Tornado and Flash Flooding in Iowa, July 6 to 7, 1998

Very warm and humid air was in place over Iowa in advance of an approaching frontal system. Dew point temperatures were approaching 80 degrees F. over a large part of Iowa. Initially, a line of thunderstorms developed from southwest Minnesota, southwest into Nebraska. This line progressed southeast during the evening. A couple of things happened during the mid evening hours. The first was the formation of a line of thunderstorms from west central Iowa into central Iowa. The storms produced winds of up to 65 MPH as they moved across Audubon County during the initial development stage. The storms formed along a convergence zone and were slow moving. The second was an acceleration of the line moving in from the west as a speed max approached. During the mid and late evening hours, the portion of the northeast to southwest oriented line over northwest Iowa turned into a bow echo and raced southeast across the state. A small tornado touched down in Carroll County. The main severe weather feature with the storms was high winds as the bow echo advanced south and east across Iowa. Winds of 60 to 70 MPH were common along the line. Numerous trees and power lines were downed by the winds. The winds resulted in scattered power outages, the worst of which was in the Des Moines metropolitan area where 9000 were without power for several hours after the storms passed. The other problem was with flooding. The main problem was with the northwest to southeast oriented line since it was very slow moving. The airmass being fairly tropical in nature resulted in large amounts of rainfall over several counties. Flash flooding occurred in Dallas, Polk, and Guthrie Counties. Widespread rainfall of 2 to 3 inches occurred, with some areas receiving around 5 inches. Much of the rain fell in about an hour, causing very rapid rises in streams as the ground was saturated at the time it rained. In Guthrie County, evacuations took place in the Lake Panorama area as water overtopped the dam there. Rainfall of 2 to 5 inches occurred in Sac, Carroll, and Greene Counties. This resulted in fairly widespread urban flooding, however rainfall in these areas took several hours to fall and with soils there not as saturated, flooding was not as serious. As the storms moved through Black Hawk County, lightning struck a house in Cedar Falls. Though no serious damage occurred, appliances were damaged and the window frame of one window was blown out. Shreds of the frame flew across the room as the lightning struck.