Tornadoes and Hail in Iowa, May 11, 2000

The seasons first tornado outbreak took place during the late afternoon and early evening hours of the 11th. During the day, very strong warm air advection took place. Temperatures warmed rapidly during the day with highs reaching the mid to upper 90s over about the southwest half of the state. Dew point temperatures were in the upper 60s to low 70s. This combination produced CAPE values in excess of 8000 J/kg. During the afternoon, a strong cap was in place with 700 mb temperatures around +16 C. This prevented convection from firing during the afternoon. Toward evening, thunderstorms broke through the cap over northeast Iowa. The storms went from initial echo to tornadic storms in less than 50 minutes. A supercell formed west of the Waterloo area. This cell became a splitting cell and produced a brief tornado in Grundy County as the right moving cell split off, and golf ball size hail in Bremer County from the left moving cell. The cell moved off quickly, however additional supercells back developed over the same area. the strongest tornado developed in the Cedar Falls area and tracked across the northern portions of Waterloo. This cell produced a strong tornado which was actually a multi-vortex tornado based on reports from storm chasers in the area. At one point, west of Dunkerton, three tornadoes each one quarter mile wide were on the ground with this storm. The tornado damaged several houses and a few farmsteads along the way. Reports indicated between 5 and 10 houses in Black Hawk County were totally destroyed, and another 15 to 20 sustained significant damage. A total of at least 41 structures sustained at least some damage. In addition to the tornado, damage also occurred south of the track as RFD winds estimated as high as 100 MPH were observed. Dunkerton was hard hit as the tornado moved through. Heavy damage was reported at a coop elevator in Dunkerton, with 10 of 16 grain elevators damaged. A roof was removed from a church as well, resulting in about $500,000 damage. The City Hall building was destroyed in Dunkerton. The City Hall was housed in a series of double wide trailers since ironically the former City Hall building was destroyed just last year in the floods of '99. As the tornado moved through the fairly densely populated area, numerous injuries were reported. Some of the injuries were severe with limbs being cut off by flying debris. One woman lost both one arm and one hand, and had her back broken. She passed away 17 days after her injuries occurred. Six of the injuries occurred at a nursing home that was hit by the tornado. They occurred as the 139 residents were being evacuated. Reports indicated the width of the tornado was up to three-quarters of a mile at its widest. The damage path from the storm was around one and one half miles wide. The tornado continued moving east and crossed into Buchanan County, about 4 miles northeast of Dunkerton. A short time later, another cell developed in the Dunkerton area and dropped a tornado. That tornado struck a farm northwest of town, destroying the farm house and seven outbuildings. In addition to the damage that occurred to the structures, the family dog and one of the five horses on the farm were killed. Reports indicate a letter from one of the farm houses destroyed was found 60 miles away from Dunkerton in the town of Monona in Clayton County. Polly Mill's rural Dunkerton farmhouse was shredded by the tornado. She lost some of her late husband's military medals, pages out of the family Bible, and part of her collection of letters and first-issue stamps. Sixty miles away, on a farm east of Monona, the letter was found in an envelope carrying a first-day issue, 5-cent stamp commemorating the Battle of New Orleans. There was yet a third round of tornadic storms in the Dunkerton area. The last tornado touched down near Dunkerton and developed southward. Two houses in the town of Dunkerton were destroyed by this tornado. In addition to the tornadoes, very heavy rain fell in a narrow area of Black Hawk County. Reports of 3 inches or more of rain was received from the area. Soil conditions were very dry at the time. The dry soil conditions helped preclude any significant flooding. By the afternoon of the 12th, Iowa Governor Vilsack had declared Black Hawk County a state disaster area.