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METEOROLOGY, HYDROLOGY, BIG THOMPSON RIVER AND CACHE LA POUDRE RIVER BASINS 43 at altitudes between 15,000 and 20,000 feet above mean sea level. Grover radar operators varied the angles of their scans, effectively obtaining a three- dimensional record of the storm's reflectivities. Scans made at 1.9-degree elevation angles intercepted the foothills storms at approximately the same altitudes as the Limon radar and are shown in figure 38. At 1845 MDT Limon radar showed Video Integrator Processor level-3 contours which corresponds to reflec- tivities between 41 and 46 dBZ, Brief periods of level-4 contours were observed later in the evening. Converse- ly, the Grover radar showed a level-5 contour (55-65 dBZ), with a maximum of 64.6 dBZ. The Grover radar data contained more detail than the Limon data because of a I-degree conical beam width as compared to the 2-degree conical beam width of the Limon radar, and because of a much closer loca- tion to the foothills region. The comparative readings suggest that the difference in beam width of the two radars enabled the Grover radar to detect small in- tense cells that were averaged out in the Limon radar signal. This difference appears to be about 15 dBZ in the center of the thunderstorms. A two-dimensional cross section through one of the largest thunderstorms was constructed, as shown in figure 39. This thunderstorm became quasi-stationary approximately 1845 MDT near Storm Mountain, which is about 5 miles north of Drake. The cross sec- tion is positioned in a line from southeast to northwest, approximately along the direction of low-level inflow. Grover reflectivity data, visual observation of cloud formation and movement, satellite observation of the areal extent of the cirrus anvil, and the interpolated sounding for Loveland were combined to give a schematic but fairly detailed picture of the storm structure. The strong inflow allowed a large amount of mass to be processed by the storm. As the low-level flow ap- proached the Front Range, a shallow layer of stratus and stratocumulus clouds formed in the layer between the lifted condensation level and the level of free con- vection. A surface observation at 1800 MDT at Fort Collins indicated a thin broken deck of clouds based at 4,000 feet above the surface. When the low-level air was forced above the level of free convection, explosive convective growth occurred_ The radar data indicated that new cells formed in the inflow and moved north- northwestward into the storm. Over the mountains, the cloud base was effectively on the ground; the high in-cloud freezing level was at about 19,000 feet above mean sea level, and the height of the -25'C isotherm was at 31,500 feet above mean sea level. This indicated an unusually deep layer for warm cloud condensation and coalescence processes to act. There was weak wind shear above the level of free convection; therefore, little entrainment of drier middle- and upper-level air into the storm. With the cloud base on or near the surface, precipitation was falling with virtually no subcloud evaporation. Neither entrainment nor evaporative processes were able to produce strong downdrafts within the storm, thus yielding a highly efficient storm. Grover radar data in- dicated that the storms which were moving into the foothills generally sloped to the east or southeast. Once the storm became quasi-stationary over the elevated terrain, they tended to slope to the northwest as shown in figure 39. The northwest slope of the up- draft allowed large precipitation droplets to form and then to fall out of the rear of the updraft. This enabled the system to exist in a nearly steady state, Efficient unloading of the updraft in the lower half of the cloud permitted large updraft velocities to develop within the ice phase upper cloud, which, in turn, pushed the cloud top to very high levels. Indeed, radar observa- tions indicated that the maximum tops of the thunderstorms were about 62,000 feet above mean sea level, or about 6,000 feet higher than the tops of any other similarly reported thunderstorms on the eastern slopes and plains of Colorado. Some 20 miles to the south of the storm portrayed in figure 39, or about 5 miles southwest of Lyons, another storm of similar size and intensity developed between 1800 and 1845 MDT. A sequence of outstanding photographs of the development of this storm was taken by Mr. John Asztalos, who was located at Mitchell Lake approximately 15 miles west-southwest of the developing thunderstorm. These photographs are shown in figure 40. Note the similarities between the photographs and the cloud model in figure 39. RAINfALL ANALYSIS The total rainfall for July 31-August 2, 1976, is shown on the isohyetal map (fig. 41). The analysis was based on rainfall records at stations in the National Weather Service climatological network and rainfall reports from 119 miscellaneous locations in the storm area_ Unfortunately, the lack of detailed rainfall- intensity data and the inaccessibility of the area over which the storm occurred resulted in data depicting