Arizona Statewide Probably Maximum Precipitation, PMP, Improving HMR 49

Recent storm analyses and site-specific Probable Maximum Precipitation (PMP) studies completed in Arizona revealed the inadequacies of HMR 49 in properly representing storm characteristics and PMP throughout the region. To address the numerous issues associated with HMR 49, Applied Weather Associates (AWA) is conducting a statewide, storm based PMP project for the state of Arizona to replace HMR 49. AWA is working directly with a consortium of stakeholders, including Arizona DWR, various dam owners, the Department of Game and Fish, the state climatologist, and others in this important project. HMR 49 was published in 1977 and is the oldest of the HMRs currently in use. More recent HMRs use updated storm data and PMP analysis procedures. HMR 49 used outdated methods and techniques that have subsequently been improved through new understanding of meteorology and updated datasets. A major issue with HMR 49 is the lack of storm data used to develop the PMP values. Only a handful of storms were investigated during the development of HMR 49, none of which were analyzed using individual storm depth-area-duration (DAD) values. This combined with the fact that the document covers such a widely varying region both climatologically and topographically, further validates the necessity for an update. AWA has incorporated updated databases and procedures as part of several recently completed and current site-specific, statewide, and regional PMP projects. These improvements include, but are not limited to: updated storm databases (the newest HMR storm analyzed is from the mid-1990's), integrating the Storm Precipitation Analysis System (SPAS) with Next Generation Weather Radar (NEXRAD) to better spatially and temporally distribute rainfall, integration of GIS to analyze orographics and controlling topography, updated dew point climatology to better represent atmospheric moisture, and updated storm analysis techniques. An extensive storm database constructed from an comprehensive storm search, previous project results, and other sources is being used to develop a database of storms. The most extreme of these events are being individually analyzed during the statewide PMP project to derive the final PMP values. The storm list contains numerous storms in each climate division as well as each storm type (local convective, general synoptic and decaying tropical) that affects Arizona. These storm analyses will allow for a storm based PMP to be developed for all points within the state, much the same as was completed by AWA for the state of Nebraska in 2008. 18 pp., 14 references.