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I I I I I I I I I I I I I I I I I I I mountains. All the ingredients for a serious flash flood threat were present awaiting a "trigger mechanism" to unleash their fateful fury. Up until 1700 MDT (2300 GMT) thunderstorm activity along the front had been confined to Kansas and southeastern Colorado, as shown in Figure 1, an SMS-l satellite photo. To the south of Colorado an organized mass of convective clouds had formed across northern New Mexico, southern Utah, and southern Colorado ahead of an upper air disturbance moving out of Arizona and New Mexico. Thunderstorm activity over eastern Colorado had been supressed by the shallow inversion of cool air near the ground caused by the shallow polar front. However, by 1730 MDT (2330 GMT) unstable air along the front was lifted through the inversion by a "gust front" passage from a thunder- storm southeast of Denver. Strong, cool southeasterly winds from this thunderstorm's downdrafts spread a wall of dust westward across Adams, Arapahoe, Jefferson and Boulder counties, south and east of the Big Thompson. This "gust front" produced local winds to 40 mph and dropped temperatures 15 - 20 degrees in minutes. As the "gust front" reached Larimer County, between 1800 - 1830 MDT (0000 - 0030 GMT), it triggered the explosive development of several large thunderstorms in Boulder and Larimer counties. The moisture laden air which was conditionally unstable burst through the frontal inversion like water through a hole in a dam as its instability was unleashed. Huge thunderstorms mushroomed 60,000 feet into the evening sky in 30 - 60 minutes (photo, page 5). Figures 4a and 4b show SMS infrared close-up photos of the explosive development taking place over the Big Thompson Canyon. The low temperatures (-70 to -740C) indicated the thunderstorm top penetrated the tropopause by 10-15,000 feet which is a very unusual 15