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<br />sensed rainfall data were also collected from the National Weather Service WSR-88D Doppler <br />radars at Cheyenne, Wyoming, and near Denver, Colorado, and also from the Colorado State <br />University CHILL dual-polarized research radar near Greeley, this stonn presented one of the <br />best opportunities in Colorado history to accurately define rainfall patterns from an extreme <br />stonn. <br /> <br />Weather Conditions Associated with the July 27-28,1997 Rains <br /> <br />From mid June into the final week of July 1997, the Fort Collins area and much of Colorado <br />had been experiencing several weeks of dry weather. At the official Fort Collins weather <br />station on the campus of Colorado State University, only 0.36 inches of precipitation had <br />fallen since June 15, just 16 percent of average for that time period Temperatures had also <br />been persistently hot since July 7. Beginning around July 18, humidity increased over <br />Colorado as tropical moisture began its typical mid-summer northward drift into the <br />southwestern U.S. Afternoon thunderstorms became more numerous, and a few were locally <br />quite heavy such as the one that dropped 3.83 inches ofrain injust over one hour at the <br />National Weather Service Forecast Office at Denver Stapleton on the afternoon ofJuly 19. <br /> <br />Despite increasing humidity, thunderstonn activity remained fairly isolated until an unusually <br />strong mid-summer cold front reached eastern Colorado on July 27. A large high pressure <br />area perched over southern Canada pushed cooler air southward toward Colorado. Early on <br />Sunday morning (July 27) the cold front extended from approximately Minnesota to southern <br />Wyoming (Figure I). At the same time, moisture continued to stream northward into <br />Colorado from the south. As the cold front moved southward over Colorado during the day, <br />easterly swface winds developed that began to push very humid surface air that had been over <br />Kansas and adjacent plains states into eastern Colorado. These conditions set the stage for <br />the very heavy rains that fell in the ensuing hours July 27-28, 1997 over the Fort Collins area <br />and were surprisingly similar to the conditions of July 31,1976 which produced the disastrous <br />Big Thompson flash flood. <br /> <br />Several atmospheric scientists are investigating the weather conditions preceding and during <br />the Fort Collins stonn. Detailed meteorological analyses will be available when these studies <br />are completed. This report does not attempt to duplicate those efforts. <br /> <br />Following the Fort Collins flood on July 28, very moist air continued to linger over Colorado <br />for several days, and more extreme rainfall events occurred. The very next evening, Fort <br />Collins experienced a near miss as heavy downpours again developed. Fortunately, these <br />stonns took a different track and dropped most of their rain north and east of Fort Collins. <br />Portions of Larimer County received more than two inches of rain during the evening of July <br />29th. But farther to the east a truly remarkable nighttime stonn inundated the Pawnee Creek <br />watershed northwest of Sterling, Colorado. This stonn likely surpassed the Fort Collins <br />stonn significantly in terms of both total rainfall and stonn area. At least 14 inches of rain in <br />6 hours is believed to have fallen over an area of perhaps 15 square miles with 4 inches or <br />more over nearly 500 square miles (Browning and Lang, 1997). The resulting flood inundated <br />large portions of Sterling and Atwood along with thousands of acres offanniand. <br /> <br />2 <br /> <br />~~ <br />