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Colorado Extreme Storm Precipitation Data Study Summary The Colorado Extreme Storm Precipitation Data Study was undertaken in Colorado in response to ongoing scientific uncertainty regarding the magnitude (intensity, duration and area) of precipitation that can conceivably occur at high elevations in the Rocky Mountain region. This uncertainty has significant implications for spillway design, sizing requirements and overall dam safety policies in Colorado and in other western states. Colorado has extensive land areas at elevations above 7,500 feet and many large and small reservoirs at high elevations. The study area for this project focused on areas in Colorado bounded by the Utah border on the western edge, the Wyoming border on the northern edge, the 5,000 foot (above mean sea level) elevation contour in eastern Colorado and the New Mexico border on the southern edge. Data on extreme storm precipitation amounts were also gathered from neighboring states having similar topography. The Extreme Precipitation Data Study is the first step in a comprehensive effort, supported by the Colorado Department of Natural Resources, Division of Water Resources, to better understand extreme precipitation as a function of location and elevation and its impact on dam safety regulations. This study focused on observational precipitation and streamflow data during the period of instrumental record which dates back approximately 125 years. The results of this study are intended to be utilized in later project phases that will focus on numerical simulation of extreme storms at high elevations leading toward a better definition of extreme storms and their spatial variations. More than 300 storms were identified by this study since the late 1800s that have produced very heavy precipitation either locally or over sizable areas in or near the mountains of Colorado based on a definition given in Section 2. Of this large set of heavy precipitation events, 36 extreme storms were identified that stand out as the heaviest storms of record for selected geographic regions of the state and the storms that must be considered when evaluating extreme precipitation and dam safety policies for high elevation areas of Colorado. This set of storms also becomes candidates for inclusion in future numerical modeling studies of extreme precipitation in elevated regions or in future deterministic studies of probable maximum precipitation (pMP). One of the nagging problems that continues to plague extreme precipitation studies is uncertainty in the reliability of precipitation and flooding reports, especially for storms that occurred long ago. Efforts were made in this study to identify storms for which precipitation reports may be suspect, and some storms were removed from consideration when lack of reliability was apparent. However, thorough evaluations of data reliability were not performed for all storms. 11