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occurrence over the Colorado Rockies. Figure 2 shows the flood pro- ducing thunderstorm system (A) at 0100 GMT (1900 MDT). The gust front hastened the development of a local inversion near the ground. Such inversions frequently cause winds near the inversion to double or triple their speeds over those of winds near the ground. Numer- ous meteorologists observed southeasterly winds estimated at 50 mph just above the surface, pump the unstable plains air into the foothills from 6 pm until 9 pm. Diagram B shows a typical summer thunderstorm weather sequence. Ini- tial cumulus clouds form over the foothills and reach maturity over the plains as westerly winds drift them eastward. On the night of the Big Thompson flood, winds near the ground were three times as strong as the southerly winds at 30,000 feet (see Diagram C). As a result of this in- verse shear situation the thunderstorm system was "locked into" the Big Thompson Canyon. Radar and satellite photos indicate that the thunderstorm system which produced the flood remained stationary over the Big Thompson area from about 1830 MDT (0030 GMT) to 2230 MDT (0430 GMT). During this time the thunderstorm towered to over 60,000 feet and reached a peak radar in- tensity of a "five-level storm" according to radar observers. Peak rain- fall estimates from such a radar intensity would indicate a rate of about 5 inches per hour for about 30 - 45 minutes of the thunderstorm's duration around 2000 MDT (0200 GMT). This result agrees well with hydrologic esti- mates. Average estimates from radar of 2 - 4 inches per hour rainfall rates were maintained by the storm until it crossed the Wyoming border around midnight. To observers on the plains the storm appeared as a huge black cloud which stretched from Virginia Dale to as far south as the eye could see. 16 I I I I I I I I I I I I I I I I I I