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4 In this derivation of PMP we assume that the record storms during. the past 80 or so years are repr~sentative of the climate of extreme precipitation. PMP estimates therefore do not allow for changes in climate. Experience gained from PMP studies in other regions gives additional guid- ance to procedures and methods used. This then points to an operational de- finition of PMP; i.e., estimates by hydrometeorologists of upper limits of rainfall, supplied to engineers for use in hydrologic design. Quoting from Operational Hydrology Report No.1 (World Meteorological Organization 1973), "Whatever the philosophical objection to the concept, the operational defini- tion leads to answers that have been examined thoroughly by competent meteor- ologists and engineers and judged as meeting the requirements of a design criterion. II 1.5 Methods of This Report Estimation of general storm PMP of this report uses basically the same pro- cedure used in two studies for adjoining regions; to the west (D. S. Weather Bureau 196]) and to the north (D. S. Weather Bureau 1966a). First, essentially nonorographic PMP, also termed convergence PMP (precipitation due to atmos- pheric processes), is estimated. Then orographic PMP (precipitation from moist air forced upward by mountain slopes and the triggering of rainfall near first upslopes) is estimated. The two components of PMP are then added to- gether. The convergence PMP is based on moisture-maximized rains of record, reduced for mountain barriers and elevations. Consideration was given to convergence PMP from the adjoining studies. Orographic PMP, for the most part, was not based on the orographic precipitation computation model used in adjoining regions (D. S. Weather Bureau 1961 and 1966a). Reasons for this departure are spelled out in chapter 3. The model is not suited for the meteorological conditions accompanying the main PMP storm prototype for much of the Southwest, par.tly because the topography is too complicated. Alter- nate methods for estimating orographic PMP are discussed in chapter 3. The method used for local or thunderstorm PMP was to adjust the most in- tense storm values for maximum moisture and develop a l-hr PMP map for 1 mi2 (2.6 km2). The regional pattern of this map took into account maximum l-hr rainfalls from recorder stations and broad-scale terrain features. Depth- duration and depth-areal variations to extend the estimates to other dur- ations and larger areas were based on record storms. 1.6 Organization of Report General-storm convergence PMP estimates are developed in chapter 2 and gen- eral storm orographic PMP in chapter 3. PMP for small areas from intense thunderstorms is covered in chapter 4. Checks on the general level of PMP are discussed in chapter 5; while chapter 6 gives procedures for and examples of use of the developed criteria. We at times refer to the study region as the Southwest or the Southwestern States. Frequent reference will be made to studies for two adjoining regions. These are the Columbia River drainage, Hydrometeorological Report No. 43 (D. S. Weather Bureau 1966a) and the Pacific Ocean drainages of