During the time period from approximately 1800 UTC 3 June to 2100 UTC 4 June 2002 heavy convective rainfall resulted in significant flash flooding and river flooding over portions of east-central Iowa and northwestern Illinois. Rainfall amounts over four inches were common in these areas with extreme amounts (as high as eight to eleven inches) reported in Delaware and Dubuque counties in Iowa. The maximum rainfall for this event equals or exceeds the 1 in 100 year event for a 48-h period. According to Storm Data, the Rock River reached flood levels near Joslin, while the Maquoketa and Wapsipinicon Rivers rose well above flood stage. The Maquoketa River rose high enough to shut down the water treatment plant in Monticello, in northeastern Jones county, which did not even occur during the historic floods of 1993. President Bush declared 17 counties in eastern Iowa disaster areas as over $7.2 million dollars of property damage occurred. In northwest and west-central Illinois rainfall of six to ten inches also resulted in significant property damage (around $3 million dollars) as heavy rainfall, rivaling that of the summer of 1993, caused flash flooding and river flooding. During the height of the storm rainfall rates of over two inches per hour were recorded.

This presentation will focus on those synoptic and mesoscale processes which contributed to this extraordinary flooding event using observed surface and upper air data, and WSR-88D and GOES imagery. WSR-88D images revealed that mesoscale convective systems (MCSs) were episodic in nature and formed north of a strong quasi-stationary boundary. The role of the low-level jet (LLJ), the upper-level jet (ULJ), and the evolution of surface moisture convergence fields will be detailed using surface observations, wind profiler data, and initialized RUC-II datasets. WSR-88D data will be examined to describe the convective-scale interactions from outflow boundaries that also affected this long period of convective activity. Operational Eta model forecasts will be discussed to see how useful they were in forecasting this event in the 12-24 h period. Finally, given the high rainfall rates experienced during the period of intense rainfall, an effort will be made to diagnose those parameters which may have proved useful in assessing the likely precipitation efficiency of the thunderstorms.