A strong push of moist air occurred during the day on the 1st with surface dewpoint readings climbing to the mid 50s to mid 60s across the state. 25 inches.
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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 push of moist air occurred during the day on the 1st with surface dewpoint readings climbing to the mid 50s to mid 60s across the state. The low level jet increased in strength during the evening with wind speeds around 45 kts by mid evening, helping to push precipitable water values to around 1.25 inches. The atmosphere became unstable with CAPE values in the 2000 to 3000 J/kg range. Downdraft CAPE was in the 900 J/kg range with a relatively low LCL of 750 meters. Wind shear, though not strong, was sufficient with 35 to 45 kts of available shear. The CAPE in the -10 to -30 C layer of the atmosphere was between 200 and 300 J/kg. The freezing level was around 13,000 feet. A line of thunderstorms formed ahead of the approaching cold front from South Dakota and moved southeast into Iowa during the mid to late evening hours. They produced gusty winds and hail as they passed. The stronger storms produced winds of 60 MPH over northwest into parts of north central and central Iowa, along with hail up to quarter size. Significant damage occurred in the Ames area as high winds of 70 to 75 MPH moved through Story County. Large trees were downed in the city. The garage doors and gutters were damaged on an apartment complex. A house was significantly damaged as a large tree was blown down onto it. The strongest winds were estimated to be around 85 MPH due to a microburst. Around the county, there were numerous reports of tree and outbuilding damage as well.
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 1, 5:45 PM CST, 3 hours 35 minutes earlier.
AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE DES MOINES IA 645 PM CDT TUE MAY 1 2012 .SHORT TERM /TONIGHT/... STRONG MOISTURE ADVECTION UNDERWAY WITH MAIN FOCUS TO THE NORTHWEST AND NORTH OF THE CWA. WITH CORFIDI VECTORS SUGGESTING THE PRECIP COMPLEX WILL DROP SEWD...WILL GO WITH THE GRADUAL EXPANSION OF TSRA SOUTHEAST ACROSS THE WHOLE CWA DURING THE NIGHT WITH THE INTENSITY DECREASING WITH TIME AND DISTANCE FROM THE SUPPORT. WILL LIKELY SEE THE CONVECTION DIMINISH LATER TONIGHT AS THE UPPER LEVEL S/WV PASSES BY AS WELL. AN AREA TO WATCH WILL BE THE EDGE OF THE COOL POOL FROM THE SMALL COMPLEX THAT BRUSHED THE SOUTHEAST PART OF THE STATE THIS AFTERNOON. WOULD NOT BE SURPRISED TO SEE SOME DEVELOPMENT ALONG THE EDGE OR A LITTLE SOUTHWEST OF THE COOL POOL AND WILL PUT IN SOME ISOLATED TSRA TO COVER THAT. OTHER THAN THAT...EXPECTED TO SEE THE LATE NIGHT FEATURED WITH ISOLATED CONVECTION AND WIDESPREAD LOW CLOUDS WITH THE DAY WED STARTING OUT SIMILAR TO THIS MORNING ONLY WARMER. .LONG TERM /WEDNESDAY THROUGH TUESDAY/... IT APPEARS THAT WE WILL REMAIN IN A WARM AND ACTIVE PATTERN FOR THE BULK OF THE SEVEN DAY FORECAST. FROM WEDNESDAY INTO FRIDAY WE WILL REMAIN BENEATH WEST SOUTHWESTERLY 500 MB FLOW...ALLOWING A SERIES OF IMPULSES MOVING OVERHEAD TO TRIGGER CONVECTIVE EPISODES. THUNDERSTORM DEVELOPMENT WILL BE ENHANCED BY THE SHORTWAVES BUT LARGELY DRIVEN BY DIURNAL HEATING...WITH INITIATION LIKELY NEAR A MORE OR LESS STATIONARY SURFACE TROUGH IN THE LATE AFTERNOON OR EVENING ON BOTH WEDNESDAY AND THURSDAY. IT APPEARS THIS TROUGH WILL LIE NEAR THE