Dissipating thunderstorms took place in a highly unstable atmosphere during the early evening hours. Downdraft CAPE values were between 1500 and 2000 J/kg.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS DMX wrote it up.
Dissipating thunderstorms took place in a highly unstable atmosphere during the early evening hours. Downdraft CAPE values were between 1500 and 2000 J/kg. There was sufficient CAPE of 3000 to 5000 J/kg with lifted indices around -10 C. High winds occurred in Audubon and Adair Counties. Winds were measured up to 60 MPH in Adair County. Tree damage was also reported. A spotter in Adair County reported a sudden temperature increase from 76 F to 96 F in a few minutes, along with a sharp drop in humidity.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Des Moines, IA wrote its last area forecast discussion before the first event at Aug 23, 5:28 PM CST, 37 minutes earlier.
AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE DES MOINES IA 628 PM CDT TUE AUG 23 2011 ...UPDATED AVIATION DISCUSSION... .SHORT TERM /TONIGHT/... COMPLEX FORECAST THROUGH THE EVENING. THERE IS A SHORT WAVE MOVING ACROSS THE NORTHERN PLAINS. FORCING WITH THIS FEATURE INTO AN UNSTABLE AIRMASS HAS BEEN SUCCESSFUL IN DEVELOPING THUNDERSTORMS OVER NORTH CENTRAL NEBRASKA. THE ENTIRE AREA HAS VERY STEEP MID LEVEL LAPSE RATES AND DECENT THETA ADVECTION BUT WILL BE FIGHTING A FAIRLY STOUT CAP IN PLACE. I AM REASONABLY CONFIDENT THAT THE CONVECTION WILL HOLD TOGETHER AND WILL DRIFT ACROSS THE SOUTHWEST CWA LATE THIS AFTERNOON AND EARLY EVENING. AS A RESULT...20/30 POPS HAVE BEEN ADDED AND A PRE FIRST PERIOD WILL BE INCLUDED IN THE ZONES FOR THE EXPECTATION OF THUNDERSTORMS. WHILE AN ISOLATED SEVERE STORM IS POSSIBLE...I DO NOT HAVE ENOUGH CONFIDENCE TO THINK THAT THERE WILL BE A WIDESPREAD SEVERE THREAT. ACROSS THE SOUTHWEST. THE CONVECTION IS MOVING INTO AN AREA OF 4000+ J/KG OF CAPE AND MODEST SHEAR BUT THE BETTER SHEAR IS OVER EASTERN IOWA AND LOCATIONS EAST. ATTENTION NOW SHIFTS TO THE NORTHEAST WHERE A WARM FRONT EXISTS FROM MSP TO DEH THEN INTO NORTHERN ILLINOIS. AS THE SHORTWAVE SHIFTS EAST LATE AFTERNOON AND EVENING IT SHOULD PROVIDE GOOD FORCING AND IN AN ENVIRONMENT WITH STRONG CAPE VALUES AND STRONG SHEAR. EVEN IF STORMS DEVELOP IN MN WHICH IS MORE LIKELY...MEAN FLOW AND CORFIDI VECTORS BOTH TAKE CONVECTION INTO EASTERN IOWA. AT THIS POINT THOUGH BASED ON THE PLACEMENT OF THE WARM FRONT...IT APPEARS AS THOUGH MOST OF THE EASTERN PORTION OF OUR CWA WILL MISS OR BE ON THE FRINGE OF THE ACTION. ALO APPEARS TO BE THE MOST AT RISK. HAVING SAID ALL OF THIS...THE SHORT WAVE IS PROGRESSIVE AND SHOULD BE EAST OF THE AREA BY 00Z THUS ENDING THE THREAT FOR STORMS OVER THE CWA. AS FOR TEMPS...CONVECTION AND ASSOCIATED CLOUD COVER IS MESSING WITH HIGHS BUT WARM ADVECTION ALOFT AND SOME MIXING THAT I EXPECT TO OCCUR SHOULD KEEP TEMPS A BIT WARMER THAN THE GFS AND CLOSER TO THE NAM. .LONG TERM /WEDNESDAY THROUGH TUESDAY/... EXTENDED PATTERN VERY SIMILAR TO PAST SEVERAL WEEKS. QUIET BEGINNING TO LONG