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<br />The study was managed by the Project Advisory Committee (PAC) and the <br />CWCB. Technical aspects of the study were reviewed by the Technical Review Panel <br />(TRP) with members from the Colorado Climate Center, Bureau of Reclamation and a <br />private engineering firm. The United States Army Corps of Engineers (USACE) and the <br />NWS were also given opportunities to review the study. During the study, two interim <br />reports were provided to the CWCB, the PAC, the TRP, the USACE and the NWS along <br />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 of PMP 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 devth of precipitation for a given duration that is physically possible over a <br />given size storm area at a particularf!eof!ravhical 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 of PMP. 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 HMRs 52 and 55A. Use <br />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 />1. Colorado extreme precipitation events are significantly smaller in their depth'! <br />duration and aerial coverage than the storm characteristics of HMR 52 that rely on <br />29 storms sampled over the eastern two-thirds of the country (See Figure 1). <br />2. The major axis of the rainfall pattern extends along the direction of the winds in <br />the storm cloud layer (+/- 10 degrees). The surface wind direction is 40 to 90 <br />degrees to the right of the cloud layer winds (See Figure 2). In effect, this <br />relationship of meteorology to rainfall pattern must be maintained during the <br />transposition of storms into a basin to k:eep the transposition process "physically <br />possible". The final PMP design storm must also maintain this relationship. <br /> <br />Figure 1 shows that Colorado storms are smaller than the HMR 52 storms. The <br />Colorado storms range from 66% to 91% of the HMR 52 storms for area sizes ranging <br />from 5,000 square miles down to 100 square miles. HMR 52 requires the use of its storm <br />sample together with their rainfall distribution curves without offering any other <br />