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canyon. According to reliable witnesses, there were no surging walls of water marking the head of the flood crest, but rather a very rapidly rising river. The waves of debris typical of flash floods such as this often give the appearance of a 'wall of water.' A question always asked in unusual natural events is: "How rare was this rainfall and flood?" It is very difficult to answer in precise terms, but the survey team has been able to establish some facts that relate to that question. First, the flood level exceeded the 100-year flood expectations. This means that the flood was in the less than 1% proba- bility of occurrence in any year. Second, the rainfall amount of 12+ inches in about 4 hours is not really unusual when viewed with other events. For example, in 1969, 32+ inches (over 810 mm) of rain fell in less than 8 hours in southwestern Virginia and rainfall amounts of 10-12 inches (250-300 mm) in a 3 to 4 hour period occur in several locations each year. Third, there are flash floods similar to this one occurring frequently during the summer in the Rocky Mountains. However, most of these flash floods go unnoticed because the canyons are devoid of people and roads. The Big Thompson flash flood effect was greatly amplified by the constriction due to the narrow channeled flood plain in the canyon. Meteorological conditions that produced the 4 l/2-hour deluge of rain were somewhat abnormal. On Friday and Saturday a surface cold front had moved slowly southward through the Central Plains states. By 6:00 p.m. Saturday, July 31, the now almost stationary front lay east-west through Missouri and Kansas into central Colorado where it curved northward along the eastern slopes of the Front Range of the Rocky Mountains from about Denver into central Wyoming. The air on both sides of the front was condition- ally unstable with abnormally high amounts of moisture in lower levels concentrated on the north side of the front. The Denver radiosonde observation taken at 6:00 p.m. Saturday, July 31, showed mixing ratios of 8-9 gms/kg to 600 millibars (60 kPa) and the observation made 12 hours earlier showed slightly higher moisture values. Convective activity that had persisted along the frontal zone into Saturday morning as light scattered showers began to intensify by early afternoon in eastern and south-central Colorado. Thunderstorms had reached severe intensities by 3:00 p.m. when severe thunderstorm warnings were issued for Kit Carson and Cheyenne counties in eastern Colorado. Also, active thunderstorms were scattered along the Front Range in a generally north-south line or zone from northern New Mexico into southern Wyoming. About 6:00 p.m., Limon weather radar first detected convective cells beginning to develop over the Big Thompson drainage basin. During the brief period from 6:30 to 7:00 p.m. the organization and intensity of the thunderstorm activity began very significant and rapid changes. The new convective cells along the Front Range began to intensify, with explosive development in north-central Colorado. Storms along the remainder of the frontal zone began a rapid decay. By 8:00 p.m. only the band of storms along the Front Range remained active. Slow northerly movement coupled with rapid development of new cells to the south combined to produce a seemingly stationary intense storm and very heavy precipitation over the Big Thompson drainage basin from about 6:30 p.m. to 11:00 p.m. A more detailed description of the synoptic situation is contained in Chapter 3. 3