Flash Flooding and Hail in Iowa, July 5, 2000

A very unstable airmass was over Iowa during the morning hours of the 5th. CAPE values were well over 4000 J/kg at sunrise with lifted indices near -10 C. Thunderstorms erupted in two clusters around 1200 UTC. The first cluster went up over north central Nebraska, the second over northwest and west central Iowa. The cluster over Iowa produced VIL levels of 80 kg/m/m over west central Iowa. Even with the VIL values so high, only pea size hail occurred with the cell. As the morning progressed, the cluster in western Iowa advanced east and a mesoscale convective vorticity center formed within the cluster. By mid to late morning, the cluster took on a small bow echo shape in its southwest quadrant. The bow echo turned southeast and caused wind damage in an area extending from Guthrie County...southeast into Monroe County. Winds of 60 to 70 MPH were common. Winds estimated around 85 MPH tore through Dallas County north of De Soto. Two airplane hangers were destroyed and several ultra light aircraft were damaged. Damage in the area was estimated at around $100,000. The same storm system struck the west side of the Des Moines metropolitan area. High winds caused considerable tree and power line damage. At one point, power was knocked out to about 25,000 homes. A major transmission line was taken down in West Des Moines as the winds toppled three power poles. One housing development lost between 50 and 60 trees as the winds blew through. Des Moines was certainly no exception. Tree and power line damage was very widespread in a broad band from west central into southeast Iowa. As the afternoon progressed, the cluster of thunderstorms that was in north central Nebraska advanced into western Iowa. The two merged as rapid development occurred along the outflow from the first cluster was overtaken by the second. Thunderstorms became severe over southwest Iowa with one inch diameter hail damaging crops in Union County. The storms were persistent with very heavy rainfall the result. Flash flooding occurred in parts of Union and Adams Counties as 3 to 5 inches of rain fell. Several roads were under water for a short time, however the water receded quickly. Southwest Iowa was not the only place to experience flooding. Considerable urban and small stream flooding took place in the Des Moines metropolitan area as well. Walnut Creek, on the west side of Des Moines, rose 7 feet in just one hour. It crested just below flood stage. Numerous city streets were covered in water, at times up to 4 feet in depth.