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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Equation (2) shows a simplified form of this relationship:

(2) Peak 60-minute rainfall =
PWI X (DeDth of uDdraft warm laver) X 2 1.5km
(3) Peak 30-minute rainfall = 0.70 X (Peak 60-min rain)
(4) Peak 10-minute rainfall = 0.60 X (Peak 30-min rain)

Where the Precipitable Water Index (PWI) is a measure of the amount of water
in the atmosphere from the surface to about 20,000 feet above the ground.
HMS generates a matrix of rainfall rates, which are derived from surface
temperature and dew point fields used to initialize the 2-D model output. For
each set of surface temperature-dew point combinations, HMS creates a unique
radar-rainfall relationship for precipitation mapping. In effect the peak 60-,30-,
and 10- minute rainfall rates are related to the 50 dBZ or greater radar
reflectivity values within the thunderstorm. Lower rain rates are logarithmically
down-stepped to the lower radar reflectivity values.

3.3 Saguache Creek Basin Rainfall Estimation Methodology

Nearby METAR weather stations temperature/dew point values were used
to initialize the HMS 2-D cloud model for rainfall estimation. For the event of
July 25th, 1999 near Saguache, Colorado HMS utilized METAR observations at
Leadville, Alamosa, Gunnison and Montrose Colorado to calculate and assign a
radar-rainfall relationship to the outlined basin (Table 1).

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Table 1
Surface Observations for Nearby Stations of Saguache, Co.
At 1900 UTC (1 P.M. Local) July 25,1999.

Montrose
Alamosa
Gunnison
Leadville

57
52
48
42

An example of this relationship and the calculations for the afternoon of
July 25, 1999 will now be discussed to illustrate the points just made. HMS
plotted the above-mentioned METAR observations on a Skew-T, Log P diagram,
containing information derived from a radiosonde, launched at Grand Junction
Colorado, to calculate the PWI. Figure 5 shows the HMS-plotted Skew-T, Log P
diagram for the afternoon of July 25th where point A is the cloud base and point
B is the point where the thunderstorm updraft cools to 00 C. The calculated PWI
is 1.16" adjusted for an elevation of 8,500 feet while the depth of the warm
updraft layer is point B (5.5 km) minus point A (3.3 km) or 2.2 km. The next
step is to solve equations (2) and (3) to calculate peak rainfall rates.

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