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<br />The Storm That Caused the Flood <br /> <br />In the weeks leading up to the flood, Fort Collins and vicinity experienced a six-week <br />period of predominantly hot and very dry weather. A total of just 0.36 inches of rain had <br />fallen from June 15 through the afternoon ofJuly 27. But in that last week ofJuly, the <br />humidity increased as moist tropical air drifted northward. Widely scattered afternoon <br />thundershowers developed in parts of Colorado each day. On July 27, a fairly strong cold <br />front moved southward into the region. This cold front served as the trigger to help <br />initiate numerous storms over northern Colorado late that afternoon, As the front pushed <br />southward across eastern Colorado, southeasterly surface winds developed that brought <br />even more humid air from Kansas into the region, The combination of a tropical air mass <br />with light winds aloft (above mountain top level), the extrernely moist near-surface air <br />from the east and the air mass boundary to help trigger storm development provided the <br />ingredients needed for copious Front Range rainfall. <br /> <br />As this weath~ pattern developed, many meteorologists noted the strong similarity with <br />weather conditions associated with previous flash floods - the Rapid City storm of June <br />9,1972, and the Big Thompson flood of July 31,1976, for example. The potential <br />certainly existed, yet no one knew exactly where or if a similar storm would develop, <br /> <br />The first round of storms late on Sunday afternoon (July 27) were not unusual. Small, <br />localized storms, complete with crashing thunder and gusty winds, brought torrents of <br />rainto areas in the lower foothills just west and northwest of Fort Collins between 5:00 <br />and 6:00 PM MDT. After dropping as much as 2.4 inches of rain near the south end of <br />Horsetooth Reservoir, the storms quickly diminished and brought only a light shower to <br />most of the city, But instead of clearing off after dark - the normal summer weather <br />pattern, moist southeasterly winds strengthened. Low, dark clouds hugged the foothills. <br />Late at night, rains developed again, but this time with a different character. Steady, <br />drumming rains without the accompaniment of lightning or thunder expanded along the <br />base of the foothills, Areas two or more miles east of the foothills again received <br />relatively light rains, but at the immediate base of the foothills, heavy rainfall was noted. <br />A gloomy dawn July 28 brought a temporary break in the rainfall, but then the <br />downpours began again, this time even more localized. While most of Fort Collins had a <br />cloudy, cool morning, rain poured along the base of the foothills from 8:00 AM to noon. <br /> <br />A detailed time history of the storm is given by Doesken and McKee (1998) as they <br />analyzed rainfall reports gathered from more than 300 locations in and near Fort Collins. <br />Unknown to most residents of Fort Collins, six to 10 inches of rain had already fallen by <br />midday July 28 from the north end of Horse tooth Reservoir to northwest of the small <br />town of Laporte (Figures 2 and 3). Areas west and southwest of Fort Collins received <br />two to four inches of rain. Flooding was severe during the day in and near Laporte. <br /> <br />Figures 2 and 3 <br /> <br />Rainfall abated across the region Monday afternoon (July 28). However, high humidity <br />air continued in place with dewpoint temperatures in the low 60s Fahrenheit, unusually <br /> <br />3 <br />