FLOOD10403

• County: Statewide
• Basin: Statewide
• Title: Colorado's 1997 Flood Season In Review
• Date: 8/1/1998
• Prepared For: CWCB
• Prepared By: Riverside Technology, Inc.
• Floodplain - Doc Type: Educational/Technical/Reference Information

I
I
I
I
I
I
I
,
I
I
I'
I'
I
I
I
I
I
I
I

Introduction

system over the plains set off thunderstorms as the moist air masses
converged over the state. As a result of these thunderstorms, major flood
damages occurred in the 13 counties declared as federal disaster areas. The
most extensive damages took place in Larimer, Logan, and Morgan counties
(Interagency Hazard Mitigation Team 1997).

The storm that occurred in Fort Collins (Larimer County) on July 28, 1997,
had some characteristics in common with historically severe storms in the
region, such as the Rapid City, South Dakota, and Big Thompson, Colorado,
events in the 1970s. It also had highly unusual characteristics that differed
dramatically from major historical events and other storms that would occur
in eastern Colorado during the summer of 1997. Initially, tropical moisture
flowed into Colorado from the southwest; moisture at the lower altitudes
was directed toward higher elevations by nearly stationary cold fronts to the
east. These conditions were similar to those for severe historical storms in
the eastern Rocky Mountain region (Forecast Systems Laboratory 1998).

Given the direction of storm cell motion at the time, the resulting
thunderstorms normally would have been expected to move off the
mountains and onto the eastern plains. However, as resulting thunder-
storms moved eastward, outflows were generated and moved back toward
the foothills, supporting new storm formation at a relatively low altitude. At
higher altitudes, tropical moisture continued to enter the area from the south.
This rare combination of circumstances produced an unusually efficient
process for developing rainfall. The Fort Collins storm presented very little
lightning or hail, and the cloud top elevations were not as high as typical
western thunderstorms. This pattern of convection is typical of tropical
events in the southern and eastern U.s. and of tropical storms making
landfall in maritime regions, but it is uncommon in Colorado. The
precipitation that resulted probably would have caused significant flooding
even in a relatively flat rural area (Forecast Systems Laboratory 1998).

In contrast, other storm events in the state lacked the extreme tropical nature
of the Fort Collins storm. In the northern and central parts of Colorado,
. high-intensity rains also fell in June and late July. In some locations, the
largest storms came on the heels of several smaller storms in preceding days.
Late July and August events in the southern part of the state were typified by
this sequential occurrence of rainfall, and flood damages resulted even
though the rainfall amounts generally were not as severe as they were to the
north.

The following are important factors in assessing flooding potential (from a
meteorological perspective):

. The intensity of rainfall

d479/chap1.doc 08/12/98

1-5