FLOOD04806

• County: Larimer
• Stream Name: Big Thompson/Cache la Poudre
• Basin: South Platte
• Title: Storm and Flood of July 31-August 1, 1976 in the Big Thompson River and Cache la Poudre River Basins
• Date: 1/1/1979
• Prepared For: Colroado Geological Survey
• Prepared By: USGS/NOAA
• Floodplain - Doc Type: Flood Documentation Report

METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 49

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Location in sn~NwI~ Sec. 31, T.8N.,R.70W" Larimer county)
I Total=1O.24 inches (260 millimeters) I
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1200
JULY 31

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Larimer County

Total=7.9 inches

(201 millimeters)
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Allenspark, Boulder County I
Total=l.~ inches (36 millimeters),

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(MDT)

1200
AUG. 2

2400

TIME, IN HOURS

FIGURE 42.-Cumulative rainfall, at three stations in Boulder and Larimer Counties. Colo.. July 31-
August 2, 1976.

representation of rainfall distribution during the storm
period for those areas. In the Big Thompson River
basin, no continuous rainfall records were available but
radar data and eyewitness accounts provide a fair
description of the storm period. The North Fork Cache
la Poudre River basin in the vicinity of Virginia Dale
lies outside the range of the Limon radar; thus, the
timing of rainfall in that area is based entirely upon
eyewitness accounts.
Limon radar-image locations with relative inten-
sities are shown in figure 43 from 1701 to 2200 MDT
on July 31, 1976. The images are shown at 20-minute
intervals prior to 1900 MDT and at 10-minute inter-
vals thereafter. The first thunderstorm cells developed

between 1800 and 1830 MDT several miles east of the
maximum rainfall zone near Glen Comfort and Glen
Haven.
The storms were moving generally north-
northwestward and reached a temporary peak in inten-
sity around 1845 MDT. Between 1900 and 1930 MDT,
the individual cells tended to merge, and the rainfall
pattern continued to shift slightly westward. By 1930
MDT the most intense rainfalls were southwest of
Drake, From about 1930 MDT until shortly after 2100
MDT, the "cloudburst" phase of the storm occurred in
the Big Thompson River basin around Glen Comfort.
The storm complex continued to shift very slowly to
the northwest, and after 2100 MDT, the most intense