Mesoscale and Radar Observations of the Fort Collins Flash Flood of 28 July 1997

Abstract

On the evening of 28 July 1997 the city of Fort Collins, Colorado, experienced a devastating flash flood that caused five fatalities and over 200 million dollars in damage. Maximum accumulations of rainfall in the western part of the city exceeded 10 in. in a 6-h period. This study presents a multiscale meteorological overview of the event utilizing a wide variety of instrument platforms and data including rain gauge, CSU-CHILL multiparameter radar, Next Generation Radar, National Lightning Detection Network, surface and Aircraft Communication Addressing and Reporting System observations, satellite observations, and synoptic analyses. Many of the meteorological features associated with the Fort Collins flash flood typify those of similar events in the western United States. Prominent features in the Fort Collins case included the presence of a 500-hPa ridge axis over northeastern Colorado; a weak shortwave trough on the western side of the ridge; postfrontal easterly upslope flow at low levels; weak to moderate southwesterly flow aloft; a deep, moist warm layer in the sounding; and the occurrence of a quasi-stationary rainfall system. In contrast to previous events such as the Rapid City or Big Thompson floods, the thermodynamic environment of the Fort Collins storm exhibited only modest instability, consistent with low lightning flash rates and an absence of hail and other severe storm signatures. Radar, rain gauge, and lightning observations provided a detailed view of the cloud and precipitation morphology. Polarimetric radar observations suggest that a coupling between warm-rain collision coalescence processes and ice processes played an important role in the rainfall production. Dual-Doppler radar and mesoscale wind analyses revealed that the low-level flow field associated with a bow echo located 60 km to the southeast of Fort Collins may have been responsible for a brief easterly acceleration in the low-level winds during the last 1.5 h of the event. The enhanced flow interacted with both topography and the convection located over Fort Collins, resulting in a quasi-stationary convective system and the heaviest rainfall of the evening.

Statistics of extremes in hydrology

“…http://ulysses.atmos.colostate.edu). Because of a flood that occurred on 28 July 1997, this data set is of special interest (although this particular rain gauge is not actually situated within the area that received the most intense precipitation) [72]. Fig.…”

Spatial Analysis of High-Resolution Radar Rainfall and Citizen-Reported Flash Flood Data in Ultra-Urban New York City

“…Furthermore, rainfall which causes urban flooding is extremely concentrated both in time and in space. The devastating Fort Collins flood of 1997 was caused by rainfall totals in excess of 10 inches near the flood site, but rainfall reached only 2 inches 4 km away [23]. The rainfall was so concentrated in space that it was missed by local weather spotters and had to be verified through residents' backyard rain gages [24].…”

A heavy rainfall event in northern Tasmania during 27 to 30 January 2004

“…In the 1983 case, however, the authors argue that coastal convergence triggered the initial convection, rather than channelling within the mountains. Petersen et al (1999) note that thunderstorms which caused a devastating flash flood event at Fort Collins, Colorado, were formed in an area where orographic focussing acted together with outflows from earlier storms. Indeed, Doswell (1985) argues that topography may in some cases be the single most important factor influencing the development of convection leading to flash flooding.…”