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<br />difference in the size distribution of the rain drops may have occurred between these two <br />regions of the stonn. <br /> <br />The second comparison was accomplished by using a combination of reflectivity and phase <br />differences between the two polarizations. This was an exploratory research methodology <br />that will be tested more in the future. It produced a ratio of observed precipitation to radar <br />derived precipitation of approximately 1.4 which was quite unifonn over the stonn area. This <br />ratio was then applied to estimate surface rainfall in the data sparse areas west and south of <br />the stonn center. Estimates of rainfall amounts were calculated on a 1 mile by 1 mile grid <br />over and south of Horse tooth Reservoir. Approximately 20 data points were added to the <br />surface observations in this data sparse region to provide enough data to complete the map of <br />precipitation for the Monday evening period (Figure 10 and 14). <br /> <br />The addition of the radar information did not change the rainfall magnitudes or observed <br />precipitation patterns over the city of Fort Collins. However, it did add detail to define the <br />patterns and gradient over the western and southwestern edges of the stonn. Further research <br />on development of rainfall algorithms for the CHILL radar have indicated closer agreement <br />with surface observations but have not changed the areal pattern of precipitation. <br /> <br />After combining radar data for data sparse areas with the measured data, fina1 computer- <br />generated maps were produced. These were carefully scrutinized, manually smoothed and <br />graphically enhanced for visual clarity. The fina1 result of this lengthy effort was a set of <br />detailed rainfall maps for each of the following four accumulation periods: <br /> <br />1) Sunday, July 27,1997, 1600-2100 MDT. <br /> <br />2) July 27, 1600 MDT through Monday July 28, 1300 MDT. <br /> <br />3) Monday, July 28, 1997, 1730-2300 MDT. <br /> <br />4) Total Rainfall, July 27,1997,1600 MDT through July 28, 1997,2300 MDT. <br /> <br />Figures 7-10 are regional maps covering much of eastern Larimer County. Figures 11-14 <br />show the Same four time periods in expanded fonn for the immediate Fort Collins area. <br /> <br />The Rain that Caused the Flood <br /> <br />The following narrative accompanies the rainfall maps (Figure 7-14) and briefly descn'bes the <br />rains that culminated in the devastating Fort Collins flood of July 28, 1997. <br /> <br />Cloudy skies greeted residents of eastern Larimer County on Sunday, July 27,1997. After <br />many days of hot, dry weather, cooler temperatures were a welcome change. Skies remained <br />cloudy into the early afternoon, but in mid afternoon the sun broke through. As the <br />temperatures rose briefly into the 80s (Fahrenheit), the air felt oppressively humid. Dewpoint <br />temperature measured on campus at Colorado State University climbed to near 60 degrees. <br /> <br />20 <br />