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<br />Mesoscale and Radar Observations <br />of the Fort Collins Flash Flood <br />of 28 July 1997 <br /> <br /> <br />Walter A. Petersen: Lawrence D. Carey: Steven A. Rutledge: Jason C. Knievel: <br />Nolan 1. Doesken:'+ Richard H. Johnson: Thomas 8, McKee:,+ <br />Thomas Yonder Haar:,# and John F. Weaver@ <br /> <br />ABSTRACT <br /> <br />On the evening of 28 July 1997 the city of Fort Collins, Colorado, experienced a devastating flash flood that caused <br />five fatalities and over 200 million dollars in damage, Maximum accumulations of rainfall in the western part of the city <br />exceeded lOin. in a 6-h period. This study presents a multiscale meteorological overview of the event utilizing a wide <br />variety of instrument platforms and data including rain gauge, CSU-CHILL multiparameter radar, Next Generarion Radar, <br />National Lightning Detection Network, surface and Aircraft Communication Addressing and Reporting System obser- <br />vations, satellite observations, and synoptic analyses. <br />Many of the meteorological features associated with the Fort Collins flash flood typify those of similar events in the <br />western United States. Prominent features in the Fort Collins case included the presence of a 500-hPa ridge axis over <br />northeastern Colorado; a weak shortwave trough on the western side of the ridge; postfrontal easterly upslope flow at <br />low levels; weak to moderate southwesterly flow aloft; a deep, moist warm layer in the sounding; and the occurrence of <br />a quasi-stationary rainfall system. In contrast to previous events such as the Rapid City or Big Thompson floods, the <br />thermodynamic environment of the Fort Collins storm exhibited only modest instahility, consistent with low lighming <br />flash rates and an absence of hail and other severe storm signatures. <br />Radar. rain gauge, and lightning observations provided a detailed view of the cloud and precipitation morphology. <br />Polarimetric radar observations suggest that a coupling between warm-rain collision coalescence processes and ice pro- <br />cesses played an important role in the rainf~ll production. Dual-Doppler radar and mesoscale wind analyses revealed <br />that the low-level flow field associated with: a bow echo located 60 Ian to the southeast of Fort Collins may have heen <br />responsible for a brief easterly acceleration in the low-level winds during the last 1.5 h of the event The enhanced flow <br />interacted with both topography and the contection located over Fort Collins, resulting in a quasi-stationary convective <br />system and the heaviest rainfall of the evenihg. <br /> <br />*Department of Atmospheric Science. Colorado State University. <br />Fort Collins, Colorado, <br />+Colorado Climate Center, Department of Atmospheric Science, <br />Colorado State University, Fort Collins, Colorado, <br />*Cooperative Institute for Research in the Atmosphere, Colorado <br />State University, Fort Collins, Colorado, <br />@NOAAlNESDIS, RAMM Branch, Cooperative Institute for Re- <br />search in the Abnosphere, Colorado State University, Fort Collins, <br />Colorado, <br />Corresponding author address: Dr. Walter A. Petersen, Depart- <br />ment of Atmospheric Science, Colorado State University, Fort <br />Collins, CO 80523, <br />E-mail: walt@olymplc.abnos,colostate,edu <br />In final form 19 August 1998, <br />@1999AmericanMeteorological Society <br /> <br />Bulletin of the American Meteorological Sociely <br /> <br />1. Introduction <br /> <br />On the evening of 28 July 19~7 the city of Fort <br />Collins, Colorado, experienced al devastating flash <br />flood that caused five fatalities and greater than 200 <br />million dollars in damage, The rainfall associated with <br />this event set new records in Fort Collins (FCL) for <br />the largest I-day, 6-h, and 3-h precipitation totals re- <br />corded at the Colorado State University (CSU) <br />weather observatory, and is one of the largest rainfall <br />events ever documented over a developed urban area <br />in Colorado (Doesken and McKee 1998), At the CSU <br />observatory, 5.30 in, (13,46 cm) of rain fell in just <br /> <br />191 <br />