The characteristics of United States rainfall climatology differ significantly across the United States and those under United States sovereignty. Summer and early fall bring brief, but frequent thundershowers and tropical cyclones which create a wet summer and drier winter in the eastern Gulf and lower East Coast. 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 and through the Great Lakes, bring cold season precipitation to from the Midwest to New England, as well as Great Salt Lake. The snow to liquid ratio across the contiguous United States averages 13:1, meaning of snow melts down to of water.Martin A. Baxter, Charles E. Graves, and James T. Moore. A Climatology of Snow-to-Liquid Ratio for the Contiguous United States. Retrieved on 2008-03-21.

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.

General

The eastern part of the contiguous United States east of the 98th meridian, the mountains of the Pacific Northwest, the Willamette Valley, and the Sierra Nevada range are the wetter portions of the nation, with average rainfall exceeding per year. The drier areas are the Desert Southwest, Great Basin, valleys of northeast Arizona, eastern Utah, and central Wyoming. Retrieved on 2008-03-09. Increased warming within urban heat islands leads to an increase in rainfall downwind of cities.

Alaska

Juneau averages over of precipitation a year, while other areas in southeast Alaska receive over . South central Alaska does not get nearly as much rain as the southeast of Alaska, though it does get more snow. On average, Anchorage receives of precipitation a year, with around of snow. The northern coast of the Gulf of Alaska receives less than of precipitation annually. Across western sections of the state, the northern side of the Seward Peninsula is a desert with less than of precipitation annually, while some locations between Dillingham and Bethel average around of precipitation. Inland, often less than falls a year, but what precipitation falls during the winter tends to stay throughout the season. La Niña events lead to drier than normal conditions, while El Niño events do not have a correlation towards dry or wet conditions. Precipitation increases by 10 to 40 percent when the Pacific decadal oscillation is positive.

West

From September through May, extratropical cyclones from the Pacific Ocean move inland into the region due to a southward migration of the jet stream during the cold season. This shift in the jet stream brings much of the annual precipitation to the region, and also brings the potential for heavy rain events. The West Coast occasionally experiences ocean-effect showers, usually in the form of rain at lower elevations south of the mouth of the Columbia River. These occur whenever an Arctic air mass from western Canada is drawn westward out over the Pacific Ocean, typically by way of the Fraser Valley, returning shoreward around a center of low pressure. Strong onshore flow is brought into the mountain ranges of the west, focusing significant precipitation into the Rocky Mountains, with rain shadows occurring in the Harney Basin, Great Basin, the central valley of California, and the lower Colorado River valley. In general, rainfall amounts are lower on the southern portions of the West Coast. The biggest recipients of the precipitation are the coastal ranges such as the Olympic Mountains, the Cascades, and the Sierra Nevada range. Lesser amounts fall upon the Continental Divide. Cold-season precipitation into this region is the main supply of water to area riverslike the Colorado River and the Rio Grande, and thus is the main source of water to people living in this portion of the United States. During El Niño events, increased precipitation is expected in California due to a more southerly zonal, storm track. Retrieved on 2008-02-28. California also enters a wet pattern when thunderstorm activity within the tropics associated with the Madden–Julian oscillation nears 150˚E longitude. During La Niña, increased precipitation is diverted into the Pacific Northwest due to a more northerly storm track.

Lake-effect snow off Great Salt Lake

The southern and southeastern sides of the Great Salt Lake receive significant lake-effect snow. Since the Great Salt Lake never freezes, the lake-effect can affect the weather along the Wasatch Front year round. The lake-effect largely contributes to the annual snowfall amounts recorded south and east of the lake, with average snowfall amounts exceeding in the Wasatch Mountains. The snow, which is often very light and dry due to the semi-arid climate away from the lake, is referred to as "The Greatest Snow on Earth" in the mountains. Lake-effect snow contributes to approximately 6-8 snowfalls per year in Salt Lake City, with approximately 10% of the city's precipitation being contributed by the phenomenon.

