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I I I I I I I I I I I I I I I I I I I AYRES /::"~''':-;~'~':~: )C~:: ;:~,~~,.,;.. >:~S~3 update of NOAA Atlas 2 for the Semiarid Southwest, Le. a regional frequency analysis approach based on L-moment. The deterministic approach will evaluate extreme rainfall Colorado storms that have been observed both along the Front Range and eastward over the plains of eastern Colorado. The Colorado Climate Center publication listing extreme rainfall storms in Colorado will be the primary source for storm identification. Additionally a storm search will be completed using the complete period of record for historic rainfall data through 2001 using our in-house data server and comprehensive meteorological database. Existing storm analyses including depth-area-duration tables for each storm will be reviewed. Where sufficient data are available, updated storm analyses will be completed using in-house software for storm isohyetal analyses and depth-area- duration analyses. These storm evaluations will identify and quantify meteorological conditions associated with extreme rainfall storms for constructing an up-to-date history of extreme rainfall events for central Colorado east of the Continental Divide. AWA will coordinate with Mr. Nolan Doesken at the Colorado Climate Center to ensure continuity with their storm data and analyses. Most of the required storm analyses are already available in-house as a result of other projects addressing site-specific PMP analyses. These studies include the Great Western Reservoir, Boyd Lake, and the Cherry Creek Reservoir. A detailed summary of the rainfall records for the SBC and surrounding area will be compiled following storm identification and analyses. The statistical approach will also use state-of-the-science methods along with our comprehensive in-house precipitation database. An extreme rainfall climatology would be constructed based on all available meteorological stations in the area including NWS Cooperative stations, FAAlNWS sites, RAWS (Remote Automated Weather Station) sites and Hourly Precipitation gauges. Spatial variability will be analyzed using gridded climatological data from the PRISM model. Data will be collected and formatted in collaboration with the Colorado Climate Center, the Western Regional Climate Center and Cooperative Mesonets in the Western United States. Data will be quality controlled using a threshold approach based on NOAA Atlas 2 for evaluating large rainfall amounts. Data quality control issues will be collaborated with Colorado Climate Center. Updated precipitation frequency estimates will be calculated using a regional frequency analysis approach based on L-moment. This is the current approach used by the National Weather Service in their update of NOAA Atlas 2 for the Semiarid Southwest (excluding Colorado). Data from individual stations will be clustered into climatologically and statistically homogenous "regions." L-moment software will be run to calculate annual maximum vs. partial duration factors. Spatial distribution of point values will use PRISM MAP (Mean Annual Precipitation) as the basis for spatial interpolation (current NWS method). For stations without hourly data, infill values will be determined using spatially and temporally interpolated estimates. Results of the analyses will be evaluated for consistency and reliability. The results will be compared to NOAA Atlas 2. High-resolution precipitation frequency maps will be created after all consistency checks have been completed. The final data base will be assembled in usable GIS format (ASCII grid and/or shapefile). 13