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<br /> <br />specific drainage basin. The guidance provided to WSFO Denver by the RFC <br />in Kansas City was of the general type. Specific guidance is not issued <br />for the Big Thompson River and many other basins in the mountains. <br /> <br />Guidance products available to WSFO Denver at noon on July 31 indicated <br />that there would be relatively little change in the upper wind flow pattern. <br />Weak upward vertical motion with mean relative humidities above 50% were <br />shown for northern Colorado. The surface high pressure center and the cold <br />front lying east-west across Kansas and into Colorado were predicted to <br />move slowly southward. Modified NMC surface guidance moved the high pres- <br />sure center a little faster and showed a stronger pressure gradient (stronger <br />easterly surface winds) in eastern Colorado by 6:00 a.m. on August 1. The <br />NMC quantitative precipitation forecasts (QPF) indicated that rainfall <br />amounts from one half to one inch were possible over the plains of north- <br />eastern Colorado. The QPF discussion received at WSFO Denver about 6:00 a.m. <br />on July 31 indicated increasing moisture for Colorado and Wyoming, but <br />discounted somewhat the amounts predicted by the numerical models. The <br />Kansas City RFC guidance indicated that more than two and one-half inches <br />of rain in a three-hour period were needed to produce flash flooding in <br />the forecast zones containing the Big Thompson Canyon. <br /> <br />Precipitation guidance available to Denver by 1:30 p.m. on July 31 showed <br />increasingly higher probabilities for measurable rainfall for northeastern <br />Colorado for the 12-hour period beginning at 6:00 p.m. on July 31. Thunder- <br />storm guidance forecasts from NMC for the 6-hour period centered at 6:00 p.m. <br />showed a relative maximum for eastern Colorado. <br /> <br />In summary, the synoptic situation as depicted by the data, maps, and charts <br />available to WSFO Denver pointed toward greater than normal thunderstorm <br />activity in Colorado. However, centrally prepared guidance forecasts of <br />thunderstorms and rainfall indicated only normal activity. The maps and charts <br />and forecast guidance as a whole did not provide a basis for specific fore- <br />casts of a catastrophic, localized storm and flash flood. <br /> <br />Local Forecasts, Warnings, and Mesoscale Considerations <br /> <br />Zone forecasts issued by WSFO Denver at 4:00 a.m. and 10:00 a.m. on July 31 <br />for the area containing the Big Thompson Canyon predicted widely scattered <br />showers and thunderstorms on Saturday becoming more numerous Saturday night <br />and Sunday. Precipitation probabilities for the nighttime hours were 50 <br />percent in the western half of Larimer County and 40 percent in the eastern <br />sections. The zone forecast issued at 4:00 p.m. indicated a chance of <br />afternoon and evening showers and thunderstorms, with nighttime precipita- <br />tion probability of 40 percent. There was no mention of abnormally heavy <br />rainfall in the forecasts. <br /> <br />During the 31st, upslope flow of moisture-laden air intensified over eastern <br />Colorado, while a concentrated surface wind maximum of mesoscale dimensions <br />moved westward across Kansas into eastern Colorado. The wind maximum <br />reached Denver by 6:00 p.m. on the 31st. At this time, thunderstorms began <br />developing rapidly along the Front Range to the west and northwest of Denver. <br /> <br />31 <br />