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<br />4.8 inches at a few locations over the region. Similarly, the likelihood of receiving 10 inches <br />ofrain in 24 hours over Fort Collins is extremely small, but the chances of receiving 10 inches <br />of rain somewhere in Colorado in any given year is quite high - about ten percent. The <br />lOO-year storm concept is useful and has an important place in design and planning, but its <br />definition must be understood. Figure 20 shows the 109 largest one-day rainfall totals <br />measured at the Fort Collins weather station from 1889 through 1997. This is called a partial <br />duration ranked series and is the foundation for precipitation frequency computations and <br />estimating the magnitude of design storms for stormwater planning. Figure 20 (top) shows <br />the partial duration series for the period 1889 through 1970 which was used to compute the <br />published design storm values in Figure 19. Updating local rainfall statistics to include the <br />recent heavy rains needs to be completed. Including recent data may result in higher estimates <br />of design rainfall, but the changes will likely be small. <br /> <br />Conclusions <br /> <br />The Fort Collins storm on the evening of July 28, 1997 in combination with the rains that fell <br />in eastern Larimer County from late afternoon July 27 through midday July 28 produced the <br />heaviest documented rainfall in Colorado since 1981 and one of the II most severe rainstorms <br />of the Twentieth Century along the Colorado Front Range. From all available data, this <br />appears to be the heaviest rainfa1l ever recorded over a developed urban area in Colorado. <br />The area of high intensity rainfall was small and covered only a few square miles. Gradients of <br />rainfall were so remarkable that many residents of Fort Collins and outlying areas were <br />unaware that flood-producing rains had fallen while other neighborhoods only a short distance <br />away were inundated with heavy rain and flooding. <br /> <br />Many large storms of the past have been documented and described based on limited rain <br />gauge data supplemented by "bucket survey" analyses. With nearly 300 gauge measurements <br />over a relatively sma1l area, the Fort Collins storm is the most thoroughly documented of any <br />extreme rainfa1l event in Colorado up to this time. <br /> <br />Acknowledgments <br /> <br />The authors wish to thank the many, many individuals and organizations who helped compile <br />rainfall information for this storm. Tun Wrrshborn of Mountain States Weather Services was <br />an incredible asset to this effort and has done so much over the past 20+ years to help the <br />citizens of Fort Collins to watch and appreciate daily weather. The City of Fort Collins, <br />including such individuals as Marsha Hilmes with the Storm Water Utility and Dennis Bode of <br />the Fort Collins Water Utility, provided prompt, cheerful and ongoing assistance. We <br />appreciated the cooperation from the Fort Collins Coloradoan in publishing appeals for rainfall <br />information. Similar thanks go to KIIX and KCOL radio stations for broadcasting our <br />requests. Thanks to Dr. Richard Johnson of the Colorado State University Department of <br />Atmospheric Science for compiling rainfall reports. Investigative work by John Weaver, <br />Poudre Fire Authority Volunteer and Meteorologist with the National Oceanic and <br />Atmospheric Administration, was very helpful. Larry Tunnell, National Weather Service <br /> <br />t <br /> <br />'''',"",-_u <br /> <br />41 <br />