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Colorado Extreme Storm Precipitation Data Study Introduction This report summarizes the results of a nearly two-year study of extreme precipitation characteristics in Colorado. The primary goal of this project was to identifY and document the heaviest storms that have occurred in or near the Rocky Mountains in Colorado. The criterion used to define heavy storms was any storm that exceeded the 100-year storm precipitation amounts for specified storm durations as published in the NOAA (National Oceanic and Atmospheric A-dministration) Atlas 2, Precipitation-Frequency Atlas of the Western United States, Volume III - Colorado (1973). The critical properties of storms that determine their potential for producing flooding are precipitation intensity, storm duration and storm area. In many of the storms included in this study, particularly the local intense summer thunderstorms, only a limited amount of information is known about storm areas. However, because of the importance of area and duration, some storms with large areas or long durations were considered even though they may not have eXI;eeded 100-year thresholds at any individual point. The format for this report is consistent with the outline of the original proposal submitted to the State of Colorado, Department of Natural Resources Division of Water Resources in the summer of 1994. Activities and accomplishments are presented in the ordl:r described in the original proposal. Most of the work for this project was conducted by personnel of the Colorado Climate Center, Department of Atmospheric Science, Colorado State University. However, some tasks were perfonned with assistance of other organizations. Activities and Accomplishments 1) Compilation of hourly and daily precipitation data. The starting point for this project was the careJill scrutiny of all archived National Weather Service precipitation records back into the 1800s and up through 1993. For each station and each month of record, the maximum observed precipitation for various durations was determined. For many stations with data going back prior to 1948, this required manual data processing and digitization. Maximum one, two and three-day precipitation totals were determined for 598 official stations when: precipitation has been measured on a daily basis. For an additional 69 stations where precipitation has been measured hourly or more frequently, maximum one-hour, two-hour, three-hour, six-hour, 24-hour, 48-hour and 72- hour precipitation totals were determined for each month of record.