Tornadoes and Flooding in Iowa, April 6, 1998

A southeast flow of moist air was in place over Iowa. This flow brought moisture north with surface dew point temperatures rising into the low to mid 50s. Lapse rates were steep with the freezing level only around 9000 feet. The low freezing level contributed to widespread hail as thunderstorms formed during the mid afternoon hours. Synoptically, an upper level low pressure system was advancing northeast from the southwest U.S. A surface low formed over Oklahoma with a trough extending northeast into eastern Nebraska. This trough served as the focusing boundary for the thunderstorms to form on. Initially , a small cluster of thunderstorms formed over southeast Nebraska and moved into southwest and west central Iowa. Most of the storms produced at least pea to marble size hail. There were several reports of hail covering the ground. In Carroll County, pea to slightly larger size hail accumulated to a depth of an inch in the town of Arcadia. Significant hail fell in Boone and Webster Counties where golf ball size hail was reported. The area around Dayton was particularly hard hit with the golf ball size hail covering the ground. During the storms strongest period, several small tornadoes were spawned. One touched down briefly in Greene County north of Churdan. A short time later, two other brief touchdowns were reported. A Ham radio spotter reported a brief touch down near Highway 169 in Webster County south of Dayton. Another brief touchdown occurred in Green County north of Scranton near the same time. All of these tornadoes touched down in open fields and caused little to no damage. They were also quite small and weak. The other problem during this event was with the rainfall. The soil in Iowa was already close to saturation. As a result, much of the rain that fell ran off, causing local and urban flooding. Rainfall of one to three inches was common from west central Iowa into parts of the north central counties. The heaviest rainfall report was received from Denison with 3.40 inches. There were several reports of one to two inches falling in one half hours time. In spite of the wet soil conditions and heavy rainfall, flooding was relatively minor, though considerable ponding did occur.