Tornadoes and Flood in Iowa, May 11, 2000

Strong warm air advection, responsible for triggering severe thunderstorms during the morning, essentially led to a strong capping inversion during the afternoon over Eastern Iowa. Mesoanalysis during the late afternoon showed low pressure west of Des Moines, with a warm front extending eastward across Northern Iowa into Southern Wisconsin. Temperatures during the afternoon climbed well into the 90s and low 100s over Southwest Iowa. The airmass was very unstable with surface based CAPE values in excess of 4000 J/Kg. Strong shear created a favorable environment for supercell development. Convergence to the east of the surface low sufficiently eroded the cap by late afternoon, leading to explosive development of an isolated supercell near Waterloo. This supercell produced a tornado in Dunkerton in Black Hawk County, which did extensive damage. This same tornado weakened as it moved into Buchanan County. Other weak tornadoes were spawned in Buchanan County. There were also indications of scattered damage produced by downbursts with the second and fourth tornadoes. Iowa governor Tom Vilsack signed a proclamation declaring Black Hawk and Buchanan Counties disaster areas. Damage was estimated at over 1 million dollars. Most of the damage in Buchanan County came on four farms. The damage was spread along an eight mile stretch south of Hazelton. Machine sheds, grain bins and outbuildings were blown apart. Damage also occurred to farm equipment with minor damage to a few houses.