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

Far to the north, a chill mass of polar air was pouring off the

Canadian prairies onto the northern Great Plains. By noon the leading

edge of this air had slipped into Wyoming and Nebraska. Behind the front

temperatures in 60s and low 70s brought relief to plains people who had

roasted in the muggy heat that still covered the central and southern

plains. Friday afternoon temperatures peaked in the 100s across Kansas

while tropical humidity made the air hard to breathe. The curtain had

begun to lift on the Colorado disaster as these three air masses converged

toward their tragic confrontation.

Saturday morning the shallow leading edge of cool Canadian air slid

"back door" into northeastern Colorado. Moist southeast winds helped blow

upslope clouds to form over Larimer County getting the weekend off to a

cloudy start. Local humidity was perceptibly higher. Extensive middle and

high cloudiness covered the Colorado high country from Arizona thunder-

storms of the previous day, A weak upper level disturbance began to move

northward out of northern Arizona and New Mexico. This disturbance would

later help trigger the flood-producing thunderstorm system.

The cool polar air held daytime highs in Larimer County in the 80s,

and steady southeast winds near the ground blew at 15 - 25 mph much of the

morning and the afternoon. The leading edge of the cool air rested on the

Palmer Lake Ridge near Colorado Springs to the south and along the foothills

from Denver to Laramie, Wyoming. Temperatures south and west of the front

rose to 96 in Grand Junction and 98 in Lamar compared to 84 in Fort Collins.

Moist air converged along the front in Kansas and Nebraska. Gusty easterly

winds sped this unstable air westward toward the Larimer County foothills.

Above mountain top level, wind speeds were very light and from the

southeast. Mountain winds help to "steer" thunderstorm systems. Unfortu-

nately the winds over Larimer County would hold evening thunderstorms on the

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