A cold front moved southeast across the state during the day on the 21st. The airmass was moderately unstable with CAPE values ahead of the front around 2000 J/kg and lifted indices of -3 to -6 C.
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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 cold front moved southeast across the state during the day on the 21st. The airmass was moderately unstable with CAPE values ahead of the front around 2000 J/kg and lifted indices of -3 to -6 C. Downdraft CAPE was between 800 and 1000 J/kg, with 500 J/kg of CAPE available in the -10 to -30 C layer of the atmosphere. Precipitable water values increased to between 1.5 and 2 inches. Sufficient shear was available with about 40 kts of shear. The freezing level was quite high, around 14,000 feet. A line of thunderstorms ahead of the cold front intensified around mid day. The storms continued to track east across the state during the afternoon. One of the storms produced high winds in the Sully area of Jasper County. The air exchanger and part of the roof was torn off the gym building of the Sully school. One of the storms produced one and one half inch diameter hail in Guthrie County, causing damage to buildings and vehicles there. The storms produced heavy rainfall in south central Iowa, with reports of 2 to 3.5 inches in a short time received. Flash flooding occurred in Decatur County with rushing water reported over the roads north of Pleasanton. A county road was washed out in southern Decatur County in Morgan Township. As the thunderstorms moved through Tama County, lightning struck a house in Traer, setting it on fire. Minor damage occurred.
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 Sep 21, 5:23 AM CST, 2 hours 30 minutes earlier.
AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE DES MOINES IA 623 AM CDT TUE SEP 21 2010 ...UPDATED AVIATION DISCUSSION... .SHORT TERM /TODAY/... COMPLEX AND ACTIVE FORECAST FOR TODAY AS COLD FRONT MOVES SOUTHEAST ACROSS THE CWA COINCIDENT WITH ONE OR MORE SHORTWAVES PASSING OVERHEAD. WIDE SWATH OF SHOWERS AND THUNDERSTORMS HAS MOVED IN FROM THE NORTHWEST OVERNIGHT AND IS CURRENTLY AFFECTING OUR NORTHERN AND WESTERN COUNTIES. AS THE LEADING SHORTWAVE THAT HAS PROVIDED SOME OF THE FORCING FOR THIS FEATURE BEGINS TO MOVE OFF THIS MORNING WE SHOULD SEE A DECREASE IN CONVECTIVE COVERAGE...HOWEVER ENOUGH LIFT WILL REMAIN ESPECIALLY NEAR THE FRONTAL ZONE TO SUPPORT A BAND OF CATEGORICAL POPS MOVING ACROSS THE AREA THIS MORNING. WILL ALSO INCLUDE A PRE-FIRST PERIOD IN THE ZONES TO ACCOUNT FOR CONVECTIVE TRENDS. THE FRONT WILL HANG UP SOMEWHERE ACROSS OUR SOUTHEASTERN HALF THIS AFTERNOON AFTER BEING ABANDONED BY THE RAPIDLY DEPARTING WAVE ALOFT. SOUTHERLY FLOW AHEAD OF THE FRONT WILL PUMP WARM AIR AND INCREASING MOISTURE INTO THE BOUNDARY WHICH WILL THEN ACTIVATE IN THE MID AFTERNOON TO EVENING HOURS PRODUCING MORE THUNDERSTORMS IN OUR SOUTH AND SOUTHEAST. STRENGTHENING SOUTHERLY LOW LEVEL WINDS BENEATH SOUTHWESTERLY FLOW ALOFT SHOULD PROVIDE SUFFICIENT SHEAR PROFILES FOR THE DEVELOPMENT OF SOME SEVERE STORMS...HOWEVER UNCERTAINTY REGARDING THE LOCATION OF THE FRONT AND TIMING OF LATE DAY ACTIVATION PRECLUDED SEVERE WORDING IN THE GRIDS/FORECAST ATTM. HEAVY RAIN WILL ALSO BE POSSIBLE WITH THE STORMS IN THE AFTERNOON GIVEN GOOD MOISTURE TRANSPORT INTO THE BOUNDARY AND HIGH PWATS FOR THIS