TERM...WITH HIGH PRESSURE AND RIDGE BUILDING AS LOW IN CANADA PUSHES EASTWARD. FOR MID WEEK...WILL BE WARM AND A BIT MUGGY...AS SOUTHERLY FLOW RETURNS AS HIGH PUSHES EAST...ALONG WITH WAA. TOWARDS END OF WEEK...ANOTHER AREA OF LOW PRESSURE IN CANADA WILL PUSH EASTWARD...WITH TRAILING FRONT EXPECTED TO PUSH SE ACROSS IOWA. MAIN FOCUS FOR EXTENDED IS WITH THIS FROPA AS MODELS HAVE CONTINUED TO FLIP FLOP A BIT WITH TIMING AND INTENSITY. APPEARS THAT MOISTURE ALONG THIS FRONT WILL BE LACKING...AND FRONT IS A BIT WEAKER THAN PREVIOUSLY THOUGHT. THEREFORE...HAVE CUT BACK SIGNIFICANTLY ON POPS. ISENTROPIC ANALYSIS DOES SHOW A PUSH OF MOISTURE WHICH COULD BE ENOUGH TO GET MORE ROBUST SHOWERS/THUNDERSTORMS...BUT KEEPS MOISTURE WELL WEST OF CWA. TIMING OF FRONT REMAINS UNCERTAIN AS WELL...BUT MODELS DO CONTINUE TO SLOW FRONT...BRINGING IT THROUGH LATE FRIDAY NIGHT...ALSO LIMITING CHANCES FOR PRECIPITATION. FOR EARLY NEXT WEEK THROUGH END OF EXTENDED...LOOKS LIKE CHANCES FOR PRECIPITATION OFF AND ON AS YET ANOTHER FRONT PUSHES EASTWARD...AS WELL AS A SHORTWAVE ACROSS NE/IA. ADDITIONALLY...TEMPERATURES WILL BE SIGNIFICANTLY COOLER AS WELL. && .AVIATION...24/00Z VFR CONDITIONS ARE EXPECTED THROUGH THE TAF PERIOD. ISOLD-SCT TSTMS WILL AFFECT FAR NERN AND SWRN IA THIS EVE BUT SHOULD NOT AFFECT ANY OF THE TERMINALS. HIGH CIGS AROUND FL150-200 CAN BE EXPECTED THROUGH TONIGHT...ALONG WITH POSSIBLY SOME BRIEF MVFR BR TOWARD SUNRISE ESPECIALLY AT KDSM/KOTM...THEN BTWN ABT 09Z AND 14Z A FRONT WILL MOVE NW TO SE ACROSS THE AREA BRINGING STRONGER NNW BREEZES ON WED. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && SHORT TERM...FAB LONG TERM...WILKINS AVIATION...LEE
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE DES MOINES IA 327 PM CDT TUE AUG 23 2011 IAZ004>007-015>017-023>026-036-037-047>049-059>062-073>075-084>086- 096-097-242030- EMMET-KOSSUTH-WINNEBAGO-WORTH-PALO ALTO-HANCOCK-CERRO GORDO- POCAHONTAS-HUMBOLDT-WRIGHT-FRANKLIN-HAMILTON-HARDIN-BOONE-STORY- MARSHALL-DALLAS-POLK-JASPER-POWESHIEK-WARREN-MARION-MAHASKA-LUCAS- MONROE-WAPELLO-APPANOOSE-DAVIS- 327 PM CDT TUE AUG 23 2011 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...THIS AFTERNOON AND TONIGHT AN ISOLATED THUNDERSTORM IS POSSIBLE THROUGH THE EVENING HOURS. SEVERE STORMS ARE NOT EXPECTED BUT STRONGER STORMS COULD PRODUCE SMALL HAIL AND WIND GUSTS TO 40 MPH ALONG WITH BRIEF MODERATE TO HEAVY RAINFALL. LOCALIZED HEAT INDEX VALUES OF 100 TO 105 ARE FORECAST THROUGH 6 PM. .DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY A COUPLE PERIODS OF THUNDERSTORMS ARE POSSIBLE FRIDAY NIGHT THROUGH SUNDAY. A FEW SEVERE STORMS MAY OCCUR. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION WILL NOT BE NEEDED TODAY. IAZ027-028-038-039-050-242030- BUTLER-BREMER-GRUNDY-BLACK HAWK-TAMA- 327 PM CDT TUE AUG 23 2011 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...THIS AFTERNOON AND TONIGHT THUNDERSTORMS ARE POSSIBLE LATE THIS AFTERNOON THROUGH THIS EVENING. A FEW SEVERE STORMS ARE POSSIBLE WITH 60 MPH WIND GUSTS AND QUARTER SIZED HAIL THE MAIN THREATS. .DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY A COUPLE PERIODS OF THUNDERSTORMS ARE POSSIBLE FRIDAY NIGHT THROUGH SUNDAY. A FEW SEVERE STORMS MAY OCCUR. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION MAY BE NEEDED THIS EVENING. IAZ033>035-044>046-057-058-070>072-081>083-092>095-242030- SAC-CALHOUN-WEBSTER-CRAWFORD-CARROLL-GREENE-AUDUBON-GUTHRIE-CASS- ADAIR-MADISON-ADAMS-UNION-CLARKE-TAYLOR-RINGGOLD-DECATUR-WAYNE- 327 PM CDT TUE AUG 23 2011 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...THIS AFTERNOON AND TONIGHT SCATTERED THUNDERSTORMS ARE EXPECTED THIS AFTERNOON AND EVENING. WHILE WIDESPREAD SEVERE STORMS ARE NOT EXPECTED...AN ISOLATED STORM MAY PRODUCE LARGE HAIL AND DAMAGING WIND GUSTS TO 60 MPH. HEAT INDEX VALUES OF 100 TO 105 ARE FORECAST THROUGH 6 PM WITH LOCAL HEAT INDIES BRIEFLY REACHING BETWEEN 105 AND 110. .DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY A COUPLE PERIODS OF THUNDERSTORMS ARE POSSIBLE FRIDAY NIGHT THROUGH SUNDAY. A FEW SEVERE STORMS MAY OCCUR. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION IS NOT EXPECTED THIS AFTERNOON OR EVENING. FAB
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE DES MOINES IA 247 PM CDT TUE AUG 23 2011 IAZ044-045-057-232200- CRAWFORD-CARROLL-AUDUBON- INCLUDING THE CITIES OF...DENISON...CARROLL...AUDUBON 247 PM CDT TUE AUG 23 2011 ...LOCALIZED HEAT INDICES BETWEEN 105 AND 110 DEGREES POSSIBLE FOR A COUPLE HOURS THIS AFTERNOON... WARMER AND MORE HUMID AIR IS MOVING INTO THE AREA FROM THE SOUTHWEST. A FEW LOCATIONS ACROSS FAR WEST CENTRAL IOWA HAVE CLIMBED INTO THE 90S WITH DEWPOINTS AROUND 80 IN THE LAST HOUR. THIS HAS PRODUCED LOCALIZED HEAT INDICES BETWEEN 105 AND 110 DEGREES. SOME THUNDERSTORMS OVER NORTHEAST NEBRASKA ARE EXPECTED TO PRODUCE CLOUD COVER AND PERHAPS SOME RAINFALL WHICH WILL LOWER TEMPERATURES LATER THIS AFTERNOON. THOSE SENSITIVE TO HIGH HUMIDITY AND HOT TEMPERATURES SHOULD SEEK SHELTER FROM THE OUTDOORS FOR THE NEXT COUPLE HOURS. FAB
PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
606 PM CDT TUE AUG 23 2011
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0557 AM HAIL 3 SE WEBSTER CITY 42.43N 93.77W
08/23/2011 E1.00 INCH HAMILTON IA PUBLIC
&&
SKOWPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
653 PM CDT TUE AUG 23 2011
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0625 AM HAIL 4 N RADCLIFFE 42.38N 93.43W
08/23/2011 E0.50 INCH HARDIN IA PUBLIC
&&
SKOWThe national radar mosaic for the moment the first event began.
1 strong wind, 1 high wind. Local standard time, as filed.
A tree reported down on a power line. Trained spotter also reported limbs down. Estimated winds around 40 MPH and temperature increase from 76 F to 96 F in a few minutes along with a sharp drop in humidity. Likely heat burst.
Measured at the Fontanelle Schoolnet site.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2011Every state in 2011This day in weather historyWarnings in Iowa, August 2011
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 339835, 325683.