An Analysis of Rainfall for the July 28, 1997 Flood in Fort Collins, Colorado

Nolan J. Doesken and Thomas B. McKee

Colorado Climate Center
Atmospheric Science Department
Colorado State University
Fort Collins, CO 80523-1371

The Rain that Caused the Flood

The following narrative accompanies the rainfall maps (Figure 7-14) and briefly describes the rains that culminated in the devastating Fort Collins flood of July 28, 1997.

Cloudy skies greeted residents of eastern Larimer County on Sunday, July 27, 1997. After many days of hot, dry weather, cooler temperatures were a welcome change. Skies remained cloudy into the early afternoon, but in mid afternoon the sun broke through. As the temperatures rose briefly into the 80s (Fahrenheit), the air felt oppressively humid. Dewpoint temperature measured on campus at Colorado State University climbed to near 60 degrees.

At that time, 4:00 p.m. (1600 MDT), cloud masses began to bubble upwards forming towers of billowing dark-bottomed cumulus clouds over the foothills northwest of Fort Collins. Thunder began to rumble, and by 5:00 p.m., the first torrents of heavy rain poured down northwest of Laporte. Close to 1.50 inches of rain fell in just over 30 minutes not far from Ted's Place (four miles northwest of Laporte), and another core of heavy rain fell farther northwest beyond Livermore. Quickly the storm expanded southward, and within minutes sheets of rain splashed onto Horsetooth Reservoir, hiding the foothills behind them. Thunder crashed and storm clouds spread eastward and southward out across Fort Collins. People ran for shelter expecting a downpour, but the heavy rains were limited to the lower foothills and diminished quickly to the east. Except for extreme western Fort Collins, most of the city received less than one quarter inch of rain, but near the south inlet bay of Horsetooth Reservoir, 2.42 inches of rain was measured from this quick storm. These first heavy rains were mostly over by 6:30 p.m. with only occasional sprinkles and light showers after that. Damp, rain-chilled air spread out across Fort Collins, but thunder continued to rumble east and south of town. Most areas of Loveland were drenched with one to two inches of welcome rains before the storm diminished after 8:00 p.m.

This first round of thunderstorms was quite typical of late July. The storms developed and struck quickly, with a commotion of lightning and thunder, only to end just as quickly. While some areas were soaked, other nearby locations were nearly dry (">Figure 7 and ">Figure 11.) This was just the beginning, though. The air remained humid and fragrant. Instead of clearing off at sunset like a normal summer evening, dense, dark clouds continued to hug the foothills and the temperature remained steady. The air was still, and sounds seemed to carry a long way.

Around 10:00 p.m. (2200 MDT), a few flashes of lightning lit up the sky just south of Fort Collins. As thunder rumbled, a brief shower of rain fell on some south-side neighborhoods in Fort Collins. The rain began to spread northward, and a similar brief but localized shower fell near Bellvue, but ended quickly. Perhaps 0.50 inches of rain fell from each of these showers, but over very small areas. The rain and thunder ended, and most residents of Fort Collins went to bed enjoying the refreshing dampness that summer storms bring – happy to know that they could skip watering their yards or gardens for at least a day or two.

After midnight, southeasterly winds, behind the cold front that had triggered the evening storms, increased and pushed more moist air up against the eastern foothills. Sometime around 0100 MDT Monday, rain began again. Without the excitement and fanfare of lightning and thunder, steady rains developed – not hard enough to wake most sleepers. At first the rains were limited to a very narrow band right along the first hogback from the southwest edge of Fort Collins northward to near Owl Canyon (15 miles northwest of Fort Collins). For a few hours the rains intensified. Between 0200 to 0400 MDT rain fell over much of eastern Larimer County. Rainfall rates in excess of one inch per hour developed northwest of the town of Laporte while rates were much lower farther east. The rains diminished again after 0400 MDT, and by the first light of dawn rain ended except in a very narrow band along the foothills from southwest of Fort Collins northward over Horsetooth Reservoir and then continuing northward approximately following U.S. Highway 287 to Livermore. Residents of this area awoke to gloomy, dark skies. Water was standing or flowing in this hilly area, and rain intensity fluctuated from light showers to occasional downpours.

