FLOOD02366

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: The Big Thompson Flood of 1976 Field Trip Guidebook
• Date: 1/1/1976
• Prepared For: Annual meeting of Geological Society
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

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fed into the Colorado high country and plains for 48 - 72 hours prior to

and during the flood. This moisture influx provided the "fuel" for the

flood and insured that a conditionally unstable and moist environment

would exist over the flood area. A chill mass of polar air spilled off

the Canadian prairies across the northern and central plans Friday and

Saturday, July 30-31. As the leading edge of this cooler air slipped

slowly across eastern Colorado Saturday, it converged very moist air to-

ward the Colorado foothills. The slow movement of the cool air boundary

helped to "direct" the strongest moisture convergence toward the thunder-

storm system which produced the flood rains. The thunderstorm system

towered over 60,000 feet in height and remained stationary over the Big

Thompson Canyon for four hours (1830 - 2230 MDT). Winds feeding the moist

plains air into the thunderstorm system were estimated to be about 50 mph

while the jet stream level winds were only 10 to 25 mph.

Clearly, the key meteorological parameters needed to produce a severe

flash flood were present. A discussion of the meteorological sequence of

events surrounding the flood points out that a unique scale interaction

of these parameters was required to produce the disastrous results.

The meteorological beginnings of the flood can be traced back to

Arizona, Canada and the Gulf of Mexico. Three important air masses were

involved in setting the stage as indicated in Diagram A. Copious amounts

of moisture were lifted by towering thunderstorms from near the Arizona

desert floor high into the atmosphere and

transported northeastward

into the Colorado high country on Friday and Saturday. This moisture

caused dew points to rise into the 45 - 55 degree range at elevations

above 8000 feet. Such high dewpoints are very unusual for the Colorado

Rockies and reflect the abnormal amount of moisture that was available

to fuel the flood-producing thunderstorm system.

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