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

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When these two figures are superimposed (not shown) it indicates that
the only region of the country in which QQt!l the 500 and 700 MB dew points are
considered very moist is the Four Corners area that includes southern and
southwestern Colorado. This indicates how deep moisture was present just to
the southwest of the flooded area 7-10 hours prior to the flash flood.

Note that the mid-level wind direction was from the southwest and speeds
were 10-15 mph, This wind direction is conducive to the formation of persistent
thunderstorms on ridges that are oriented southwest to northeast. The low wind
speeds are conducive to slow storm movement.

These two factors work together for the formation of either a large
singular storm system anchored into topography by the cloud-layer winds or the
development of successive storm formation that results in rainfall cores "training"
over the same basin. In the case of the Saguache storm, the former event
occurred as one large, persistent storm formed over the southwest facing
mountain ridges just to the northwest of the town of Saguache,

3.0 Data Sources, Analysis Techniques, and "Gridding" Procedures

3.1 Data Sources

The procedures, techniques, and data utilized in the analysis of this event
will be briefly described and further detailed in the following sections, Data
utilized in this reconstruction includes, NEXRAD WSR-88D base reflectivity data
from the National Weather Service Radar located near Pueblo, Co., upper air
atmospheric soundings from Grand Junction and Denver, Co, along with
Albuquerque, N,M" surface observations from numerous stations in Colorado and
New Mexico, and c1oud-to-ground lightning data from the National Lightning
Detection Network (NLDN).

In order to better analyze some of the meteorological components of this
event in a more efficient and accurate manner, a "grid" is established over the
area in which the flash flood occurred, The size of one of the squares in the grid
is coincidental with a pixel depicted in the NEXRAD data utilized in the rainfall
analysis. Use of this grid is very helpful in deriving the rainfall totals, which will
be described in more detail in the following section, In addition, this grid is
extremely helpful in analyzing the c1oud-to-ground lightning data and the
relationship it has with the storm

3.2 Use of Radar to Calculate Stonn Rainfall

The utilization of radar to estimate rainfall has been in use for over 30
years by meteorologists in both the government and the private sector. In
general, most current radar-derived rainfall techniques rely an assumed
relationship between the strength of the