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I I I I I I I I I I I I I I I I I I The cloud caused a "premature sunset" by 7 pm as an ominous thick anvil spread out the thunderstorm's top over the mountains and plains. Con- tinuous lightning and deep peals of thunder rolling in the foothills made it appear as if a life and death battle were taking place to the west. The thunderstorm reached peak maturity from 1930 MDT (0130 GMT) to 2130 MDT (0330 MDT) over the Big Thompson Canyon. By 2130 MDT the upper air which was over southern Colorado at 1800 MDT had begun to increase mountain top winds over central Colorado. Begrudgingly the thunderstorm system "un- locked" its circulation from the flooded canyon about 2200 MDT (0400 GMT) in response to the increasing upper level winds. By 2230 MDT (0430 GMT) the storm moved over northern Larimer County and began to flood the Cache la Poudre River basin (Figure 3). Fortunately the storm had weakened by this time, and coupled with its northerly movement, lesser rainfall amounts fell over these northern basins, Prior to 10 pm 1/3 to 1 1/2 inches of rain had fallen in the foothills and been welcomed by local residents. However, between 10 pm and midnight, 4 - 6 inches of rain fell into small streams feeding the Poudre. By mid- night the Poudre Canyon, Rist Canyon and Soldier Canyon had been trans- formed into raging torrents. The rainfall over the Poudre was just as in- tense as over the Big Thompson. The thunderstorm system still towered to over 60,000 feet. Between 11 pm and midnight 1/2 to 1 1/2 inches of rain had fallen over Fort Collins and northeastern Larimer County as the heavy thunderstorm system headed north. By midnight the thunderstorm system had lost its source of moist un- stable plains air as night-time cooled the air, Tremendous amounts of rain- cooled air filled the lower atmosphere choking the thunderstorm from below. Lightning that was almost continuous prior to midnight became occasional by 2 am. 17