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. . generated by closed low pressure systems; and summer storms (monsoons) derived from the Gulf of Mexico and tropical Pacific storm systems (Sellers and Hill 1974; Rykaczewski 1981). Spring. As the rotational axis of the earth swings during spring, closed low pressure systems develop in the West (e.g., the Tonopah low pressure system centered over southern Nevada), producing a northward movement of moist, low-lying air and consequent early spring precipitation as wet snow. Much of the difficulty in predicting melt-off water volume for the upper Colorado River basin has been attributed to these erratic, late winter storms. May and June fall between the spring and summer storm periods and are often the driest months, especially in the southern part of the Plateau. Summer. The southwest receives a considerable portion of its annual precipitation in the form of torrential summer monsoons. During July and August, northward movement of the California high pressure system permits advection of moist air from Mexico and from tropical Pacific storm systems. These moist air masses pass over the hot southwest landscape and are driven aloft, causing condensation, cloud build-up, and precipitation in the form of intense thundershowers. One such storm deposited 89 mm (3.5 inches) of rain on Mesa Verde National Park in 45 minutes on 3 August 1924 (Hansen et al. 1977). Lifting of these air masses and consequent precipitation is greatest over mountains because both convective and orographic lifting take place simultaneously. Differential area and perhaps rock color relative to the surrounding landscape are factors that contribute to increased precipitation in mountainous regions. Autumn. A return of low pressure systems in late fall increases autumn precipitation. Winter. Winter frontal storms are spin-offs from the Aleutian low pressure system which move southeastward from the Alaskan coast and diagonally across western North America. These storm systems lose moisture as they travel across the continent, depositing snow and rain on the Wasatch Front and the west slope of the Rocky Mountains (Fig. 9). Snow is common from October through May at higher elevations, but the amount of snowfall varies greatly between years (Table 4). Snow is generated by three types of winter storm systems: arctic storms in early and mid-winter, closed low pressure storms in late winter, and southwestern Pacific fronts in late winter and early spring. Protected settings at high elevations retain permanent snow. Total annual snowfall accumulations exceed 5 m at the higher elevations. . . . . . . . . 17 .