Midwest U.S. Trends in Precipitation Extremes and Event Return Periods

Recent analysis of the frequency of heavy precipitation events in the Midwest U.S. indicates a significant upward trend during the past 20-30 years. Although frequencies were also high around the beginning of the 20th Century, the decade of the 1990s experienced the highest frequency in the available instrumental record. Major societal impacts have been associated with some of the flooding caused by recent heavy precipitation, most notably during 1993 in the upper Mississippi River basin and during July 1996 in northern Illinois. Analysis of return period thresholds indicates that precipitation design values have increased in many areas of the Midwest. The apparent non-stationary character of the time series of heavy precipitation raises questions about the appropriate period to use for determination of design values. Nevertheless, the recent changes indicate a need for regular updates of these design values. Otherwise, inappropriate design and construction can create increased risk of future societal losses to flooding. The potential that anthropogenic forcing of the climate forcing may be a factor in the observed changes further emphasizes this need as such forcing increases in the future.