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. 'OJ ...~, . Executive Summary Applied Weather Associates (A W A) assembled a team of experts to conduct an independent site-specific Probable Maximum Precipitation (PMP) study for the Cherry Creek drainage basin. In addition, the A W A team performed a technical review of the 1995 National Weather Service (NWS) PMP study titled "Site-Specific PMP for the Cherry Creek Drainage in Colorado". This project was supported by the Colorado Water Conservation Board (CWCB), Arapahoe and Douglas Counties, and the cities of Aurora and Greenwood Village. The study was managed by the Proj ect Advisory Committee (PAC) and the CWCB (PAC members are listed in Appendix C). Technical aspects of the study were reviewed by the Technical Review Panel (TRP) with members from the Colorado Climate Center, Bureau of Rec1amation and a private engineering firm. The United States Army Corps of Engineers (USACE) and the NWS were also given opportunities to review the study. During the study, two interim reports were provided to the CWCB, the PAC, the TRP, the USACE and the NWS along with a draft Final Report. Both the NWS study and the A W A team study were conducted using the same definition ofPMP used by the NWS, the World Meteorological Organization and the National Research Council provided below: "Probable Maximum Precipitation (PMP) is defined as theoretically the greatest deoth of precipitation for a given duration that is physically possible over a given size storm area at a particular geof!raphicallocation at a certain time of year". The A W A team study produces approximately 2S percent less precipitation than the NWS study using the same definition ofPMP. The A W A team used a storm- based, site-specific PMP methodology. The NWS relied on the general PMP methodologies contained in Hydrometeorological Reports (HMR) 52 and 55A. The primary difference between the two approaches is that the A W A team approach was based on the use of storm characteristics of Colorado extreme rainfall events while the NWS incorporated the characteristics of29 storms listed in HMR 52 that occurred in the eastern two-thirds of the country. Most of the methodologies used are more similar than dissimilar. The A W A team performed a detailed review of the fourteen largest Colorado extreme precipitation events that included Colorado storms referenced in HMRs 52 and 55A. Use of Colorado storms maintains the "geographic location" requirement of the PMP definition. The following two findings form the basis for the primary differences in the two study results: V1ll