MESOSCALE DISCUSSION 1797
   NWS STORM PREDICTION CENTER NORMAN OK
   1110 PM CDT SAT AUG 18 2007
   
   AREAS AFFECTED...PORTIONS SERN SD...NERN NEB...NWRN/N-CENTRAL IA.
   
   CONCERNING...TORNADO WATCH 615...
   
   VALID 190410Z - 190600Z
   
   CORRECTED FOR WW TYPE
   
   THE SEVERE WEATHER THREAT FOR TORNADO WATCH 615 CONTINUES.
   
   CONVECTION IS SLOW TO DEVELOP ACROSS SERN SD...LIKELY RELATED TO
   LACK OF LIFT RESULTING FROM COMBINATION OF SHALLOW SLOPE OF FRONTAL
   ZONE IN THIS AREA AND LOSS OF SFC DIABATIC HEATING.  SHOWERS AND
   WEAK TSTMS ARE MOVING ENEWD APPROXIMATELY 20 KT OVER PORTIONS BOH
   HOMME/HUTCHINSON COUNTIES -- AS OF 445Z -- WITH SOME POTENTIAL FOR
   INTENSIFICATION DURING NEXT 2-3 HOURS AS ACTIVITY MOVES TOWARD FSD
   AREA...NWRN CORNER IA AND SWRN CORNER MN.  MESOANALYSIS SHOWS
   FRONTAL-WAVE LOW BETWEEN MHE-YKN...WITH QUASISTATIONARY FRONTAL ZONE
   EWD OVER NWRN IA THEN SEWD ACROSS CENTRAL IA.  LOW LEVEL VERTICAL
   SHEAR WILL REMAIN MAXIMIZED AND MOST FAVORABLE FOR SUPERCELLS ALONG
   AND JUST N OF FRONT...WITH 0-3 KM SRH AOA 300 J/KG ESTIMATED FROM
   FSD AREA EWD.  MODIFIED RUC SOUNDINGS SUGGEST THAT EFFECTIVE PARCELS
   WILL REMAIN ROOTED AT OR VERY NEAR SFC ALONG FRONT FOR ANOTHER 2-3
   HOURS...PARTICULARLY NEAR AND ESE OF SFC WAVE WHERE STRONGEST SFC
   THETAE PERSISTS.  RICH LOW LEVEL MOISTURE WILL CONTINUE OVER
   REGION...ESPECIALLY E AND SE OF SFC LOW WHERE NARROW CORRIDOR OF
   1500-2500 J/KG MLCAPE WILL CONTINUE THROUGH ABOUT 05Z.  SBCINH IS
   FCST TO INCREASE GRADUALLY...HOWEVER...AS MOIST SFC AIR MASS
   SLOWLY/DIABATICALLY COOLS.
   
   OTHERWISE...ALTHOUGH AIR MASS REMAINS FAVORABLY MOIST AND SHEARED
   ALONG/S OF FRONT...OTHER FACTORS -- INCLUDING WEAK FORCING AND LACK
   OF SIGNIFICANT SHALLOW CONVECTION EVIDENT IN IR IMAGERY  -- INDICATE
   SEVERE PROBABILITIES ARE DIMINISHING.  LOOSELY ORGANIZED BAND OF
   STG-SVR TSTMS -- MOVING EWD 20-25 KT INTO WRN-MOST CHERRY COUNTY NEB
   -- SHOULD NOT REACH WW BEFORE SCHEDULED EXPIRATION.  THEREFORE...PER
   COORDINATION W/LBF/OAX...SOME COUNTIES S OF SD/NEB BORDER MAY BE
   CLEARED BEFORE 5Z.
   
   ..EDWARDS.. 08/19/2007
   
   
   ATTN...WFO...ARX...MPX...DMX...FSD...OAX...ABR...LBF...UNR...
   
   42090014 42980020 42999949 43519956 43559931 43959934
   43929895 44249885 44179641 43849645 43839482 43519487
   43509304 42189298 42219640 42429744 42429832 42069828