Hail in Iowa, June 10, 1994

Thunderstorms formed in Iowa ahead of an approaching weak cold front. There were a total of three factors contributing to the formation of the storms. First off, Iowa was in the left front quadrant of the upper level jet. The second factor was of course the weak frontal boundary. The third factor that became important during the early evening hours was the outflow boundary from the afternoon convection. The airmass was moderately unstable with CAPE values in the 1300 J-Kg-1 range. Thunderstorms fired in northwest Iowa first. The main feature with the storms was hail. The freezing level was around 10,000 feet MSL, with the wet bulb zero in the 7,000 foot range. Most of the storms produced pea- to marble-size hail as they passed. The stronger storms produced hail from dime-size to golf ball-size. Golf ball-size hail was reported in Clay and Audubon Counties. Most other reports were of hail three-quarters of an inch or less in diameter. The storms also produced gusty winds, but winds were generally below 50 mph with the strongest storms. One funnel cloud was observed northeast of Audubon. It did not last long, and did not become a tornado. As the evening progressed, the initial line of storms advanced southeast across central Iowa. The next severe storms developed at the intersection of the outflow boundary from the afternoon convection and the weak frontal boundary. The outflow boundary was easily detected by the WSR-88D radar. The storms that formed in southwest Iowa at the intersection point became severe and produced large hail. One of the storms produced the golf ball-size hail in Audubon County. Though damage was spotty, significant damage was reported to the field crops due to the hail.