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- The characteristics of United States rainfall climatology differ significantly across the United States and those under United States sovereignty. Late summer and fall tropical cyclones bring precipitation which falls across the Gulf and Atlantic states. During the winter, and spring, Pacific storm systems bring Hawaii and the western United States most of their precipitation. Low pressure systems moving up the East coast bring cold season precipitation to the Mid-West and Northeast states, as well as Great Salt Lake and the Finger Lakes region. The snow to liquid ratio across the contiguous United States averages 13:1, meaning 13 inches (330 mm) of snow melts down to 1 inch (25 mm) of water. During the summer, the North American monsoon combined with Gulf of California and Gulf of Mexico moisture moving around the subtropical ridge in the Atlantic Ocean bring the promise of afternoon and evening air-mass thunderstorms to the southern tier of the country as well as the Great Plains. Equatorward of the subtropical ridge, tropical cyclones enhance precipitation across southern and eastern sections of the country, as well as Puerto Rico, the United States Virgin Islands, the Northern Mariana Islands, Guam, and American Samoa. Over the top of the ridge, the jet stream brings a summer precipitation maximum to the Great Plains and western Great Lakes. Large thunderstorm areas known as mesoscale convective complexes move through the Plains, Midwest, and Great Lakes during the warm season, contributing up to 10% of the annual precipitation to the region. The El Niño–Southern Oscillation affects the precipitation distribution, by altering rainfall patterns across the West, Midwest, the Southeast, and throughout the tropics. There is also evidence that global warming is leading to increased precipitation to the eastern portions of North America, while droughts are becoming more frequent in the western portions. Furthermore, global La Niña meteorological events are generally associated with drier and hotter conditions and further exacerbation of droughts in California and the Southwestern and to some extent the Southeastern United States. Meteorological scientists have observed that La Niñas have become more frequent over time. (en)
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