Between 0600 and 0730 MDT, the rain tapered off along the foothills and stopped completely elsewhere. Around 0800 rain began again. A brief but soaking shower caught many morning commuters in Fort Collins, while to the northwest the heaviest rain of the morning began to cause major flooding around Laporte. From southwest Fort Collins near Hughes Stadium northward to Ted's Place (northwest of Laporte), one to two inches of rain fell between 0800 and 0900 MDT. The area of heavy rain shrank after 0930, but remarkably heavy and highly localized torrents continued northwest of Laporte until after 1100 MDT. People driving northwestward out of Fort Collins were shocked to go from dry roads in town to pouring rains and flood waters covering highways near Laporte - all of this without the accompaniment of lightning or thunder.

The rains ended across all of Larimer County by noon, but not before six to eight inches of early morning rain had fallen northwest of Laporte. North and south of this storm center, three to six inch rain totals were common in the narrow band along and east of U.S. Highway 287 northward to Owl Canyon (15 miles northwest of Fort Collins) and south to Lory State Park and Horsetooth Reservoir immediately west of Fort Collins. Rainfall totals dropped off quickly to the east (Figure 8 and Figure 12), but two to four inches fell over portions of west Fort Collins west of Taft Hill Road. All of the Fort Collins area received some rain early Monday morning but most totals were only 0.50 to 0.75 inches over the eastern half of the city. South of Fort Collins in the vicinity of Loveland and Berthoud, only a few scant showers had fallen.

Many irrigation canals exit the Poudre River near Laporte. While the localized rains had little effect on the morning flow rates on the main river, irrigation canals were a different story. Headgates were shut early that morning, and still the ditches filled with runoff from the Laporte and Bellvue floodwaters. Many downstream residents were surprised by the high water, many of whom were not aware of the heavy rains near Laporte.

Skies remained cloudy over the Fort Collins area Monday afternoon. Clouds hung low along the foothills, as steady southeasterly surface winds continued to push very moist surface air into Larimer County. Dewpoint temperatures, a measure of the water content of the air, stayed in the low 60s all day. With weather conditions not unlike those that preceded the infamous Big Thompson flood in Larimer County 21 years previous, weather forecasts called for "locally torrential rains." Hallway, phone and e-mail conversations among climatologists, meteorologists and hydrologists along the Front Range from Denver to Cheyenne speculated on where the flash flood might occur this time. Weather forecasters familiar with Front Range flash flood conditions knew that this situation was potentially dangerous. But with all the rain that had already fallen, no one knew where or even if another flash flood producing storm would erupt.

During the afternoon of July 28th, heavy thunderstorms began to develop west of Denver and in other areas of the state. In eastern Larimer County, the first showers appeared around 5:00 p.m. (1700 MDT). They began innocently enough – brief showers moving from south to north with little or no lightning and thunder. Just before 6:00 p.m. (1800 MDT) the first wave of heavy showers moved into Fort Collins. Like the early rains, these evening showers seemed to hug the base of the foothills. The rains increased in both area and intensity between 1800 and 1900 MDT with hourly accumulations of close to one inch in southwest Fort Collins, but with lighter rains over most of the Fort Collins area from the lower foothills out onto the plains. A few bolts of lightning accompanied these rains, but electrification was surprisingly little considering the intensity of the rains. Unlike so many Colorado storms, no hail was reported. Many individuals independently noted how warm the rain seemed. Raindrop sizes were not large, considering the intensity of the rain, and no strong winds accompanied the rains – at least not in the immediate Fort Collins area. One report of strong winds blowing out of the north (out from the storm center) was filed by a weather watcher a few miles northwest of Loveland.

Extreme rainfall rates may have begun earlier, but the first burst to reach the recording rain gauge at the Atmospheric Science Department on the Foothills Campus of Colorado State University (extreme west Fort Collins just east of Horsetooth Reservoir) began shortly before 7:00 p.m. (1900 MDT). For a few minutes, rainfall rates approached three inches per hour and then tapered off again. Surges of extremely heavy rains seemed to emanate from southwest Fort Collins and spread northward in waves over the west side of the city. Heavy rains also reached northern portions of Fort Collins and continued northward into the county, while only light to moderate rains were observed over southeast Fort Collins.

