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19 Through most of the Southwest, the decadent tropical cyclone is considered the PMP prototype for the period from the end of June to mid-october. Examples of record are the storms of September 1939, August 1951, and September 1970. In the southern portion of the region during the cool season, fronts and storm centers from the Pacific Ocean produce major rains. Slow- moving to stagnant frontal situations, as in December 1955 and January 1916, are examples. The summer tropical cyclone is not likely to penetrate into the northwest or extreme northeast corners of the study area. For all-season PMP in the northwest portion, storms with more westerly moisture flows can enter the region around the north end of the Sierra Nevada range. This has led to the conclusion that northwest Nevada would have a seasonal influence more closely allied to northern California, where the October 1962 storm produced extreme rains. The northeast corner, particularly north of the Uinta Mountains and east of the Wasatch Mountains, can be influenced by moisture flows from the east that have spilled around the northern end of the Rocky Mountains in Colorado. Al- though no prototype storm for this northeast corner has yet been observed, the June 1964 storm that struck the Montana Rockies is an example of the type of storm that could affect this portion of the Southwest. Thus, seasonally, the northeast corner is similar to the eastern boundary region in HMR No. 43. Exact boundaries for the zone of influence of each type of storm have not been delineated. Rather, their influence has been incorporated in part by adjustments in the barrier elevation chart (see section 2.3) to account for the expected flows, and in part by the seasonal variations built into the convergence PMP analyses through tie-ins to peripheral studies. To understand the result and effectiveness of these methods, see the discussion in chapter 5 on checks on PMP level.. 2.2.6 Development of 10_mi2 (26-km2) 24-hr Convergence PMP In the development of seasonal maps of convergence PMP a number of consider- ations were used as guidance. Not necessarily in the order of importance, These were to: a. Envelop all maximized values of observed rainfall in least-orographic areas without explicit transposition. b. Recognize trends in seasonal variations established by data from least-orographic stations. c. Recognize the potential summertime maximum precipitation represented by the seasonal variation of maximum precipitable water. d. Fit a pattern that is in accord with tracks of extreme rain-producing storms. e. Observe regional variations caused by influences of different prototype storms. _c,_, ---~--