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

32

FLOOD, JULY 3I-AUGUST I, 1976, BIG THOMPSON RIVER, COLORADO

FIGURE 27.-Geostationary Operational Environmental Satellite photograph, 1800 MDT. July 31, 1976. Bright areas are clouds.

ANALYSIS OF CONDITIONS IN REGION OF FLASH FLOODING
Surface data were recorded almost continually at
Stapleton International Airport, Rocky Flats, and
Table Mountain. Detailed time series were constructed
using these data (figs. 33-35). At Table Mountain,
about 6 miles north of Boulder, Colo., the east-west
component of the wind was measured from the surface
to about 2,000 feet by a Doppler acoustic-echo sounder
operated by the Wave Propagation Laboratory of the
Environmental Research Laboratories. These time
series, hourly observations from the remaining sites,
and radar data from Limon, Colo., were used to con-
struct the analyses of surface conditions shown in
figure 36. Radar echoes for Video Integrator Processor
levels 1, 2, and 3 are shown on the maps (fig. 361. These
echoes correspond to the minimum detectable signal,
30 and 41 dBZ, respectively.
In the Fort Collins, Loveland, and Greeley, Colo.,
areas, an increase in wind speed and gustiness were the

only indications of the passage of the trailing front.
The sky over the areas remained partly cloudy during
the afternoon with southeasterly winds, resulting in
small temperature differences across the front. In the
Denver-Boulder, Colo., area, however, afternoon
cloudiness was minimal and the resulting heating and
mixing had increased the surface-air temperatures and
decreased the dewpoints. The winds also were more
southerly. The trailing front passed both Stapleton In-
ternational Airport and Table Mountain at about 1730
MDT. At these sites, the passage of the front was
accompanied by a significant increase in easterly
or southeasterly winds. Dewpoint temperatures
increased 10'-13'F and air temperatures decreased
lOo-12'F in 30 minutes.

Prior to 1730 MDT, a large thunderstorm had
developed southeast of Denver, Colo., as the trailing
front moved into this region (fig. 36A). The
thunderstorm moved northwestward and merged with