Lightning activity increased around 2000 MDT as heavy rains continued to fall. What began as minor street flooding became increasingly more serious over the western half of Fort Collins as the downpours continued. Then for a few minutes parts of town experienced a lull in rainfall intensity before 8:30 p.m. (2030 MDT). Soon after that, the rains let up or ended completely just a few miles south of town, in the vicinity of Masonville southwest of Fort Collins and over southeastern portions of the city. At the same time, cloud watchers east of Fort Collins noted the storm clouds that had seemingly been spreading eastward and northward appeared to retreat back towards the west and condense into a small but ominous cloud mass over and west of the city.

Most summer thunderstorms would have begun to dissipate or move away by this time, but this storm was an exception. Instead of weakening, rainfall intensities increased again, and the most intense rains were still ahead. From about 8:30 to 10:00 p.m. (2030 to 2200 MDT) extremely heavy rain, of a magnitude rarely experienced in northern Colorado, was localized over an area of a few square miles centered not far from the corner of Drake Road and Overland Trail in extreme southwestern Fort Collins. Based on a variety of individual observations and numerous reports of over-topped rain gauges, it appeared that rainfall totals for this 90-minute period approached or exceeded five inches over the approximate area delineated by Taft Hill Road on the east, the crest of the hogback formation that forms the eastern edge of Horsetooth Reservoir on the west, the western extension of Horsetooth Road on the south and approximately Elizabeth Street on the north (see Figure 13). This area includes much of the Spring Creek watershed. Maximum instantaneous rainfall rates likely exceeded 5-6 inches per hour at times. With these extreme rainfall rates falling on a surface already covered with flowing water, incredible volumes of water accumulated that moved downhill from approximately west to east across Fort Collins initiating the devastating flooding. Other reports are being written by hydrologic experts that focus on the flooding produced by this remarkable rainstorm.

Mercifully, the rains came to a sudden halt. In southwest Fort Collins near Harmony Road, eyewitnesses reported the rain ended abruptly a few minutes before 10:00 p.m. (2200 MDT). Farther north along Drake Road the rain ended just after 10:00 p.m. On the campus of Colorado State University, the rain ended right about 10:30 p.m. (2230 MDT) while in extreme northern parts of the Fort Collins area, lighter rains continued until at least 11:00 p.m. (2300 MDT). As the rain ended, a cool, damp stillness again covered the area, just like the night before – except for the sounds of sirens and the roar of local flood waters searching for a path to the Poudre River. Some distant lightning flashed, but the Fort Collins storm was over.

The rainfall pattern from the July 28, 1997 evening storm over eastern Larimer County is shown in Figure 9 with an expanded view over the immediate Fort Collins area in Figure 13. Compared to many U.S. storms, the rains that produced the severe flooding in Fort Collins were remarkably localized. The heaviest documented rainfall totals exceeded ten inches in less than five hours and were found in extreme southwest Fort Collins. An extremely tight rainfall gradient was observed southeastward from the storm center, with less than two inches of rain reported less than three miles east and southeast from the maximum. This is an excellent demonstration of the huge variations in rainfall over short distances that are possible with summer convective storms.

The core of the heaviest rains was immediately east of the hogback that marks the beginning of the foothills and serves as the eastern bank of Horsetooth Reservoir. The band of heaviest rains reached northward from the storm center toward the towns of Laporte and Bellvue and remained oriented parallel to the foothills. This positioning strongly suggests that topography played an important and perhaps controlling role in positioning this storm. North of Fort Collins, the rainfall diminished and spread out, but a secondary rainfall maximum was observed in the upper Boxelder watershed east of Virginia Dale.

Total accumulated rainfall for the period beginning at 1600 MDT on July 27, 1997 through 2300 MDT July 28 is shown in Figure 10 and Figure 14. While probably not a coincidence, it is still quite remarkable that each episode of heavy rain during this 31-hour period produced its maximum rainfall totals right at the base of the foothills. As a result, the accumulated totals look very much like a single storm rainfall pattern. Three separate maxima appear on the map all in a similar location with respect to the local topography: 1) a 14.5 inch total in southwest Fort Collins produced primarily by the July 28 evening storm but with significant contributions from both the Sunday evening and the Monday morning rains, 2) a 13.1 inch total northwest of Laporte produced primarily by the morning rains July 28 but with contributions from the two other major rainfall periods, and 3) A total of over 12 inches near Claymore Lake northwest of Fort Collins which coincidentally received nearly the same amount of rain both Monday morning and then again Monday evening.

The composite rainfall pattern shows that rainfall totals exceeded ten inches over an area approximately 12 miles long and two to three miles wide for a total area approaching 30 square miles.