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38 FLOOD, JULY 3I-AUGUST I, 1976, BIG THOMPSON RIVER, COLORADO Win ~>- _ a: "'::::J ZU E ~ '"l U) "- 30.20 a:o UJ tii ~ 30.10 :::;::I: _u f-z -'- <( 90 f- u:; :I: 80 z UJ a: 70 :I: <( u.. U) 60 UJ UJ a: 50 '" UJ 0 40 1600 ~1 Surface air temperature Surface dewpoint temperature 2100 2200 2300 1800 1900 2000 TIME, IN HOURS (MOT) 1700 ~ EXPLANATION WINO-DIRECTION AND SPEED OBSERVATION. -Shaft indi- cates wind direction; north is at top. Barbs on shaft indicate wind speed, in knots. Long barb=10 knots; short barb=5 knots. FIGURE 33.-Time series of surface winds. altimeter setting. and surface-air and dewpoint temperatures. Stapleton International Airport, Denver, Colo.. 1600-2300 MDT, July 31.1976. thunderstorms that had formed ahead of the trailing front in the region west of Denver between 1730 and 1800 MDT. The resulting arc of thunderstorms (fig. 36B) moved over the Boulder, Colo., area at about 1830 MDT (fig. 36C). The data for the Rocky Flats plant (fig. 34) shows that the passage of the thunderstorms was marked by strong wind gusts and air and dewpoint-temperature changes similar to those occur- ring about 1 hour earlier at Stapleton International Airport and at Table Mountain. Eyewitness accounts and the rawinsonde data from Denver, Colo., at 1800 MDT indicated that the clouds which formed ahead of the trailing front south of Boulder had higher bases than the clouds which developed along the foothills to the north. At about 1830 MDT, a pressure increase of about 1 millibar was observed at the Rocky Flats plant and at Boulder, Colo. This pressure increase was ac- companied by a wind shift to the southwest which in- dicated that rain showers and evaporative cooling in drier air along the foothills south of Boulder had pro- duced a small high-pressure center in that area. The arc of thunderstorms east of Boulder dissipated rapidly after 1830 MDT. The western part of the thunderstorms moved over the foothills southwest of Boulder, but rainfall amounts were much less than observed amounts 20-80 miles to the north. Radar echoes shown in figure 36D indicate the thunderstorms southwest of Boulder were not strong- ly affected by terrain as some of them moved westward almost to the Continental Divide. From the meager data available, it appears that the small high-pressure center developed sufficiently to cause the trailing front to become quasi-stationary between Denver and the foothills south of Boulder, thereby preventing the very moist unstable air from reaching the elevated terrain southwest of Boulder. The winds at Boulder and the Rocky Flats plant remained light southerly to westerly until about 2200 MDT on July 31.