SD/MN/IA JUNCTION LATE ON WEDNESDAY...WITH CONVECTIVE INITIATION LIKELY SOMEWHERE IN THAT VICINITY BETWEEN ABOUT 20-22Z. STORMS SHOULD THEN TRAVERSE NORTHERN IOWA OVERNIGHT...WITH SOME QUESTION AS TO HOW FAR SOUTH THEY MAY BE ABLE TO TURN OR PROPAGATE TOWARD CENTRAL IOWA. ON THURSDAY THE TROUGH WILL LIE MORE ACROSS OUR AREA...PROBABLY ACROSS OUR NORTHWESTERN HALF SOMEWHERE...WITH STORMS INITIATING A BIT SOONER IN THE AFTERNOON AS CONVECTIVE INHIBITION APPEARS WEAKER ON FORECAST SOUNDINGS. HAVE MAINTAINED TIME SPLITS IN THE DAYTIME POPS FOR ANTICIPATED LATE DAY TIMING...AND THE HIGHEST POPS IN THE NORTH AND NORTHWEST WEDNESDAY AND WEDNESDAY NIGHT BUT MORE WIDESPREAD LATE ON THURSDAY DUE TO UNCERTAINTY IN THE LOCATION OF THE BOUNDARY BY THAT TIME. SEVERE WEATHER WILL BE POSSIBLE THROUGH THE PERIOD GIVEN AMPLE INSTABILITY AND DECENT WIND SHEAR...ESPECIALLY AT TIMES WHEN THE SHEAR MAY BE ENHANCED BY AN APPROACHING MID-LEVEL WAVE AND/OR A NOCTURNALLY ENHANCED LOW-LEVEL JET FEATURE. SPC HAS US INCLUDED IN SLIGHT RISKS FOR BOTH DAYS ACCORDINGLY WHICH SEEMS VERY REASONABLE. TEMPERATUREWISE THE PERIOD FROM WEDNESDAY THROUGH FRIDAY ALL APPEARS QUITE WARM AS WE WILL SEE MORE OR LESS SOUTHERLY SURFACE FLOW FOR THE MAJORITY OF THAT PERIOD WITH INCREASING LOW-LEVEL MOISTURE PROMOTING HIGHER OVERNIGHT LOWS AS WELL. ONCE THE LAST OF THE MID-WEEK SHORTWAVES MOVES AWAY FROM OUR AREA ON THURSDAY NIGHT IT APPEARS WE WILL SEE A PERIOD OF WEAK RIDGING FROM FRIDAY INTO SATURDAY. HAVE TRIMMED BACK POPS TO SLIGHTS ON FRIDAY AND SATURDAY...BUT KEPT CHANCES IN ON FRIDAY NIGHT AS SEVERAL MODELS INDICATE THE POTENTIAL FOR A 500 MB RIPPLE OVERTOPPING THE RIDGE THAT COULD PROMOTE SOME OVERNIGHT STORM DEVELOPMENT. BY SATURDAY NIGHT ANOTHER DEEP TROUGH WILL BE BUILDING DOWN THE WESTERN U.S. AND WE WILL SEE A RETURN OF WEST SOUTHWESTERLY 500 MB FLOW THROUGH THE REMAINDER OF THE PERIOD. LONG RANGE MODELS BECOME INCREASINGLY DIVERGENT BY NEXT WEEK...AS IS TO BE EXPECTED IN SUCH A RELATIVELY WEAK STEERING PATTERN...AND HAVE NOT STRAYED FROM INHERITED FORECAST IN THE OUT PERIODS. IT DOES APPEAR THAT A SHORTWAVE MOVING THROUGH WITH THE RETURN OF WEST SOUTHWESTERLY MID-LEVEL FLOW LATE ON SUNDAY WILL FINALLY PUSH OUR PESKY SURFACE TROUGH SOUTHEASTWARD INTO THE OHIO RIVER VALLEY...SO WE WILL LIKELY SEE COOLER WEATHER EARLY NEXT WEEK AND HAVE STUCK WITH ONLY ISOLATED THUNDER MENTION WHICH MAY STILL BE OVERDONE. && .AVIATION...02/00Z SOUTHERLY FLOW ACROSS CWA WILL CONTINUE THROUGHOUT THE PERIOD...WITH INCREASING INSTABILITY ACROSS THE CWA AHEAD OF ADVANCING LINE OF THUNDERSTORMS OFF TO WEST/NORTH OF THE STATE. WEAK DISTURBANCE TO MOVE THROUGH IOWA TONIGHT...WITH SHOWERS AND THUNDERSTORMS TO SPREAD ACROSS CWA BY LATE EVENING INTO THE OVERNIGHT HOURS. ACTIVITY COULD CONTINUE THROUGH MUCH OF THE LATE NIGHT HOURS. MAY HAVE SOME LINGERING LOW VFR CIGS BEHIND THE ACTIVITY...AND POSSIBILITY OF SOME LOWER STRATUS INTO THE MORNING HOURS. HAVE HINTED AT THIS WITH A SCT010-015 DECK. OTHERWISE INCREASING MOISTURE TO ALLOW ANOTHER LOW VFR/POSSIBLY HIGHER MVFR CU DECK TO FORM ACROSS CWA BY LATE MORNING INTO THE AFTERNOON HOURS WEDNESDAY. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && SHORT TERM...MS MAY 12 LONG TERM...LEE AVIATION...BEERENDS
