FLOOD10114

• County: Saguache
• Stream Name: Saguache Creek
• Basin: Rio Grande
• Title: Radar Rainfall Reconstruction of Saguache Creek Flash Flood
• Date: 7/25/1999
• Prepared For: CWCB
• Prepared By: HDR Engineering, Inc.
• Floodplain - Doc Type: Flood Documentation Report

1. The first disturbance (1) is located across central Nebraska and can be
seen as the decaying meso-scale convective complex (MCC).

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later another intense mountain thunderstorm complex formed over the Dallas
Divide and produced a monumental high country flash flood, These three
events occurred during a prolonged, deep intrusion of monsoon moisture into the
state,

The purpose of this report is to document the meteorological causes, re-
construct the estimated spatial coverage, temporal duration and rainfall depth of
the thunderstorm complex that produced the flash flood. The general steps
followed to acquire this information are:

1. A detailed meso-synoptic evaluation was completed of the weather causes
of the storm using conventional surface and upper observations taken by
the National Weather Service (NWS),

2, NWS WSR-88D Doppler radar observations from the Pueblo radar site
were analyzed to determine the duration, location and depth of rainfall
produced,

3. Cloud-to-ground lightning observations were obtained from the National
Lightning Data Network and analyzed for the relationship to the rainfall
production and aerial coverage of the storm,

4. Rnally, the storm location and amounts as determined by radar were
compared to the actual topography and an independent paleo-
hydrological estimate.

2.0 Synoptic Situation

The large-scale atmospheric pattern in which the Saguache Creek flood
occurred was very typical of conditions conducive for producing flash flooding in
central Colorado Mountains, Flash floods in this area typically occur in the late
June to mid-September time frame when large amounts of sub-tropical moisture
are drawn up into Colorado via south to southwesterly flow in the lower-to-mid-
levels of the atmosphere,

A brief examination of a regional satellite photo 1600 UTC (10:00AM) on
July 25th, 1999 (Figure 2) shows three disturbances in this unstable sub-tropical
flow:

2, The second disturbance (2) is located along the northern Front Range and
Continental Divide just to the west of Denver.

3, The third disturbance is located near Durango in southwestern Colorado.
This third disturbance is the one associated with the Saguache flash flood,

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