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

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experiences. Many recollections were shared, but only those remarks pertinent to the
actual observations were recorded.

In all, 307 rainfall reports were obtained. At least two-thirds of these involved phone
conversations or e-mail messages. This process of information gathering consumed, on
average, about 30 minutes per rainfall report. With one full-time professional working on this
activity with additional part-time support, it was stilI only possible to process about 20 to 25
rainfall reports each day. Driving surveys were even more time consuming since it often took
several stops and conversations in order to track down a single report. The easiest data
collection was the existing National Weather Service cooperative stations and those
maintained by Mountain States Weather Services. The advantages of these sites were that
they recorded their observations on standardized forms, used known types of rain gauges and
their locations were already known and indexed. There were simply not enough of these
existing stations, however, to adequately describe the stonn. Despite being consistent and
well documented, follow-up calls were sometimes still needed to obtain more detailed
information from these observing sites.

Of the 307 rainfall reports, 281 were actual gauge measurements. Measurements came from a
variety of rain gauges ranging from official National Weather Service eight-inch diameter
standard manual rain gauges and a variety of electronic recording gauges to several types of
low-cost hardware-store-type gauges. While the Fort Collins area did not have an existing
special precipitation network, 19 recording gauges were identified in the region which offered .
hourly or finer time resolution data for documenting rainfall rates and time distributions. Of
the many manual gauges, nearly one third were quality gauges meeting National Weather
Service guidelines for quality. The majority of these were the 4-inch diameter clear view
gauge that is thought to be as accurate as the official NWS 8-inch diameter standard gauge.

This large number of high quality gauges made a remarkable contribution to the validity of
data collected about this stonn. The unusually large number of privately owned high quality
rain gauges may have been the result of many years of promotion of volunteer weather
observing by Tun Wirshbom, Mountain States Weather Services. During the past twenty
years he has circulated dozens of the 4-inch diameter gauges in Larimer County. Credit could
also be given to the relatively large number of working or retired foresters and hydrologists in
this area, many with professional experience recording precipitation. But even low cost
calibrated gauges can provide fairly accurate information when read properly and mounted in
representative locations. For this reason, Colorado Climate Center personnel visited many of
the observing sites where very heavy rainfall amounts were reported in order to check and
verifY the readings. Observers that were not visited were still asked to describe their gauge
location and potential obstructions. Some observations were found to be of poor quality or
low confidence due to close proximity to obstacles such as trees, fences and buildings.
Obstacles can either decrease or exaggerate rainfall totals depending on factors such as wind
speed, direction and rainfall intensities.

The Colorado Clirnate Center used the information about types and exposures of rain gauges
and other information about each observation to qualitatively assign a data quality evaluation
to each rain report. Excellent observations using quality gauges were assigned "A" status.

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