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Publication Abstract

The Record Rainstorm on July 17-18, 1996 in Northern Illinois Angel, James R., Stanley A. Changnon, David Changnon, Floyd A. Huff, Paul Merzlock, Steven R. Silberberg, and Nancy E. Westcott, 1997  Illinois State Water Survey, Champaign, IL,  ISWS MP-182    Full Text Available

By the afternoon of July 18, 1996, Midwestern radio and television stations were issuing reports of extremely heavy rains and flash flooding in northeastern Illinois. The news reports indicated that a raingage in Aurora, Illinois, had received nearly 17 inches of rain during the rainstorm on July 17-18, 1996. In the storm center, hundreds of thousands of persons in the area were in shock because of the massive flooding and the sudden devastation with 35,000 homes experiencing flood damage in a matter of hours.

The maximum point rainfall value of 16.94 inches within a 24-hour period occurred at a raingage operated by a cooperative weather observer of the National Weather Service in Aurora. Inspection of historical precipitation records for all parts of the state indicated that this amount established a new record 24-hour rainfall for Illinois. The previous record rainfall was 16.5 inches in a storm in the East St. Louis area in 1957 (Huff et al., 1958). Clearly, the July 17-18, 1996, rainstorm was of record proportions. Information available by July 19 indicated that the resulting flood damages were extensive, particularly in south Chicago and across extensive suburban and rural areas located west, southwest, and south of the city.

Figure 1-1 is an isohyetal map showing the total storm rainfall for the July 17-18 storm. Note that heavy rainfall was produced across a three-state area—Illinois, Indiana, and Wisconsin. This was an exceptionally large storm with rainfall amounts above 10 inches across wide areas of Illinois and a portion of southern Wisconsin. The axis of the storm was more than 600 miles long. The major axis of the storm rainfall was oriented from northwest to southeast, a somewhat unique position for a severe rainstorm. As part of the present study, the unusual weather conditions that created this storm and its odd placement were investigated (see Chapter 2).

Scientists of the Illinois State Water Survey and its Midwestern Climate Center answered a variety of questions about this severe rainstorm. These requests sought hydrometeorological and climatological interpretations of the significance of the heavy rains on July 17-18, along with detailed information on the rainstorm's dimensions and causes.

The Water Survey has a history of performing field-based studies of extreme rainstorms, an effort that began in 1951 with the study of two rainstorms (Larson et al., 1952; Illinois State Water Survey, 1952). These and subsequent studies of intense rainstorms, including the field collection of rainfall data in "bucket surveys," have provided unique data about the characteristics and dimensions of severe Midwestern convective rainstorms (Huff et al., 1955, 1958; Huff and Changnon, 1961; Changnon et al., 1977). The objectives of such storm studies were not only to collect data and make exhaustive scientific analyses to better define causes and dimensions, but also to provide a database on storm rainfall that could be used by others to...

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