HAZARDOUS WEATHER OUTLOOK NATIONAL WEATHER SERVICE DES MOINES IA 344 PM CDT TUE MAY 1 2012 IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-081>086- 092>097-022045- EMMET-KOSSUTH-WINNEBAGO-WORTH-PALO ALTO-HANCOCK-CERRO GORDO- POCAHONTAS-HUMBOLDT-WRIGHT-FRANKLIN-BUTLER-BREMER-SAC-CALHOUN- WEBSTER-HAMILTON-HARDIN-GRUNDY-BLACK HAWK-CRAWFORD-CARROLL-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- 344 PM CDT TUE MAY 1 2012 THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA. .DAY ONE...TONIGHT THUNDERSTORMS WILL SPREAD ACROSS THE AREA FROM THE NORTHWEST TONIGHT. SOME STORMS MAY BE SEVERE...MAINLY NORTH AND WEST...WITH LARGE HAIL AND DAMAGING WINDS THE PRIMARY THREATS. .DAYS TWO THROUGH SEVEN...WEDNESDAY THROUGH MONDAY VARIED THUNDERSTORM CHANCES WILL LINGER THROUGH EARLY NEXT WEEK. THE SLIGHT RISK OF SEVERE WEATHER IS EXPECTED TO CONTINUE THROUGH THURSDAY WITH LARGE HAIL AND DAMAGING WINDS STILL THE PRIMARY THREATS. THE SEVERE WEATHER POTENTIAL FOR THE END OF THIS WEEK AND EARLY NEXT WEEK WILL BE BETTER DETERMINED IN THE COMING DAYS. .SPOTTER INFORMATION STATEMENT... SPOTTER ACTIVATION MAY BE NEEDED AT TIMES THROUGH THURSDAY...MAINLY DURING THE LATE AFTERNOON AND EVENING HOURS. SMALL/LEE
The Sioux Falls radar (KFSD), 160 km away, was scanning. The volume nearest the first event began at 03:20 UTC.
18 thunderstorm wind, 3 hail. Local standard time, as filed.
Horizontal rain and numerous six inch tree branches down.
Trained spotter driving on Highway 15 between Whittemore and Fenton estimated 60 MPH winds and had trouble controlling his car. Several cars were reported in the ditch with quarter size hail beginning at 2158 CST.
Trained spotter reported quarter size hail blowing horizontally against the window.
Several branches down, a few four inches in diameter.
Several large tree limbs down with damage to fences. Damage began about a mile south of Fraser and continued sporadically for about 2 miles.
Several six inch tree limbs down.
The emergency manager reported that four 1 1/2 foot diameter evergreen trees were downed and the roof was torn off of a barn in the northwest corner of Boone. Also from a mile north of Boone to the eastern border of the county sheeting was strewn about from various buildings.
Large trees were uprooted along 13th street in Ames.
Garage doors were blown in with gutters torn off of an apartment complex in north Ames.
Damage report within two blocks of Bloomington Rd. and Grand Ave. Shingles off of house roofs, privacy fences down, apartment buildings lost partial balcony, traffic light pole bent over, and trees of one to two foot diameter snapped at bases.
A tree landed on a house and a car in the area of Nixon Circle in Ames. The tree damaged the gas line to the house.
Wind speed measured at an Ames mesonet site.
A pine tree was uprooted and multiple tree limbs were downed.
Power lines were reported down in Ames.
Wind damage reported to trees. Observer measured wind speed.
The top of a tree that was two feet in diameter was snapped off in the town of Roland.
Several ten inch limbs down.
Large limb toppled a power line which fell across IA 175 in Reinbeck. Several other large limbs down across the west side of town.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Iowa in 2012Every state in 2012This day in weather historyWarnings in Iowa, May 2012
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: 368613, 368464, 368468, 368463, 368466, 368612, 368497, 368614, 368504, 368498, 368499, 381482, 368500, 368495, 368501, 368502, 368496, 368503, 368615, 368616, 368494.