Author(s):
Thomas B. Williams, Associate Professor* - Department of Geography, Western Illinois University
Jongnam Choi, Associate Professor - Department of Geography, Western Illinois University
Abstract:
The Great Flood of the summer of 1993 featured widespread catastrophic flooding of unprecedented magnitude that affected a nine-state area in the Upper Midwest along the Mississippi and Missouri River systems. Only 15 years later, another extreme flood event in the summer of 2008 produced flood crests on many rivers throughout the same area which exceeded records previously set in 1993. The purpose of this study is to compare the prevailing synoptic weather patterns responsible for the flooding.
The 1993 flood was the direct result of exceptionally heavy precipitation in the months preceding the flooding, which saturated soils in the region, followed by a persistent wet weather pattern. A convergence zone and quasi-stationary frontal boundary developed between cool, dry air dropping down from Canada and abundant warm, humid air pumped northward from the Gulf of Mexico. These conditions were associated with a stronger jet stream displaced further south than normal over the Rockies, combined with an unusually strong Bermuda High located over the Southeast.
Similar to 1993, the flooding in 2008 can be traced back to antecedent conditions beginning in late summer 2007 when heavy rains and significant flooding occurred in portions of the upper Mississippi River basin. The pattern of above normal precipitation continued through fall into winter, when record snowfall hit southern Wisconsin. Gradual snow melt mitigated the threat of spring flooding but left river levels abnormally high with additional spring rainfall. When recurrent heavy rains hit in June, the end result was record flooding, especially in Iowa.
