A strong line of thunderstorms formed along a cold front to the west of Iowa. As it approached Iowa, the line weakened as there was a very strong cap in place over Iowa and low level winds became divergent.
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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.
A strong line of thunderstorms formed along a cold front to the west of Iowa. As it approached Iowa, the line weakened as there was a very strong cap in place over Iowa and low level winds became divergent. The atmosphere was quite unstable as the line approached with about 2000 J/kg of CAPE available and a lifted index of -5 C. The freezing level was quite high, around 15,000 feet. Downdraft CAPE was very high and was between 1400 and 1600 J/kg. In spite of a 65 to 70 kt low level jet, the storms weakened as they moved into Iowa. One of the storms bowed out in the Denison area of Crawford County and produced a 63 MPH wind gust at the Denison Airport. Tree and power line damage occurred in Dow City and the town of Denison. A shed was blown over near Dow City.
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 May 30, 10:42 PM CST, 40 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DES MOINES IA 1142 PM CDT MON MAY 30 2011 ...UPDATED AVIATION DISCUSSION... .SHORT TERM /TONIGHT/... A STRONG AREA OF LOW PRESSURE WITH DYNAMIC FLOW ALOFT IS SET UP ACROSS THE WESTERN PLAINS AND THE MID MISSOURI RIVER VALLEY THIS AFTERNOON. THE UPPER LEVEL LOW OVER WYOMING WILL CONTINUE TO LIFT INTO MONTANA AND NORTH DAKOTA OVERNIGHT. AS THIS OCCURS...THE SFC LOW PRESSURE WILL LIFT NORTHEAST THROUGH THE NEBRASKA AND THE EASTERN DAKOTAS THIS EVENING WITH A TRAILING FRONTAL BOUNDARY MOVING INTO IOWA OVERNIGHT. TWO FOCUS AREAS FOR STORM INITIATION THIS AFTERNOON AND EVENING WITH THE FIRST OVER EASTERN SOUTH DAKOTA AND NORTH CENTRAL NEBRASKA AND THE OTHER OVER SOUTH CENTRAL NEBRASKA. THESE TWO COMPLEXES MAY SPLIT AS THEY MOVE INTO WESTERN IOWA THIS EVENING AND THEREFORE PORTIONS OF THE EASTERN FORECAST AREA MAY REMAIN. OVERALL...DISCRETE STORMS SHOULD DEVELOP TO THE WEST INITIALLY BEFORE MERGING INTO A FORWARD PROPAGATING LINE THAT WILL MOVE INTO IOWA THIS EVENING AND INTO CENTRAL IA AROUND 06-08Z. A DAMAGING WIND THREAT WILL ACCOMPANY THE STORMS WITH 50KTS OF WIND IN PLACE JUST ABOVE THE SFC. IN ADDITION...EFFECTIVE HELICITY VALUES OF 200-300 M2/S2 WILL BE IN PLACE WEST OF INTERSTATE 35 BEFORE WEAKENING TO THE EAST. THEREFORE...A POSSIBILITY EXISTS OF A BRIEF TORNADO ALONG THE LEADING EDGE OF THESE STORMS. .LONG TERM /TUESDAY THROUGH MONDAY/... THE MAIN CONCERN FOR THE EXTENDED CONTINUES TO BE AROUND WEDNESDAY NIGHT INTO THURSDAY MORNING WHEN A SHORTWAVE RIDING THE UPPER LEVEL RIDGE WILL IMPACT A FRONTAL BOUNDARY ACROSS NORTHERN IOWA THAT IS TIED TO A SURFACE LOW OVER THE NORTHERN PLAINS. THERE IS STILL SOME QUESTION AS TO THE EXACT PLACEMENT OF THE FRONTAL BOUNDARY BUT CONSENSUS WOULD PLACE THE FRONT ALONG AN ALGONA TO DUBUQUE LINE