FLOOD05944

• County: Saguache
• Basin: Rio Grande
• Title: Radar - Rainfall Reconstruction of the Saguache Creek Flash Flood of July 25, 1999
• Date: 6/1/2001
• Prepared For: Saguache County
• Floodplain - Doc Type: Flood Documentation Report

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Daytime heating of the local topography initiated the development of the storms.
The storm began raining in the basin about 300PM MDT, reached its peak rain
production between 330PM MDT and 430PM MDT and moved off the basin and
dissipated between 500PM MDT and 600PM MDT. The duration of the peak
rainfall was about two hours.

The storms were "locked" into the terrain by slow moving winds that flowed in
the cloud layer directly into an area of steep elevation gradient as new cells
developed over a small mountain immediately to the west of the basin. The axis
of the heaviest rainfall was along the mean winds in the 10,000 to 20,000 foot
layer of the atmosphere. The heaviest rainfall area was located over the middle
of the basin at elevations below 10,000 feet. The average elevation of the 5.00-
inch isohyetal was about 8,750 feet.

The storm produced peak radar derived rainfall of just over 5 inches at
elevations of 8,000 to 9,000 feet and produced rainfalls exceeding 4.00 inches in
elevations to almost 10,000 feet. This rainfall is among the largest estimated at
such high elevations in the Colorado Mountains.

The storm covered an area of roughly 72 square miles with a basin average
rainfall of 1.84 inches in this area. A smaller area of average rainfall of 3.50
inches covered an area of roughly 30 square miles.

Cloud-to-ground lighting production of the storm produced several interesting
observations. First, the peak lightning areas hugged the terrain gradient areas to
the north and to the east of the storm rainfall area with over 80 percent of the
c1oud-to-ground lightning strikes in the 9,000 to 10,000 foot elevation band.
Most of these strikes were to the north of the heaviest rainfall area in an area of
decreasing radar reflectivity gradient,

A casual comparison of the radar-derived (RD) and run-off derived (RO) rainfall
patterns showed the RD areas of 2.00 inches or more rainfall as roughly twice as
large as the comparable RO area but the RO area of heaviest rainfall was about
twice the RD heavy rainfall area of 5.00 inches of rain or more. In general, the
two rainfall estimates identified the same areas of the basin impacted by the
flooding. A more quantitative comparison of these differences begs to be
accomplished.

In closing the rainfall values presented in this study should be of assistance in
providing an estimated but quantitative description of the storm. The advent of
radar coverage by National Weather Service W5R-88D Doppler radars allows
reasonable estimates of the spatial and temporal characteristics of heavy rain
producing thunderstorms across the Colorado Mountains. Additionally, the radar
observations afford the opportunity to provide quantitative estimates of the
amount and volume of rainfall.

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