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28 FLOOD, JULY 31-AUGUST 1, 1976, BIG THOMPSON RIVER, COLORADO FIGURE 23.-Geostationary Operational Environmental Satellite photograph, 1600 MDT, July 31, 1976. Bright areas are clouds. was 0.55 inch, substantially lower than 12 hours earlier. However, precipitable water in the layer from the surface to the 500-millibar level was 0.97 inch, nearly the same as the morning value. Heating and mixing had redistributed the moisture through a thicker layer. The trailing front moved through Denver, Colo., 15-20 minutes after the rawinsonde was released. Therefore, the rawinsonde provided no infor- mation about the extremely moist airmass just a few miles to the northeast. CONDITIONS DURING THE STORM Prior to 1800 MDT, there had been almost no precipitation falling on the foothills in northeastern Colorado. Two to three hours later, catastrophic flooding was occurring. This section describes meteorological conditions along the Front Range from Denver, Colo., to north of Fort Collins, Colo., from 1700 MDT to about 2200 MDT on July 31,1976. Data were available from the following locations: Fort Collins, Colo. (Colorado State University, At- mospheric Science Building); Greeley, Colo. (Universi- ty of Northern Colorado, Ross Hall); Table Mountain near Boulder, Colo. (National Oceanic and At- mospheric Administration, Environmental Research Laboratories); Rocky Flats plant near Boulder, Colo. (Rockwell International Corp.l; Jefferson County and Arapahoe County Airports, Colo. (Federal Aviation Administrationl; and Stapleton International Airport in Denver, Colo. (National Oceanic and Atmospheric Administration, National Weather Service). Radar reflectivity data were available for the entire storm from the radar operated by the National Weather Ser- vice at Limon, Colo., located about 125 miles southeast of the Big Thompson River area. Reflectivity data for 45 minutes at the beginning of the storm were available from the radar used for the National Hail Research Experiment at Grover, Colo., located about 70 miles east-northeast of the Drake-Estes Park area.