This report documents the findings of regional precipitation-frequency analyses of 24-hour and 2-hour precipitation annual maxima for western Washington. It also describes the procedure used for spatial mapping of the precipitation-frequency estimates for selected recurrence intervals.This study is an update of the information contained in the precipitation-frequency atlas published by the National Weather Service in 1973. Data collection for the NWS study ended in 1966, and this study includes the 34-years of record collected since 1966. These additional data provide a precipitation database with more than double the record length than was available in the original NWS study.Since the original study, major advances have been made in the methods for statistical analysis of precipitation annual maxima, and for spatial mapping of precipitation in complex terrain. Specifically, L-Moment statistical analysis techniques conducted with a regional framework have greatly improved the reliability of precipitation magnitude-frequency estimates, particularly for rare storm events. Development of the PRISM model incorporating digital terrain data has also improved the spatial mapping of precipitation and increased the reliability of estimating precipitation in the broad areas between precipitation measurement stations. These methodologies are particularly effective in areas with high topographic and climatic variability such as in western Washington, where mean annual precipitation varies from less than 20-inches to over 200-inches. Both of these methodologies have been utilized in conducting the precipitation-frequency analyses and in developing the isopluvial maps for selected recurrence intervals.
MGS Engineering Consultants, Inc.; Oregon Climate Service.
Hours, Mapping, Measurement, Precipitation (Meteorology), Time duration.