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<br />Most summer thunderstonns, and the 1992 event was a good example, do not last long. It is <br />rare for such stonns to continue at a high intensity much beyond one hour. The Fort Collins <br />stonn of July 1997 stands out because of its prolonged high intensity. At durations of three <br />and six hours (Figure 17) this recent stonn is unprecedented among observed rains on <br />campus. 4.42" was measured in three hours 2000-2300 MDT July 28, 1997 which was nearly <br />1.50 inches more than the previous three-hour record established August 3, 1951. The six- <br />hour total was 5.30" on campus compared to 3 inches from the August 1951 stonn. While <br />impressive in its own right, the campus measurement was only about one-half of the rainfall <br />total for that period at the stonn's center three miles southwest from the main campus. <br /> <br />Flooding from heavy rains September 2-3, 1938 is well documented in the Fort Collins area. <br />However, the observed rainfall for that stonn of3.54 inches in one day and 4.68 inches in two <br />days places that stonn as only the 5th largest rainstonn in 109 years of Colorado State <br />University measurements. The reason that flooding was severe appears to be that the stonn <br />was more intense over the foothills immediately west and southwest of Fort Collins. <br /> <br />Comparison to the 100-year storm <br /> <br />The 100-year stann is a statistical rainfa1l amount derived from historical rainfall observations <br />and used by hydrologists, engineers and planners. Figures 18 and 19 show the 100-year <br />rainfa1l estimates for the Larimer County area for 6-hour and 24-hour periods, respectively. <br />Interpolation from these federally-produced maps indicates that for central Fort Collins, the <br />6-hour 100-year rainfall is approximately 3.5 inches while the 24-hour 1000year event is 4.8 <br />inches. These maps were produced in the early 1970s and have served as the primary <br />information source for estimating design rainfall amounts. An update to these maps is <br />available for neighboring states south and west of Colorado and is planned for Colorado and <br />neighboring states to the east and north. <br /> <br />Based on the campus rainfall measurements, the maximum 6-hour rainfa1l on the evening of <br />July 28, 1997 of 5.30 inches exceeded the 100-year value for that time period by a significant <br />amount, 1.80 inches. However, since most of the rain on campus was confined to a 6-hour <br />period, the stonn only slightly exceeded the 100-year, 24-hour value. Near the center of the <br />stonn over southwest Fort Collins, these comparisons were much more impressive. For the <br />intense rainfall period during the evening hours of July 28, 1997, the maximum rainfa1l was <br />three times greater than the l00-year 6-hour value. A direct comparison with the 100-year <br />24-hour stonn rainfall is tricky since the stonn totals from Figures 10 and 14 were actua1ly <br />31-hour totals. However, for those areas of maximum rainfall along the base of the foothi1ls, <br />the stonn total rainfall was 2 to 3 times greater than the 100-year 24-hour values. <br /> <br />A great deal of misunderstanding has developed regarding the 100-year stonn (Doesken, <br />1996) and what it means. In many people's minds, it appears that 100-year and greater <br />stonns occur "all the time" and they question the accuracy and usefulness of the values. In <br />applying stonn statistics like these, it is necessary to realize that these are "single point" <br />values. A rainfa1l of 4.8 inches in 24 hours may, in fact, have only a 1 percent likelihood of <br />occurrence on the Colorado State University Campus. However, if you look at all stonns <br />each summer over all of northeast Colorado, it is almost a certainty that rainfall will exceed <br /> <br />39 <br />