MESOSCALE DISCUSSION 1991
   NWS STORM PREDICTION CENTER NORMAN OK
   0840 PM CDT THU SEP 20 2007
   
   AREAS AFFECTED...FAR NE NEB...SE SD AND FAR SW MN
   
   CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 
   
   VALID 210140Z - 210315Z
   
   AN ISOLATED SEVERE THREAT SHOULD CONTINUE ACROSS FAR NE NEB
   SPREADING NEWD INTO SERN SD OVER THE NEXT 1 TO 2 HOURS. THE ACTIVITY
   SHOULD REMAIN RELATIVELY ISOLATED WITH LARGE HAIL AND ISOLATED WIND
   DAMAGE LIKELY. A TORNADO WILL ALSO BE POSSIBLE. IF CONVECTIVE
   COVERAGE INCREASES MORE THAN EXPECTED...A WW COULD BECOME NECESSARY
   ACROSS THE REGION LATE THIS EVENING.
   
   ISOLATED THUNDERSTORMS ARE ONGOING NEAR A MAXIMUM IN INSTABILITY
   WITH MLCAPE VALUES LOCALLY EXCEEDING 3000 J/KG. THE LATEST RUC MODEL
   SUGGESTS THE ACTIVITY MAY BE ASSOCIATED WITH A VERY SUBTLE SHORTWAVE
   TROUGH. RECENT RADAR TRENDS SUGGEST THE STORMS HAVE BEEN INCREASING
   AND THE INSTABILITY IN PLACE SHOULD SUPPORT A CONTINUED SEVERE
   THREAT FOR SEVERAL MORE HOURS. VERTICAL SHEAR AROUND 40 KT AND STEEP
   MID-LEVEL LAPSE RATES WILL BE SUFFICIENT FOR SUPERCELLS WITH LARGE
   HAIL. IN ADDITION...THE LATEST WSR-88D VWP FROM SIOUX FALLS SHOWS A
   LOOPED LOW-LEVEL HODOGRAPH SUGGESTING AN ISOLATED TORNADO THREAT MAY
   ALSO EXIST. WIND DAMAGE WILL ALSO BE POSSIBLE ESPECIALLY IF A SHORT
   BOWING LINE SEGMENT CAN ORGANIZE LATE THIS EVENING.
   
   ..BROYLES.. 09/21/2007
   
   
   ATTN...WFO...FSD...OAX...LBF...UNR...
   
   44399633 44169707 43569881 43009968 42199942 41949915
   41959859 42289791 42759700 43619555 44209562