SEL7
   
   URGENT - IMMEDIATE BROADCAST REQUESTED
   SEVERE THUNDERSTORM WATCH NUMBER 617
   NWS STORM PREDICTION CENTER NORMAN OK
   1255 AM CDT TUE JUL 13 2004
   
   THE NWS STORM PREDICTION CENTER HAS ISSUED A
   SEVERE THUNDERSTORM WATCH FOR PORTIONS OF 
   
          NORTHERN AND CENTRAL IOWA
   
   EFFECTIVE THIS TUESDAY MORNING FROM 1255 AM UNTIL 700 AM CDT.
   
   HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70
   MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.
   
   THE SEVERE THUNDERSTORM WATCH AREA IS ALONG AND 100 STATUTE MILES
   EAST AND WEST OF A LINE FROM 40 MILES NORTHEAST OF SPENCER IOWA TO
   45 MILES EAST SOUTHEAST OF DES MOINES IOWA.
   
   REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE
   FAVORABLE FOR 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. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY DO PRODUCE
   TORNADOES.
   
   OTHER WATCH INFORMATION...THIS SEVERE THUNDERSTORM WATCH REPLACES
   TORNADO WATCH NUMBER 613...SEVERE THUNDERSTORM WATCH NUMBER 615.
   WATCH NUMBER 613 615 WILL NOT BE IN EFFECT AFTER 1255 AM CDT.
   CONTINUE...WW 616...
   
   DISCUSSION...MCS LIKELY WILL CONGEAL TONIGHT ACROSS WW AS SWLY LLJ
   FOCUSES PERSISTENT CONVECTION ALONG AND ON COOL SIDE OF SURFACE
   OUTFLOW BOUNDARY CURRENTLY EXTENDING SE-NW ACROSS IA.  VERY STEEP
   LAPSE RATES EVIDENT ON 00Z SOUNDINGS SUGGEST AIR MASS WITHIN WARM
   SECTOR WILL BECOME INCREASINGLY CAPPED.  HOWEVER...THIS WILL ALSO
   SUPPORT STRONG ELEVATED INSTABILITY.  THUS...LOW LEVEL WAA SHOULD
   SUSTAIN A THREAT OF LARGE HAIL WITH STORMS OVERNIGHT.  SHOULD STORMS
   CONGEAL ALONG A COMMON OUTFLOW...BOW ECHO AND DAMAGING WINDS WOULD
   BECOME DOMINANT THREAT.
   
   AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT
   TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60
   KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500. MEAN STORM
   MOTION VECTOR 29025.
   
   
   ...EVANS