LIFTING SLOWLY NORTH THROUGH THURSDAY EARLY AFTERNOON. THE ATMOSPHERE IS FAIRLY JUICED WITH DEWPOINTS CLIMBING INTO THE 60S TO AROUND 70 BY LATE THURSDAY. THE ATMOSHPERE IS UNCAPPED WEDNESDAY NIGHT WITH CAPES OF 2500-3000 J/KG. SHEAR IS FAIRLY UNIMPRESSIVE BUT THERE IS AN AREA OF STRONG THETA ADVECTION THAT LIFTS ACROSS THE FORECAST AREA LATE WEDNESDAY AFTERNOON AND NIGHT. SOME FRONTOGENETICAL FORCING AS WELL LATE WEDNESDAY NIGHT INTO THURSDAY MORNING. EXPECT STORMS TO FIRE MAINLY ALONG THE WARM FRONT WEDNESDAY EVENING THEN LIFT ACROSS THE CWA OVERNIGHT AND BE OVER THE EAST THURSDAY MORNING. BY THE AFTERNOON A WEAK CAP BUILDS INTO MUCH OF THE CWA WHICH SHOULD END THE STORMS...UNLESS SOME ELEVATED STORMS ARE ABLE TO MAINTAIN THEMSELVES. THE RAIN ENDS AND THE HEAT IS ON FOR FRIDAY BUT FOR THE WEEKEND SEVERAL WEAK RIDGE RIDER SHORTWAVES ARE PICKED UP BY THE MODELS ONCE AGAIN BUT TIMING OF THESE SHORTWAVES AND LACK OF EVIDENCE OF A FRONTAL BOUNDARY MAKES IT HARD TO PUT POPS IN THE EXTENDED. THERE WAS SOME MID LEVEL FORCING PROGGED TO PUSH ACROSS THE FORECAST AREA AT TIMES AND I TRIED TO TIE ANY CHANCE OF PRECIP WITH THE FORCING BUT VERY LOW CONFIDENCE SO I DID NOT GO MORE THAN SLIGHT CHANCES. TEMPS AT OR ABOVE SEASONAL NORMALS THROUGH THE EXTENDED. && .AVIATION...31/06Z SKIES WILL REMAIN VFR WITH GUSTY WINDS AHEAD OF AN APPROACHING LINE OF STORMS MOVING EAST FROM NEBRASKA. ANTICIPATE STORMS TO BEGIN TO DIE OFF AS THEY ENTER IOWA...ONLY AFFECTING KFOD. COULD SEE AFFECT AT KDSM...KMCW AND KALO...BUT MOST LIKELY WILL HAVE MINIMAL IMPACTS. WINDS BEHIND SYSTEM WILL BE FROM NORTHWEST...WITH VFR CEILINGS. VFR CEILINGS WILL PREVAIL THROUGH TUESDAY...WITH WINDS AGAIN GUSTING IN THE AFTERNOON HOURS BEFORE DECREASING LATE IN THE PERIOD. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && SHORT TERM...DONAVON LONG TERM...FAB AVIATION...WILKINS
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE DES MOINES IA 747 PM CDT MON MAY 30 2011 IAZ004>006-015-016-023>025-033>036-044>047-057>059-070>072-081-082- 092-093-010100- EMMET-KOSSUTH-WINNEBAGO-PALO ALTO-HANCOCK-POCAHONTAS-HUMBOLDT-WRIGHT- SAC-CALHOUN-WEBSTER-HAMILTON-CRAWFORD-CARROLL-GREENE-BOONE-AUDUBON- GUTHRIE-DALLAS-CASS-ADAIR-MADISON-ADAMS-UNION-TAYLOR-RINGGOLD- 747 PM CDT MON MAY 30 2011 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...TONIGHT THUNDERSTORMS ARE EXPECTED ACROSS THE AREA TONIGHT. SOME OF THESE STORMS MAY BE SEVERE WITH DAMAGING WINDS IN EXCESS OF 60 MPH AND QUARTER SIZED HAIL POSSIBLE. AN ISOLATED TORNADO OR TWO MAY ALSO OCCUR. THE PRIMARY THREAT TIME IS BETWEEN 9 PM AND 4 AM. RIVER FLOOD WARNINGS REMAIN IN EFFECT ACROSS PORTIONS OF THE NORTHERN DES MOINES RIVER BASIN. PLEASE REFER TO THE NATIONAL WEATHER SERVICE WEBSITE AT WEATHER.GOV/DESMOINES FOR FURTHER DETAILS. .DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY ADDITIONAL THUNDERSTORMS ARE EXPECTED BETWEEN LATE WEDNESDAY AFTERNOON AND THURSDAY MORNING. A FEW SEVERE STORMS WITH LARGE HAIL MAY OCCUR. HEAVY RAINFALL IS ALSO A THREAT. