Damaging Winds in Iowa, June 9, 1999

The synoptic situation was very much like it was the previous day. A cold front had become nearly stationary from northern Minnesota into central Nebraska. An upper level disturbance lifted northeast into Iowa during the afternoon hours. The airmass was primed with dew point temperatures in the upper 60s to near 70 and surface temperatures in the upper 80s to low 90s. As a result, CAPE values were once again in excess of 5000 J/kg by mid afternoon. A line of thunderstorms formed over the west part of the state in a north to south line. Initially these storms were not severe. The southern end of the line was more broken up with several individual cells scattered along the line. The storms became pulse severe with one of the cells collapsing over Atlantic in Cass County. The microburst that resulted was recorded by a local radio station there at 108 MPH. Significant damage occurred over a 30 square block within the city with both structural damage as well as tree damage reported. Several homes sustained major structural damage. Another report indicated trees as large as three feet in diameter were toppled. After that storm dissipated, another cell formed northeast of Atlantic. This storm produced 60 MPH winds about 9 miles to the northeast of the city. Minor damage occurred in Audubon in Audubon County as 60 MPH winds blew into the city. Several farm buildings were damaged in Greene County as high winds struck southwest of Grand Junction. Yet another cell formed just west ot Taylor County and moved into the area. This cell produced winds of 70 to 80 MPH as it moved into the New Market area. After a period of relatively quiet weather, the storms became stronger as they headed into central Iowa. High winds were recorded at 61 MPH at the NWS office in Johnston, with the winds causing tree damage across northern Polk County into the southern part of Story County. There were numerous reports of hail with the storms as they crossed the west two thirds of the state, though the hail size was generally around one half inch or less.