Mesoscale and radar observations of the Fort Collins flash flood of 28 July 1997

On the evening of 28 July 1997 the city of Fort Collins, Colorado, experien ced a devastating flash flood that caused five fatalities and over 200 mill ion dollars in damage. Maximum accumulations of rainfall in the western par t of the city exceeded 10 in. in a 6-h period. This study presents a multis cale meteorological overview of the event utilizing a wide variety of instr ument platforms and data including rain gauge, CSU-CHILL multiparameter rad ar, Next Generation Radar, National Lightning Detection Network, surface an d Aircraft Communication Addressing and Reporting System observations, sate llite 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. Prominen t features in the Fort Collins case included the presence of a 500-hPa ridg e axis over northeastern Colorado; a weak shortwave trough on the western s ide 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 thermody namic environment of the Fort Collins storm exhibited only modest instabili ty, consistent with low lightning flash rates and an absence of hail and ot her severe storm signatures. Radar, rain gauge, and lightning observations provided a detailed view of t he cloud and precipitation morphology. Polarimetric radar observations sugg est that a coupling between warm-rain collision coalescence processes and i ce processes played an important role in the rainfall production. Dual-Dopp ler radar and mesoscale wind analyses revealed that the low-level flow fiel d 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-lev el winds during the last 1.5 h of the event. The enhanced flow interacted w ith both topography and the convection located over Fort Collins, resulting in a quasi-stationary convective system and the heaviest rainfall of the e vening.