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2 1.2 Authorization Authorization for the study was given in a memorandum from the Office of Chief of Engineers, Corps of Engineers, dated July B, 1971. In conferences between representatives of the Corps of Engineers and the National Weather Service it was agreed the study should cover the Colorado River drainage and interior drainages of Nevada, Utah, and California. As thunderstorm PMP had not been previously considered for the Pacific Ocean drainages in California, it was subsequently agreed to expand this portion of the study. 1. 3 Scope Estimates of general-storm probable maximum precipitation (~ in this re- port cover the region between the crest of the Sierra Nevadas on the west and the Continental Divide on the east. To the north, the region extends to the southern limits of the Columbia River drainage and to the south to the U. S. border. This study region is shown in figure 1.1. The shaded portion of the study region in figure 1.1 is a ZOne (to the west of the Continental Divide) where the PMP values are considered least certain. Detailed generalized PMP estimates including seasonal variation are not avail- able for the slopes immediately east of the Continental Divide. PMP gradients in this region can influence PMP estimates west of the Divide. A future PMP study covering the area east of the Divide is needed before there will be comparable confidence in PHP over the contiguous portion of the Southwestern States. General-storm PHP estimates may be obtained for basin sizes from 10 to 5,000 mi2 (26 to 12,950 km2) for durations from 6 to 72 hours. Values can be computed for each month. Intense local summer thunderstorms can produce rain for short durations over small basins that exceed the rain potential from general storms. Chap- ter 4 gives these criteria for durations from 15 minutes to 6 hours covering basin sizes up to 500 mi2 (1,295 kml). The thunderstorm PMP estimates cover not only the primary study region defined above but also the remainder of California except a small section of the northern coastal region. The meteorological background and discussions have been kept to a minimum. A companion report (Schwarz and Hansen 1978) contains detailed descriptions of the meteorology of storms and other major meteorological analyses. 1.4 Definition of Probable Maximum Precipitation Probable maximum precipitation (PMP) is defined (American Meteorological Society 1959) as "...the theoretically greatest depth of precipitation for a given duration that is phYSically possible over a particular drainage basin at a particular time of year." We recognize there are yet unknowns in the complicated atmospheric processes responsible for extreme rainfalls. Thus, methods used for deriving PMP include making judgments based on record storms and meteorological processes related to them. Results of studies are con- sidered estimates because changes are likely as our understanding increases.