LATE IN THE SUMMER...HOWEVER THE STORMS SHOULD HAVE A DECENT FORWARD MOTION AND BE SPOTTY ENOUGH THAT FLASH FLOOD THREAT WILL BE TOO LOCALIZED TO JUSTIFY A FFA OR HVY RAIN WORDING THIS MORNING. OTHER THAN POP/WX TIMING TRANSITIONS ACROSS THE CWA OTHER FORECAST CHALLENGE IS TEMPERATURES. DECENT CAA BEHIND THE FRONT SHOULD BE MITIGATED SOMEWHAT BY DIURNAL HEATING AS THE FRONT STALLS LATER IN THE DAY...AND SOME CLEARING IN OUR NW MAY ACTUALLY CAUSE DECENT TEMPERATURE RISES IN THAT AREA. CONCURRENTLY TEMPS WILL BE HELD DOWN BY SHOWERS AND STORMS BUT THE AREA OF SAID STORMS WILL BE MOVING...SO HAVE MADE BEST ATTEMPT AT HOURLY TEMP TRENDS TODAY BUT THERE IS A LARGE DEGREE OF VARIABILITY IN THAT FORECAST. .LONG TERM /TONIGHT THROUGH MONDAY/... MADE NO LARGE CHANCES TO THE LONG TERM FORECAST. MODELS REMAIN IN GOOD AGREEMENT THROUGH THE WEEK...WITH A FEW CHANGES IN HOW MODELS HANDLE THE WEEKEND SYSTEM. THROUGH THE WEEK...MAIN FORECAST CONCERN WILL BE THE PLACEMENT OF BOUNDARY WHICH HANGS ACROSS THE SOUTH...AND HOW FAR NORTH IT WILL MAKE IT BEFORE LOW PUSHES NORTHEAST FROM COLORADO. BELIEVE BOUNDARY WILL STALL ACROSS NORTHERN MO...WITH GREATEST CHANCES FOR HEAVIEST RAIN BEING SOUTH OF THE CWA OVERNIGHT TUESDAY. FRONT WILL PUSH BACK NORTH THROUGHOUT THE DAY WEDNESDAY...WITH CO LOW BEGINNING TO INFLUENCE THE CHANCE FOR RAIN LATE WEDNESDAY. THIS LOW WILL TRACK ACROSS NEBRASKA AND THE DAKOTAS. ALONG WITH RAIN CHANCES...LOW WILL BRING QUITE BREEZY CONDITIONS ON THURSDAY WITH STRONG PRESSURE GRADIENT. LOW WILL PUSH OUT OF AREA BY FRIDAY...AND DRY WEATHER WILL RETURN...WITH WEAK SURFACE HIGH PRESSURE BUILDING IN. FOR THE WEEKEND...MODELS HAVE BEGUN TO COME INTO AGREEMENT WITH LOW PRESSURE SWEEPING SE FROM CANADA. GFS PREVIOUSLY DID NOT HAVE THIS FEATURE...BUT NEW RUN HAS BROUGHT LOW PRESSURE SE ACROSS WISCONSIN. ECMWF...WHICH HAS BEEN DEVELOPING THE LOW THE PAST FEW RUNS...HAS PUSHED IT FURTHER EAST AS WELL THIS RUN. HAVE OPTED TO LEAVE THE FORECAST DRY FOR THE WEEKEND. WITH THIS EASTWARD PUSH OF THE MODELS...IF THE LOW DOES DEVELOP...IT IS UNLIKELY TO PROVIDE MUCH FORCING FOR PRECIP ACROSS THE CWA. HOWEVER...IT IS A SYSTEM TO WATCH CLOSELY...AS UNCERTAINTY STILL EXISTS SO FAR OUT. && .AVIATION... 21/12Z...COLD FRONT OVER NW IA ATTM WILL SLOWLY ADVANCE SEWARD AND STALL OVER SRN IA BY THIS AFTERNOON. SWATH OF SHRA/TSRA EVIDENT ON RADAR WILL DIMINISH DURING MORNING HRS BUT STILL AFFECT KMCW/KFOD/KALO AT TIMES...THEN REDEVELOP FURTHER SOUTH THIS AFTERNOON WITH PEAK HEATING. HAVE INCLUDED TEMPO GROUPS AT NRTHRN TERMINALS FOR THE MORN CONVECTION BUT STUCK WITH VCTS IN THE AFTN AS DETAILS OF CONVECTIVE TIMING/LOCATION ARE UNCLEAR. WINDS WILL BECOME LGT AND VBL FOR MUCH OF THE DAY WITH THE FRONT WASHING OUT OVHD...AND COMBINED WITH MOISTURE POOLING ALONG THE STALLED BOUNDARY WILL SUPPORT DVLPMNT OF STRATUS AND FOG AFTER SUNSET TONIGHT...ESP AT KDSM/KOTM. HAVE ADVERTISED MVFR CONDS IN 12Z TAFS BUT THIS MAY NEED TO BE TAKEN DOWN ON SUBSEQUENT ISSUANCES. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...LEE LONG TERM...WILKINS AVIATION...LEE