North American Monsoon

The North American Monsoon (NAM) occurs from late June or early July into September, originating over Mexico and spreading into the southwest United States by mid-July. This allows the wet season to start in the Southwest during the summer rather than early fall as seen across the remainder of the West. Within the United States, it affects Arizona, New Mexico, Nevada, Utah, Colorado, West Texas, and California. The North American monsoon is known to many as the Summer, Southwest, Mexican or Arizona monsoon. It is also sometimes called the Desert Monsoon as a large part of the affected area is desert.

When precipitable water values near , brief but often torrential thunderstorms and the hurricane force winds and hail can occur, especially over mountainous terrain. This activity is occasionally enhanced by the passage of retrograding (westward-moving) upper cyclones moving under the subtropical ridge and the entrainment of the remnants of tropical storms. Tropical cyclones from the eastern Pacific contribute to the moisture within the monsoon system, and bring to southern California up to 20 percent of the average annual rainfall. Flash flooding is a serious danger during the monsoon season. Dry washes can become raging rivers in an instant, even when no storms are visible as a storm can cause a flash flood tens of miles away. Lightning strikes are also a significant danger. Because it is dangerous to be caught in the open when these storms suddenly appear, many golf courses in Arizona have thunderstorm warning systems.

As much as 45% of the annual rainfall across New Mexico occurs during the summer monsoon. Many desert plants are adapted to take advantage of this brief wet season. Because of the monsoons, the Sonoran and Mojave are considered relatively "wet" when ranked among other deserts such as the Sahara. Monsoons play a vital role in managing wildfire threat by providing moisture at higher elevations and feeding desert streams. Heavy monsoon rain can lead to excess winter plant growth, in turn a summer wildfire risk. A lack of monsoon rain can hamper summer seeding, reducing excess winter plant growth but worsening drought.

Great Plains

Downslope winds off the Rocky Mountains can aid in forming the dry line. Major drought episodes in the midwestern United States are associated with an amplification of the upper tropospheric subtropical (or monsoon) ridge across the West and Plains, along with a weakening of the western edge of the "Bermuda high". During the summer, a southerly low-level jet draws moisture from the Gulf of Mexico. Additional moisture comes from more local sources, especially transpiring vegetation. Maximum precipitation generally occurs in late spring and early summer, with minimum precipitation in winter. During La Niña events, the storm track shifts far enough northward to bring wetter than normal conditions (in the form of increased snowfall) to the Midwestern states, as well as hot and dry summers.

The convective season for the Plains ranges between May and September. Organized systems of thunderstorms known as mesoscale convective systems develop over the region during this period, with a bulk of the activity occurring between midnight and 6 a.m. local time. The time of maximum precipitation during the day gradually varies from late afternoon near the slopes of the Rockies to early morning near the Ohio River valley, in part reflecting the west-to-east propagation of mesoscale convective systems. Mesoscale convective systems bring 30 to 70 percent of the annual warm season rainfall to the Plains. An especially long-lived and well-organized type of mesoscale convective system called a mesoscale convective complex produces on average 8% to 18% of the annual warm season rainfall across the Plains and Midwest. Squall lines account for 30% of the large thunderstorm complexes which move through the region.

Gulf Coast and lower Atlantic Coast south of New England In general, northern and western portions of this region have a winter/spring maximum in precipitation with late summer/early fall being drier, while southern and eastern portions have a summer maximum and winter/early spring minimum in precipitation.

Most locations on the East Coast from Boston northward show a slight winter maximum as winter storms drop heavy precipitation. South of Boston, convective storms are common in the hot summer months and seasonal rainfall shows a slight summer maximum (though not at all stations). As one moves from Virginia Beach southward, summer becomes the wettest season, as convective thunderstorms created in the hot and moist tropical air mass drop brief but intense precipitation. In winter these areas still sees precipitation as low pressure systems moving across the southern United States often tap moisture from the Gulf of Mexico and drop cold season precipitation from eastern Texas to the New York area.

On the Florida peninsula, a strong monsoon becomes dominant, with dry winters and heavy summer rainfall. In winter the strong subtropical ridge creates the stable air over Florida with little convection and few fronts. Along the Gulf Coast, and the south Atlantic states, decaying tropical systems added to summers peak rainfall.

Cold season