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METEOROLOGY. HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 25 FIGURE 20.-Geostationary Operational Environmental Satellite photograph, 1400 MDT. July 31. 1976. Bright areas are clouds. Sterling, Colo. (fig. 24). By this time, the trailing front was located about 75 miles southwest of Sterling. The rawinsonde recorded a mean mixing ratio of 12.5 g/kg, a Lifted Index of - 2, and a level of free convection at the 600-millibar level. Evidently, the air with the greatest potential for strong convection had moved southwestward in a narrow band behind the trailing front. Conditions that existed at 1800 MDT while the storms that caused the flash flooding were forming are depicted in figures 25-32. In western Colorado, the surface low had reached its maximum intensity. The trailing front had moved into the foothills in north- eastern Colorado and had merged with the leading front everywhere except in the Arkansas River valley in southeastern Colorado. Easterly surface winds behind the trailing front were 15-30 knots in a broad band from central Kansas to eastern Wyoming. At the 700-millibar and 500-millibar levels Ifigs. 29, 30), the large ridge had increased in amplitude over Montana and southwestern Canada. The trough at the 700-millibar level had moved only slightly while the two troughs at the 500-millibar level had evolved into a single northward-moving trough extending from cen- tral Nevada to northern New Mexico. The radar sum- mary for 1735 MDT (fig. 261 and the satellite photograph for 1800 MDT (fig. 27) indicated that the squall line in Utah and most of the widespread thunderstorms over the mountains of New Mexico and Colorado were alined along and to the northeast of the trough at the 500-millibar level. The stability analysis for 1800 MDT (fig. 311 in- dicated that the area having the thermodynamic potential for strong thunderstorms had increased dur- ing the day. Very unstable conditions extended from northern New Mexico to Montana. For operational purposes, rawinsonde are released about 45 minutes prior to the 0600 and 1800 MDT standard upper-air analyses time. On the evening of July 31, the rawinsonde from Denver, Colo., was released at 1715 MDT. Data from that rawinsonde are shown in figure 32. Diurnal heating had modified the airmass over Denver, Colo., significantly. The inver- sion had risen to the 590-millibar level with the lapse rate below the inversion near dry adiabatic. The mean mixing ratio in the lowest 100-millibar layer had decreased from 12.0 to 9.5 g/kg; the Lifted Index was -2. Precipitable water in the lowest 150-millibar layer