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<br />I <br />I <br />I <br />I <br />I <br />I <br />,I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />,I <br /> <br />The Big Thompson Flash Flood: <br />What If It Happened Again? <br /> <br />John F. Henz <br />Henz Meteorological Services <br />Denver, Colorado 80211 <br /> <br />Introduction <br /> <br />During the early evening hours of July 31,1976 a massive and unforgiving thunderstorm system <br />developed over the foothills of the Rocky Mountains of Boulder and Larimer Counties, The <br />thunderstorm complex grew quickly as a surging stream of steamy plains air focused lhe storm's <br />wrath over the Big Thompson river basin in the Larimer County foothills. Countless tourists had <br />chosen to spend the long three day weekend celebrating Colorado's Centennial year of statehood <br />in the cool canyon picnic and camping grounds to escape the unrelenting heat of the Colorado <br />Drought of 1976, No flash fiood watches were issued that day or evening to alert the masses of <br />the deluge that would shortly come. <br /> <br />The storm complex transformed from billowing cumulus clouds into a massive rain-making <br />machine in less than 90 minutes between 600PM and 730PM, It reached peak storm maturity as <br />it paused over the Big Thompson basin for a three hour period dropping a deluge of 12 to 15 <br />inches of rain. The ensuing flash flood killed at least 139 people In the canyon and injured scores <br />of others. Millions of dollars of damage was done to canyon roads, businesses, local government <br />facilities, homes and tourist facilities that night. <br /> <br />The storm system detached itself from the Big Thompson River basin about 1030PM and slowly <br />moved northward, As it did, it produced an estimated 5-10 mile wide swatch of 4 inches or more <br />of rainfall which stretched from northern Boulder County's North St. Vrain basin across the Cache <br />La Poudre River basin of northern Larimer County into south-central Wyoming just southwest of <br />Laramie, Wyoming, According to satellite photos the storm system slowly dissipated shortly <br />before sunrise on August 1, 1976 over southeastern Wyoming. <br /> <br />The combined runoff from this rainfall produced serious flooding on the North St. Vrain River, <br />Cache La Poudre River and the South Platte River downstream from its confluence with the Big <br />Thompson in Weld County. While the flooding on these rivers was overshadowed by the Big <br />Thompson disaster, the potential loss of life, especially along the Cache la Poudre, was reduced <br />by fortuitous actions by the observant public rather than planned action. <br /> <br />r <br /> <br />The Big Thompson flash flood disaster hammered home several important lessons as relevant <br />today as 20 years ago: <br /> <br />1. Marginal flood plain regulation and development pressure had allowed an expanding <br />population to build in and along a designated flood plain in a haphazard manner. It literally <br />created a population at risk to flooding. <br /> <br />2. Poor prediction and detection of the thunderstorm system and the flash flooding rainfall by <br />the operational meteorological community provided little or no advance awareness of the <br />potential or magnitude of the deadly flash flood. <br /> <br />1 <br />