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I I I I I I I I I I I I I I I I I I I 1.0 Introduction Considerable interast has developed in recent years concerning the rainfall characteriatica of thunderatoras which occur at elevations at or above 7500 feet. It is the intent of this survey to identify soae of the key characteristica of thunderatora rainfall generated at elevations above 7,500 feet in the centrsl Colorsdo 'ront Range foothills. The source of tha interest is related to the incressing developaent and population located at elevations above 7500 faet along Colorado's Front Range. Since the Big Thoapson Flash Flood of 1976 speculation has existed on the probability of flash flooding occurring at elevations above 7500 feet along the Colorado Pront Range (Jarrett, 1989). Jarrett speculates on the basis of a aulti-disciplinary approach which relies partially on the non-occurrence of rainfall exceeding 2 inches at elevations above 7500 feet. It is not the intent of this survey to research this proble. but to review existing observational evidence of the validity of this assuaptiDn. The priaary focus of the survey is a study of heavy thundsrstora rsinfall (2 inch or aore total or an equivalent intensity) over a period froa 1979 to 1988 which occurred at elevations above 7500 feet in the central Colorado Pront Range in Jefferson, Clear Creek, Gilpin and Teller Counties. The priaary data source for the study is called Stora Data, which is published aDnthly by NOAA. Stora Data chronicles severe weather reports by state as repDrted and verified by the local National Weather Service Porecast Office and the State Cliaatologist office, if available. A recent study of severe weather events in Colorado by Maddox et aI, 1981 also relied on Stora Data as its aain data source. The li.itations of Stora Data are recognized as priaarily related to pDpulation biases which favor reports near large or outdoor populations. This bias results in a conservative reporting of the actual nuaber of events that occur in IDw population density areas. This point is especially well-taken considering the low population density of the central Colorado Pront Range foothills above 7500 feet. The period chosen for study was concurrent with several field experiaents run by the PROPS (PrDtotype Regional Observing snd forecasting Service) based at the Environaental Research Lsboratory Headquarters in Boulder, Colorado. During field experiaents in 1983, 1985 and 1987 .obile chase teaas of aeteorologists and experienced "stora chssers" were used tD observe .ost heavy thunderstoras which occurred near the Denver aetro area including the Pront Range foothills. As a result of these field prograas the observations of heavy rainfall events noted in Stor. Data are judged to be of good value because of the care used to obtain verifications for the PROPS field prograas.