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<br />METEOROLOGICAL ASPECTS OF THE BIG THOMPSON CANYON FLOOD <br /> <br />Mr. John Henz <br />Geophysical Research and Development Corp. <br />Fort Collins, Colorado <br /> <br />The Big Thompson flood of 31 July 1976 was one of the most costly <br /> <br />flash floods in terms of both lives and property damage ever to occur in <br /> <br />Colorado and the Western United States. Only the disasterous Black Hills <br /> <br />Flood of 9 June 1972 in South Dakota is of a comparable magnitude. Al- <br /> <br />though destruction was the greatest in the Big Thompson Canyon, serious <br /> <br />flooding occurred on the Cache la Poudre River and on several streams <br /> <br />draining the adjacent foothills, including Soldier and Rist Canyon areas. <br /> <br />The meteorological conditions necessary to produce a heavy localized <br /> <br />rainfall near elevated topography are reasonably well known. <br /> <br />a. The atmosphere should be conditionally unstable and very moist <br /> <br />through a deep layer. <br /> <br />b. A stationary cloud system capable of processing rapidly a <br /> <br />significant volume of atmospheric moisture should develop <br /> <br />over topography. <br /> <br />c. A strong "feeding mechanism" capable of converging the moisture <br /> <br />into the flood-producing cloud system and triggering cloud <br /> <br />development should exist. <br /> <br />d. Inverse vertical wind shear should exist, i.e., winds should be <br /> <br />very strong in the sub-cloud layer and very light in the cloud <br /> <br />layer. In effect this wind structure anchors the cloud system <br /> <br />over the topography. <br /> <br />In the case of the Big Thompson Flood, all of these conditions were <br /> <br />met. Moisture from both the Gulf of Mexico and the Arizona monsoon had <br /> <br />8 <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />