
 URGENT - IMMEDIATE BROADCAST REQUESTED
   TORNADO WATCH NUMBER 251
   NWS STORM PREDICTION CENTER NORMAN OK
   535 PM CDT SAT MAY 5 2012   
   THE NWS STORM PREDICTION CENTER HAS ISSUED A
   TORNADO WATCH FOR PORTIONS OF    
          EXTREME NORTHWEST IOWA
          CENTRAL AND NORTHEAST NEBRASKA
          EXTREME SOUTHEAST SOUTH DAKOTA   
   EFFECTIVE THIS SATURDAY AFTERNOON AND EVENING FROM 535 PM UNTIL
   1100 PM CDT.   
   TORNADOES...HAIL TO 3 INCHES IN DIAMETER...THUNDERSTORM WIND
   GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE
   AREAS.   
   THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 35 STATUTE
   MILES NORTH AND SOUTH OF A LINE FROM 50 MILES WEST SOUTHWEST OF
   BROKEN BOW NEBRASKA TO 35 MILES NORTH NORTHEAST OF SIOUX CITY
   IOWA.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED
   WATCH OUTLINE UPDATE (WOUS64 KWNS WOU1).   
   REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR
   TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
   AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
   THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
   AND POSSIBLE WARNINGS.   
   OTHER WATCH INFORMATION...CONTINUE...WW 248...WW 249...WW 250...   
   DISCUSSION...STRONG SURFACE HEATING/DEEP MIXING IN THE WARM SECTOR
   HAS REDUCED CONVECTIVE INHIBITION ACROSS NEB...AND VISIBLE IMAGERY
   IS BEGINNING TO SHOW DEEPENING CUMULUS ALONG THE SURFACE FRONT FROM
   SE OF LBF TO SW OF YKN.  THE SITUATION IS COMPLICATED BY LOWER
   DEWPOINTS IN THE WARM SECTOR AND A QUICK TRANSITION TO STRONGER
   SHEAR/COOLER TEMPERATURES ACROSS THE FRONT...LEAVING ONLY A NARROW
   CORRIDOR WITH SURFACE-BASED SUPERCELL/TORNADO POTENTIAL. 
   STILL...THE ORIENTATION OF THE FRONT APPEARS TO BE ROUGHLY PARALLEL
   TO EXPECTED STORM MOTION...SO ANY STORMS THAT FORM ALONG THE
   IMMEDIATE SURFACE FRONT COULD REMAIN IN THE NARROW FAVORABLE
   CORRIDOR LONG ENOUGH TO BE A TORNADO THREAT.  IN ADDITION TO THE
   TORNADO RISK...VERY LARGE HAIL WILL BE POSSIBLE WITH ANY SUSTAINED
   STORMS THROUGH LATE EVENING.   
   AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
   SURFACE AND ALOFT TO 3 INCHES. EXTREME TURBULENCE AND SURFACE
   WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
   550. MEAN STORM MOTION VECTOR 24025.      
   ...THOMPSON