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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

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