERN FRINGE OF POLAR TROUGH.  MIXED LAYER CAPE IN
   EXCESS OF 4000 J/KG WILL MAINTAIN INTENSE UPDRAFTS...AND MODERATELY
   LARGE LOW-LEVEL HODOGRAPHS BENEATH 30+ KT SOUTHWESTERLY 850 FLOW
   WILL SUPPORT A CONTINUING RISK FOR TORNADOES...IN ADDITION TO LARGE
   HAIL...THROUGH AT LEAST 01-02Z...PARTICULARLY WITH ACTIVITY
   WEST/NORTHWEST OF NORFOLK.  THROUGH 01-02Z... CONTINUED UPSCALE
   CONVECTIVE GROWTH INTO AN EXPANDING CONVECTIVE CLUSTER IS
   ANTICIPATED.  AND...UNSATURATED LOWER/MID TROPOSPHERIC PROFILES WILL
   PROMOTE STRONG EVAPORATIVE COOLING NEAR HEAVY PRECIPITATION...WITH
   AN INCREASING RISK FOR DAMAGING DOWNBURSTS AND SURFACE COLD POOL
   DEVELOPMENT EASTWARD INTO NORTHERN/WESTERN IOWA THEREAFTER.
   
   ..KERR.. 08/21/2007
   
   
   ATTN...WFO...DMX...FSD...OAX...GID...LBF...
   
   42159887 42519871 42949796 43439642 43419573 43009461
   41999531 41549689 41439830 41409895