Flood and Hail in Iowa, July 30 to 31, 1999

A very hot and humid airmass was in place over Iowa. Surface temperatures were in the mid 90s to low 100s with dew point temperatures in the low to mid 70s. CAPE values were in the 7000 to 9000 J/kg range over most of the state. The upper air soundings were in the form of an inverted V sounding as thunderstorms developed over an approaching cold front from the northwest. The airmass was capped over Iowa. As a result, thunderstorm development was quite limited over the state. A large area formed to the north of the state over Minnesota and became an MCC. During the early evening hours, the southwest tail of the MCC developed into north central Iowa. With the inverted V sounding, the main weather feature with these storms was high winds. As the line sunk into Iowa, 70 MPH winds were reported north of Forest City. As the line moved east, 60 MPH winds were reported in Worth County from Northwood south to Manly. The storms moved out of the Des Moines CWA after that. Another cell developed a short time later and dropped dime size hail in Forest City. This was an isolated event from a pulse severe storm. The thunderstorms continued into the evening and remained fairly well behaved. There were spotty reports of hail west of the Des Moines CWA but nothing within the CWA. Around midnight a line of thunderstorms developed in central Iowa. Though most of the storms were not severe, one storm did become a pulse severe storm over Boone and Story Counties. The storm produced high winds as a microburst developed in the storm and lasted about 30 minutes. High winds initially occurred in the Boone area as trees were downed on the west side of town at the Boone Golf and Country Club. High winds destroyed a barn in Boone County at Napier. High winds also blew 15 mobile homes over in Ames, destroying three of them, and 3 in the town of Kelley, just south of Ames. Six people, including one family of four, was left homeless. Power poles were snapped across a half mile wide area by the high winds with estimates of wind speeds around 100 MPH from the microburst. There were numerous reports of trees and power lines downed and one car was crushed in Ames. Two newly constructed, but unoccupied, homes were destroyed by winds in Ames as well. Following the storm, a Presidential Disaster Declaration was made for Story County to aid in cleanup. Of the 15 counties in Iowa to receive federal disaster aid so far this summer, Story County was the only one that received aid for damage other than flooding. Lightning struck two houses in Ames as well, causing some minor damage. Damage stopped from the storm as quickly as it began with the high winds never reaching the next county downstream. Rainfall from the storms was spotty, but some areas did receive very heavy rainfall. Rainfall of 4 to 6 inches fell over a several hour period over Taylor County as repeat thunderstorms moved over the county. The rain caused urban and small stream flooding, and eventually resulted in the 102 River jumping its banks by later in the morning.