FLOOD00626

• County: Larimer
• Community: Fort Collins
• Title: An Analysis of Rainfall for the July 28, 1997 Flood in Fort Collins
• Date: 2/1/1998
• Prepared For: State of Colorado
• Prepared By: CSU
• Floodplain - Doc Type: Flood Documentation Report

these analyzed precipitation patterns is very high in the immediate Fort Collins area due to the
concentration of data. However, outside of Fort Collins, data were much sparser, and the
confidence in the analysis was lower.

To increase confidence and to add important details in data-sparse areas, an independent
source of data was obtained. The National Weather Service (NWS) weather surveillance
radar (WSR-88D) just east of Denver had operated continuously throughout the storm period,
and the radar reflectivity data were archived. With excellent assistance from Larry Tunnell,
the hydrologist at the Denver NWS forecast office, maps of estimated precipitation were
computed from radar reflectivity patterns. An example is provided in Figure 6. Radar-derived
rainfall estimates may differ significantly from actual measured values since reflectivity is a
function of not only rainfaIl rate, but also raindrop size, the presence of hail, and the distance
from the radar transceiver (which detennmes the height above ground and the sampling
volume). In this case, rainfa1l totals compared exceptionally well during the early portion of
the storm on Sunday afternoon. The relationship between rainfall and radar reflectivity
changed dramaticaIly later Sunday evening. From late Sunday evening through the end of the
storm Monday night, the WSR-88D radar severely underestimated surface rainfa1l. The
general spatial patterns, however, were captured well. These patterns were used to
qualitatively adjust precipitation isohyets (rainfall contour lines) in areas with little or no
surface data. Data from the Cheyenne, WY NWS radar were also available but were not
included in this analyses because of time limitations.

Colorado State University operates a special research radar facility northeast of Greeley. The
CHILL radar was operated during a portion of the storm and captured most of the intense
rainfall period on the evening of Monday July 28. Due to its proximity to Fort Collins, and
some of the special features of this research radar, it afforded a particularly detailed remotely
sensed view of rainfa1l characteristics that fateful evening. We will not go into special detail
about the CHllL radar results, since other publications on that topic are planned by CHllL
scientists and engineers. However, the radar output proved very helpful in completing the
fina1 high resolution rainfa1l map of the immediate Fort Collins area. From the few rainfaIl
reports from the lower foothills immediately southwest and west of the city, it was apparent
that rainfa1l totals diminished rapidly west of the city. There were not enough surface
measurements, though, to positively define the gradients. The CHILL radar data provided an
objective approach to estimating these gradients and more accurately defining the storm's
area.

The NWS and the CSU-CHllL are both Doppler radars. However, the CHILL is a dual
polarization radar which aIlows more information about the precipitation particles to be
determined. Two comparisons were made of radar derived precipitation to surface
observations. The radar estimates were provided by Lawrence Carey and Walt Petersen
members of Professor Steve Rutledge's research group. One comparison was made using the
CHllL reflectivity only in a manner similar to the method used by the NWS. The resuhs
showed a ratio of observed surface precipitation to the radar derived precipitation with values
of approximate 2.6 in the northwest portion of the storm and 1.5 in the southeast portion of
the storm. These large variations in the ratio of radar reflectivity to surface rainfaIl suggest a

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