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<br />of 10 September 1938. Each of these flash floods was produced by intense, <br />short-term thunderstorm rainfalls which occurred within 150 miles of the Boul- <br />der Creek watershed. Reasonably accurate historical weather, rainfall and <br />discharge information is available for each of these storms. <br /> <br />I <br /> <br />The thunderstorm rain1'"lls produced by each of these storms resulted in <br />significant flash flooding in storm systems that are climatologically germane <br />to the Boulder Creek watershed. These storms are particularly well suited for <br />the development of simplif:ll!d guidance for mountain flash flood warning within <br />the UDFCD Flash Flood prediction Program (F2P2) and the assessment of the <br />relative magnitude of flood hazard caused by these infrequent severe storms. <br /> <br />r' <br /> <br />The data used to support the reconstitution were National Weather Service <br /> <br /> <br />surface observations and upper air observations, upper air radiosonde observa- <br /> <br /> <br />tions and storm related references and rainfall surveys. In the case of the <br /> <br /> <br />Masonville storm of 10 September 1938, use was made of aircraft-derived upper <br /> <br /> <br />air temperature, moisture and wind profiles gathered at Cheyenne, Wyoming. <br /> <br /> <br />These observations provided a unique opportunity to reconstruct the possible <br /> <br /> <br />spatial, temporal and quantitative rainfall for a historical storm using <br /> <br /> <br />today's technology. Mr. Verne Leverson of the Flood Hydrology Branch of the <br /> <br /> <br />U.S. Bureau of Reclamation kindly provided the Masonville storm data for our <br /> <br /> <br />analysis. <br /> <br />, <br /> <br />The methodology of the reconstitution and transposition of each storm <br />required the fOllowing steps: <br /> <br />A. Reconstitution of storm rainfall. <br /> <br />1. A complete set of surface, upper air and historical weather records <br /> <br /> <br />were assembled for eEch storm case. Surface and upper air features <br /> <br /> <br />known to influence rainfall production were identified. <br /> <br />2. Next, the vertical temperature and moisture structure of the atmo- <br />sphere over the basir were prepared and entered into the Convective <br /> <br />8 <br />