Tornado and Flood in Iowa, June 1, 1999

A frontal system stalled to the south of Iowa during the night of the 1st and began to lift north as a warm front during the day on the 2nd. At the same time an upper low moved northeast into the state. A surface low also formed under the upper low and moved along the front. Initial convection kicked off south of the surface low in the warm sector as a dry line punched east under the low. The storms that formed remained below severe levels, but did produce heavy rain and pea to marble size hail. As the upper low advanced east, a comma precipitation pattern formed. The head of the comma was over Butler County as heavy thunderstorms moved into the county and then stopped. As the vorticity maximum moved through the area, the storms began to sag slowly south and weaken. In the process, upwards of 2 to 3.1 inches of rain fell over much of Butler County. This caused urban flooding problems, but not too serious. Water was reported over some roads for a while, but the water receded once the rain tapered off. Meanwhile, over western Iowa, small thunderstorms formed in what would be the deformation zone under the cold pool aloft. The storms were popcorn variety and lasted for 15 minutes or less. A few of the storms produced cold air funnels. One storm in Calhoun County produced a brief spin up tornado. The tornado did not do any damage and would better be described as a landspout rather than a tornado.