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION MAY BE REQUIRED TONIGHT. IAZ007-017-026>028-037>039-048>050-060>062-073>075-083>086-094>097- 010100- WORTH-CERRO GORDO-FRANKLIN-BUTLER-BREMER-HARDIN-GRUNDY-BLACK HAWK- STORY-MARSHALL-TAMA-POLK-JASPER-POWESHIEK-WARREN-MARION-MAHASKA- CLARKE-LUCAS-MONROE-WAPELLO-DECATUR-WAYNE-APPANOOSE-DAVIS- 747 PM CDT MON MAY 30 2011 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...TONIGHT SCATTERED THUNDERSTORMS ARE POSSIBLE LATE TONIGHT. A SEVERE STORM WITH 60 MPH WINDS IS POSSIBLE. .DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY ADDITIONAL THUNDERSTORMS ARE EXPECTED BETWEEN LATE WEDNESDAY AFTERNOON AND THURSDAY MORNING. A FEW SEVERE STORMS WITH LARGE HAIL MAY OCCUR. HEAVY RAINFALL IS ALSO A THREAT. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION MAY BE NEEDED WEDNESDAY NIGHT AND THURSDAY. AW
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE DES MOINES IA 702 PM CDT MON MAY 30 2011 IAZ004-005-015-023-024-033>035-044>047-057>059-070>072-081>083- 092>094-310200- EMMET-KOSSUTH-PALO ALTO-POCAHONTAS-HUMBOLDT-SAC-CALHOUN-WEBSTER- CRAWFORD-CARROLL-GREENE-BOONE-AUDUBON-GUTHRIE-DALLAS-CASS-ADAIR- MADISON-ADAMS-UNION-CLARKE-TAYLOR-RINGGOLD-DECATUR- INCLUDING THE CITIES OF...ESTHERVILLE...ALGONA...EMMETSBURG... POCAHONTAS...HUMBOLDT...SAC CITY...ROCKWELL CITY...FORT DODGE... DENISON...CARROLL...JEFFERSON...BOONE...AUDUBON... GUTHRIE CENTER...ADEL...ATLANTIC...GREENFIELD...WINTERSET... CORNING...CRESTON...OSCEOLA...BEDFORD...MOUNT AYR...LEON 702 PM CDT MON MAY 30 2011 ...GUSTY SOUTH WINDS WILL CONTINUE THROUGH EVENING... STRONG GUSTY SOUTH WINDS WILL CONTINUE ACROSS CENTRAL IOWA THROUGHOUT THE EVENING HOURS. SUSTAINED WINDS WILL BE AROUND 25 MPH...WITH GUSTS OF 35 TO 40 MPH POSSIBLE. WINDS WILL GRADUALLY BEGIN TO DIMINISH AFTER SUNSET. STRONG WINDS MEAN THAT ANY UNSECURED OUTDOOR ITEMS MAY BLOW AROUND. STRONG WINDS WILL ALSO MAKE DRIVING DIFFICULT...ESPECIALLY FOR HIGH PROFILE VEHICLES AND ALONG EAST AND WEST ROUTES. AW
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE DES MOINES IA 1157 PM CDT MON MAY 30 2011 IAZ033-044-310700- SAC-CRAWFORD- INCLUDING THE CITIES OF...SAC CITY...DENISON 1157 PM CDT MON MAY 30 2011 ...LINE OF STRONG THUNDERSTORMS THROUGH 2 AM CDT TUESDAY MORNING... A LINE OF STRONG THUNDERSTORMS WILL BE MOVING ACROSS THE REGION EARLY THIS MORNING. THE STORMS WILL PRODUCE BRIEF HEAVY RAINFALL... SOME SMALL HAIL AND ALSO HAS THE POTENTIAL TO PRODUCE WIND GUSTS UP TO 50 MPH. THE STORMS ARE MOVING TO THE EAST AT 45 MPH. REV
The Omaha radar (KOAX), 100 km away, was scanning. The volume nearest the first event began at 05:20 UTC.
3 thunderstorm wind. Local standard time, as filed.
Trees were blown down across the highway and a shed was blown down.
Trees were blown down along with power lines.
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, May 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: 294961, 294959, 294962.