United States Rainfall Climatology - West

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 Willamette Valley, 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 rivers, such as the Colorado River and Rio Grande, and also acts as 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. California also enters a wet pattern when thunderstorm activity within the tropics associated with the Madden-Julian Oscillation nears 150E longitude. During La Niña, increased precipitation is diverted into the Pacific Northwest due to a more northerly storm track.