FLOOD02473

• County: Weld
• County: Logan
• Basin: South Platte
• Title: A Post Evaluation of Rainfall Reports Associated with Pawnee Creek Flood of July 29-30, 1997 in Eastern Weld and Western Logan Countie in Northeastern Colorado
• Date: 8/1/1998
• Prepared For: Weld County/Logan County
• Prepared By: CSU
• Floodplain - Doc Type: Flood Documentation Report

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journals or calendars that make it easy for them to look up past storms. One full year after
the storm, it was still feasible to gather data.

Results

1) National Weather Service Radar data

A detailed radar analysis of this storm was originally considered beyond the scope of this
post-analysis project. However, with the help of the National Oceanic and Atmospheric
Administration in Boulder and meteorologist Matt Kelsch, a relatively thorough review
was performed. The data for that storm had been captured and saved for local analysis
and research almost by accident last year as a part of the archival of data associated with
the Fort CoIlins flash flood of July 28, 1997.

Using NOAA system software, maps of radar-derived estimated precipitation were
produced for a variety of time intervals. Some maps were also prepared for two different
estimation procedures: 1) the standard radar reflectivity versus rainfall relationship and 2)
a special reflectivity versus rainfall relationship for a tropical airmass. The tropical
analysis was performed since meteorological analysis had shown that the airmass over the
region had many traits of a tropical humid airmass.

Four maps are included in this report to show the general spatial patterns and timing of
rainfall as estimated by radar for successive three-hour period, Keep in mind that some
heavy rain had already fallen in extreme northeastern Weld County and adjacent areas
during the afternoon of the 29th, and this is not included on the maps provided here,
Readers should please be aware that radar is an excellent tool for tracking storms, defining
storm areas, motions and relative intensities. However, the technology is not always
capable of accurately determining the rainfall that reaches the ground. Therefore, the
results below need to be interpreted carefully with that limitation in mind.

There are many interesting features of the storms shown by these radar products. For
example, a narrow band of very heavy precipitation can be seen extending southwestward
from the center of maximum rainfall to a point west of New Raymer. Another feature is
the secondary rainfall maximum that appears over extreme northeast Weld County. Heavy
rains also fell that night over Morgan County but were not the focus of this study.

The processed radar data will be archived and available for an unknown period of time in
Boulder, Colorado, Additional analyses and maps could be produced, if needed, Original
data will be permanently archived at the National Climatic Data Center, but access to that
data and re-anaIysis will be very time consuming and expensive.

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