FLOOD02053

• County: Weld
• County: Boulder
• Community: Weld, Boulder Counties
• Stream Name: Big Thompson River
• Title: Meteorological Aspects of the Big Thompson Flash Flood of 31 July 1976
• Date: 7/1/1977
• Prepared For: State of Colorado
• Prepared By: NOAA
• Floodplain - Doc Type: Flood Documentation Report

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Figure 3. Mean July rainfall (inches) for Colorado (data from NOAA-EDS Climatological Summary
for Colorado). The area shown in Fig. 2 is shaded.

hy the flash flooding, and cumulative rain-
fall isohyets are depicted in Fig, 2, The pre-
cipitation summary covered the period 31
July through 2 August 1976, Mean July
precipitation for Colorado is shown in Fig,
3, Note that within a few hours the Big
Thompson storm produced rain amounts 5
to 10 times that normally expected for the
entire month of July.
On July 31 a large thunderstorm com-
plex developed over the mountains from
west of Fe Collins to southeast Estes Park
just before dark when strong, unusually
moist, easterly winds pushed upslope into
the Front Range of the Rocky Mountains,
The thunderstorm intensified very rapidly
and hy 0045 GMT radar measured tops
exceeded 62,000 ft MSL (18,9 km) (NWS
radar at Limon, Colorado) and reflectivities
exceeded 60 dBZ (National Hail Research
Experiment radar at Grover, Colorado),
During its intense phase, the storm com-
plex remained nearly stationary for 2 to 3 h;

subsequently, it moved slowly northward
into \Vyoming, The behavior and charac-
teristics of the Big Thompson storm were
markedly different from those of typical
summer afternoon storms in northeastern
Colorado. These storms usually develop
over the mountains and ridges in early af-
ternoon and then move eastward across the
plains, Rainfall is most often localized and
of short duration, and is frequently accom-
panied by haiL
Stream flow records (Grozier et aI.,
1976) indicate that the precipitation that
produced the flood crest on the main fork of
the Big Thompson River at Drake probably
fell within the first 2 h of the storm: Grozier
et al. also reported a maximum measured
streilm flow on the Big Thompson River at
the mouth of the canyon of 31,200 ft3 S-I
(884 m3 S-l) from an effective drainage
basin of approximately 60 mi2 (155 km2),
Table 1 compares maximum measured
stream flows with drainages of varying

3