<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
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