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE DES MOINES IA 507 AM CDT TUE SEP 21 2010 IAZ004>007-015>017-023>028-033>035-044-045-221015- EMMET-KOSSUTH-WINNEBAGO-WORTH-PALO ALTO-HANCOCK-CERRO GORDO- POCAHONTAS-HUMBOLDT-WRIGHT-FRANKLIN-BUTLER-BREMER-SAC-CALHOUN- WEBSTER-CRAWFORD-CARROLL- 507 AM CDT TUE SEP 21 2010 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...TODAY AND TONIGHT SHOWERS AND THUNDERSTORMS WILL SPREAD NORTHWEST TO SOUTHEAST ACROSS THE AREA THROUGHOUT THE MORNING AND AFTERNOON HOURS. THE THREAT FOR SEVERE WEATHER IS LOW. .DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY INTERMITTENT THUNDERSTORM CHANCES WILL PERSIST THROUGH FRIDAY MORNING. A FEW SEVERE THUNDERSTORMS AND LOCALLY HEAVY RAINFALL WILL BE POSSIBLE AT TIMES. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION IS NOT ANTICIPATED. IAZ036>039-046>050-057>062-070>075-081>086-092>097-221015- HAMILTON-HARDIN-GRUNDY-BLACK HAWK-GREENE-BOONE-STORY-MARSHALL-TAMA- AUDUBON-GUTHRIE-DALLAS-POLK-JASPER-POWESHIEK-CASS-ADAIR-MADISON- WARREN-MARION-MAHASKA-ADAMS-UNION-CLARKE-LUCAS-MONROE-WAPELLO-TAYLOR- RINGGOLD-DECATUR-WAYNE-APPANOOSE-DAVIS- 507 AM CDT TUE SEP 21 2010 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...TODAY AND TONIGHT SHOWERS AND THUNDERSTORMS WILL SPREAD IN FROM THE NORTHWEST LATE THIS MORNING...PERSISTING INTO TONIGHT. SOME OF THE STORMS IN THE AFTERNOON COULD BE SEVERE...WITH LARGE HAIL AND DAMAGING WINDS THE MAIN THREATS. LOCALLY HEAVY RAINFALL IS ALSO POSSIBLE WITH THESE STORMS. .DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY INTERMITTENT THUNDERSTORM CHANCES WILL PERSIST THROUGH FRIDAY MORNING. A FEW SEVERE THUNDERSTORMS AND LOCALLY HEAVY RAINFALL WILL BE POSSIBLE AT TIMES. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION IS POSSIBLE THIS AFTERNOON. AW
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE DES MOINES IA 514 AM CDT TUE SEP 21 2010 IAZ007-016-017-211045- CERRO GORDO-HANCOCK-WORTH- 514 AM CDT TUE SEP 21 2010 ...THUNDERSTORM PRODUCING HAIL... AT 509 AM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A STRONG THUNDERSTORM 25 MILES SOUTHWEST OF MASON CITY...MOVING NORTHEAST AT 60 MPH. NICKEL SIZE HAIL IS POSSIBLE WITH THIS STORM. * THE THUNDERSTORM WILL BE NEAR... VENTURA BY 520 AM CDT... CLEAR LAKE BY 525 AM CDT... MASON CITY AIRPORT BY 530 AM CDT... MASON CITY BY 535 AM CDT... PLYMOUTH AND ROCK FALLS BY 540 AM CDT... GRAFTON BY 545 AM CDT... SEEK SHELTER IN A STURDY STRUCTURE UNTIL THIS STORM HAS PASSED. LEE
The Des Moines radar (KDMX), 58 km away, was scanning. The volume nearest the first event began at 13:55 UTC.
2 heavy rain, 2 flash flood, 1 hail, 1 lightning, 1 thunderstorm wind. Local standard time, as filed.
Both vehicles at a house received considerable hail damage.
An air exchanger on the rooftop was ripped off. Around one quarter of the roof of the gym of the Lynnville-Sully school was peeled back with a section of the roof thrown 50 to 100 feet to the east of the building, causing some tree damage. One window was blown out.
Spotter measured 3.2 inches of rain.
Two inches of rain had fallen since 1430 CST with 3-4 inches of water flowing pretty quickly across County Road 230th just northwest of Seymour.
Spotter reported water rushing over Quarry Road near R46 with water too deep to cross. The Little River was out of its banks in a few areas.
Law enforcement relayed that 290th Ave was washed out in Morgan Township, Section 5.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2010Every state in 2010This day in weather historyWarnings in Iowa, September 2010
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: 252168, 259896, 251910, 251903, 251905, 251902, 251993.