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1.0 Introduction Henz Meteorological Services (HMS) was retained by Black & Veatch (BV) to prepare a review of existing Mason Rese/voir Probable Maximum Precipitation (PMP) studies, evaluate the validity of the existing Hydrometeorological Report (HMR)-based PMP assumptions as they apply to Mason Reservoir and prepare a site-specific PMP for the Mason Reservoir drainage basin, Special considerations were made for the unique location of Mason Reservoir which is influenced both by the rolling High Plains to the east and by the surrounding topography of the Rocky Mountains to the south through north. Mason Reservoir is located at an elevation of 11,600 feet immediately south and east of the base of Pikes Peak which towers just west of Colorado Springs, Colorado and is presented in Figure 1. Its drainage basin elevations range from 11,100 feet to 13,500 feet and its area is less than 10 square miles in size, It is located in a high alpine area which consists of mostly rock outcroppings above 12,500 feet and a combination of bristle cone pine, spruce, Douglas fir, aspen and pines in alpine meadows at lower elevations, The task of determining the site-specific F'IVIP for Mason Reservoir consisted of four steps which were split into two phases: Phase I: 1. HMS evaluated the BV local storm and geneml storm PMP calculations for Mason Reservoir and the HMR 55A assumptions used to calc:ulate them. HMS evaluated the use of these historically accepted methodologies and identified opportunities tel combine them with new technologies and methodologies (i.E~., the HMS Convective Storm Methodology) in a manner which allowed continuity with previollls accepted studies while providing incremental improvements where enhanced data sets and new technolclgical adlvances allowed. Phase I of this study, a preliminary site-speGific PMP evaluation, was begun in February 1995. During this Phase HMS evaluated the prior BV site-specific PMP calculations which had been completed for the Mason Reservoir basin, An internal memorandum was issued in April, 1995 which summarized the results of meteorological and analytical evaluations of the standard HMR 55A methodologies which are used to prepare site-specific PMP's relative to both enhanced data sets and new technologies and the high elevation of Mason Reservoir, The conclusion of Phase I recommended that an enhanced site- specific PMP which makes use of these new data sets and methodologies should be undertaken.