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. . In June 1965 a storm over eastern Colorado produced some ve rainfalls. Some of the amounts were new maxima of record for va rations. How much difference did these large values make in the precipitation- and rainfall-frequency maps prepared in this investigation? Not all of the extreme values occurred at regular rainfall reporting stations. Some of the largest values were obtained from "bucket surveys". Several stations with long records did receive record values, e.g., Holly and Two Buttes. Comparison of the data at these stations indicated that the 2-year value for both the 6- and 24-hour duration would increase from 0.1 to 0.2 of an inch, and that the 100-year value would increase by approximately 1-1/2 to 2 inches. Only about five percent of the stations in southeastern Colorado experienced outstanding values in this storm. At these stations the 1965 storm produced precipitation amounts more than twice as high as the previous maxima of record. These values probably have return periods greater than 100 years, and the frequency maps have not been uniformly increased by the 1-1/2 to 2 inches that these few stations would indicate. The isopluvials in the portion of the state,like all those for all other regions, are based on a regional generalization of the data from all stations ~_ .a-\.._ ___.1__ .Lll I.lll:: I. l::l!; .LUll. ry intense rious du- The major centers of rainfall-frequency values are located on the most exposed and steepest slopes of the mountains. Where the crest of the range under investigation was 3,000 to 4,000 feet above the plains region to the windward of the mountains, the highest isopluvial line should extend past the crest and include a little of the lee side of the mountain. Where the crest of the range was 8,000 to 10,000 feet above the plains region to the windward of the mountain range, then the center of the highest isopluvial line would generally be at about 4,000 to 6,000 feet above the plains region. For mountain ranges with crests between these extremes, the placement of the highest rainfall-frequency values would depend on the degree of exposure of the mountain range to moisture-bearing wind, the steepness of the slope and other orographic factors, which could be utilized only in subjective reasoning based on experience. The maximum isopluvials are located farther downslope on the May-September maps than they appear on annual maps. This is presumably due to the heavy contribution to the May to October values given by summer showers and thunderstorms. In general, isopluvial centers for the longer return periods tend to occur at lower elevations than the centers for the sl~orter return periods. The distance downslope that the center is displaced depends on the exposure and steepness of the slope. Centers will be displaced less on a steep slope than on a gentle slope similarly exposed. This is a result of the downwind carry of precipitation formed over lower elevations. It is not always possible to depict these variations on maps, however, because of the steep isopluvial gradients along mountain slopes. Smoothi data read from the maps. The complex patterns and steep gradients of the isopluvials combined with the difficulties of interpolation and accurate location of a point on a series of maps might result in incon- sistencies in data read from the maps. Such inconsistencies can be minimized by fitting smooth curves to a plot of the data on logarithmic paper. 15