Tornado and Flash Flooding in Iowa, June 18, 1998

A warm front extended from a low pressure area over northeast Nebraska, southeast through central into southeast Iowa. A cold front extended south from the low into Kansas. South of the warm front, dew point temperatures were in the low 70s. A speed max passed to the northwest of the state, placing Iowa in the right rear entrance region of the speed max. This set the stage for thunderstorm development. A strong low level jet was in place with speeds in excess of 50 knots. Initially, a small like of strong, but not severe, thunderstorms formed from southwest into north central Iowa. As the morning progressed, the storms initially produced hail of three quarters inch up to golf ball in size. The hail event quickly turned into a wind event as the storms accelerated along the line. Wind damage was widespread. A roof was taken off of a house in Des Moines as one of the cells moved through. A later storm took a roof off of a building in West Des Moines later in the morning. Wind damage was also significant in Webster and Hamilton Counties as the storms moved through. Numerous reports of sheds blown over were received. Some of the pieces from the sheds were found over a mile downwind. A school roof was 40 percent removed in Blairsburg by high winds. Damage to the building was placed at $150,000. The next problem was with the storms training over the same areas. The cells moved northeast along the line with little eastward progress of the line itself. This caused urban and small stream flooding over a several county area as widespread rainfall of one to two inches occurred in several central Iowa counties. The situation became more serious in a several county area. Rainfall of three to five inches was recorded in parts of Dallas, Madison, Polk, Story, and Warren Counties. This resulted in flash flooding as all creeks and streams swelled over their banks. The situation became very serious in the larger cities. In Des Moines, a state of emergency was declared as water of 6 feet or greater in depth choked many of the city intersections. Hundreds of cars were stalled in the streets and many basements were flooded. Between the high winds and water, electricity was knocked out to 15000 homes in Des Moines during the height of the storm. Damage estimates were in the hundreds of thousands of dollars in all of the flash flood counties, and over one million dollars in Polk County, where the Governor declared a state disaster. A disaster declaration was also made for Dallas County. Reports indicated that in the Des Moines metropolitan area, up to 10,000 homes had water in the basement. The city of Des Moines reported that infrastructure losses alone around $1 million. Crop damage estimates from this event, combined with the event of just a few days earlier, totalled around $100 million. During the early afternoon hours, the cold front over Nebraska began moving east, resulting in a change in the thunderstorm configuration. Several bow echo elements formed along the line which caused another round of high winds and also began shifting the line east. From Des Moines east, numerous places reported winds of 60 to 75 MPH, along with considerable damage. Seventy two MPH winds buffeted Grinnell in Poweshiek County, breaking out windows in homes there. Winds approached 85 MPH west of Montezuma causing extensive damage on several farms there. Two semi-tractortrailor trucks were blown off of I-380 southeast of Waterloo in Black Hawk County. The highest wind reported was in Tama County, at Montour where 90 MPH winds struck the city, resulting in widespread damage. One tornado touched down south of Des Moines. The tornado was small and rain wrapped and produced an intermittent track of minor damage from near Martensdale to northeast of Norwalk in Warren County. Scattered thunderstorms formed ahead of the main area of thunderstorms during the late morning hours. Initially, they were not severe, but one did produce a lightning stroke in Grinnell that touched off a minor fire in a house there.