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000825 The rainfall totals over the Wetzel Creek basin accumulated over a period from 1500MDT to 1900MDT. This four hour period was not a period of continuous rainfall but was concentrated durin, the period 1550MDT through 1800MDT, Figure 3 shows the 5 minute rainfall estimated for the north, central and south basins of Wetzel Creek. It is easy to see that the rainfall varied considerably in time and intensity across the basin. Figure 4 shows the time accumulated rainfall for the north, central and south basins of Wetzel Creek which clarifies further the differences in the rainfall over the basin. The occurrence of thunderstorm complexes over eastern Colorado during the monsoon season from late June through mid-August is fairly common during the wet decades and rarer during dry decades. A wet decade has been observed in eastern Colorado with regualrity during the even numbered decades since the turn of the century, ie, the 1920's, 1940's, the 1960's and now the 1980's. By contrast the odd numbered d~cades, the 1910's, the 1930's, the 1950's and most recently, the 1970's were accented by general plains' drought conditions. It is our experience that the weather patterns which occur during the periods of drought and over abundance of rain are different and this leads to an increased opportunity for heavy rain events during the wet decades. Another factor affecting the occurrence of thunderstorms over eastern Colorado is the documented existence of preferred places for the formation of thunderstorms along the Front range of the Rockies and Palmer Lake Divide. These hot spots of thunderstorm formation are shown in Figure 5. The Wetzel Creek basin is immediately downstream from one of these hot spots, Number VII, and with southwesterly winds, storms that form over the hot spot drift over the basins and adjoining ones with more regularity than basins further east. Additionally the Wetzel Creek basin is affected by storms which form to the north and drift southeastward off hot spots in the vicinity of Fort Collins(I and II). A graphic example of the wet decade and hot spot effects can be seem in the repeated occurrence of heavy rain and hail events in the Denver Metro area to the west during the 1960's and 1980's. In 1967 and 1969 the western suburbs of Denver experienced estimated 2-5 inch rainfalls accompanied by 1-3 feet of hail four times from late May through July. The dates of these events is 5-30-67, 7-3-67, 7-7-67 and 6- 8-69. On June 13, 1984 the same area of Denver experieinced a 2-4inch rainfall with pea-size to softball-size hail and suffered $350 million damage. The next year a similar but faster moving storm occurred on July 19. 1985 and produced and estimated $50-100 million dollars damage. If history repeats itself another round of this form of storm is likely over the next two summers. Each of these events were over 100 year 3-hour events and their repeated occurrence points out the inadequacies that exist in assigning return frequencies to events in geographic areas affected by cyclic rainfall patterns and topographic influences on storm formation.