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<br />Executive Summary <br /> <br />Applied Weather Associates, LLC (A W A) assembled a team of experts to <br />conduct an independent site-specific Probable Maximum Precipitation (PMP) study for <br />the Cherry Creek drainage basin. In addition, the A W A team performed a technical <br />review of the 1995 National Weather Service (NWS) PMP study titled "Site-Specific <br />PMP for the Cherry Creek Drainage in Colorado". This proj ect was supported by the <br />Colorado Water Conservation Board (CWCB), Arapahoe and Douglas Counties, and the <br />cities of Aurora and Greenwood Village, Colorado. <br /> <br />The study was managed by the Project Advisory Committee (PAC) and the <br />CWCB (PAC members are listed in Appendix C). Technical aspects of the study were <br />reviewed by the Technical Review Panel (TRP) with members from the Colorado <br />Climate Center, Bureau of Reclamation and a private engineering firm. The United <br />States Army Corps of Engineers (USACE) and the NWS were also given opportunities to <br />review the study. During the study, two interim reports were provided to the CWCB, the <br />PAC, the TRP, the USACE and the NWS along with a draft Final Report. <br /> <br />Both the NWS study and the A W A team study were conducted using the same <br />definition ofPMP used by the NWS, the World Meteorological Organization and the <br />National Research Council provided below: <br /> <br />"Probable Maximum Precipitation (PMP) is defined as theoretically <br />the greatest depth of precipitation for a given duration that is <br />physically possible over a given size storm area at a particular <br />geographical location at a certain time of year ". <br /> <br />The A W A team study produces approximately 25 percent less precipitation <br />than the NWS study using the same definition ofPMP. The A W A team used a storm- <br />based, site-specific PMP methodology. The NWS relied on the general PMP <br />methodologies contained in Hydrometeorological Reports (HMR) 52 and 55A. <br /> <br />The primary difference between the two approaches is that the A W A team <br />approach was based on the use of storm characteristics of Colorado extreme rainfall <br />events while the NWS incorporated the characteristics of 29 storms listed in HMR 52 that <br />occurred in the eastern two-thirds of the country. Most of the methodologies used are <br />more similar than dissimilar. <br /> <br />The A W A team performed a detailed review of the ten largest Colorado extreme <br />precipitation events that included Colorado storms referenced in HMR 52 and HMR 55A. <br />Use of Colorado storms maintains the "geographic location" requirement of the PMP <br />definition. The following two findings form the basis for the primary differences in the <br />two study results: <br /> <br />X11 <br />