Climate

See also: Climate change in Washington

Köppen climate types of Washington state

As described above, Washington's climate varies greatly from west to east. An oceanic climate (also called "west coast marine climate") predominates in western Washington, and a much drier semi-arid climate prevails east of the Cascade Range. Major factors determining Washington's climate include the large semi-permanent high pressure and low pressure systems of the north Pacific Ocean, the continental air masses of North America, and the Olympic and Cascade mountains. In the spring and summer, a high pressure anticyclone system dominates the north Pacific Ocean, causing air to spiral out in a clockwise fashion. For Washington this means prevailing winds from the northwest bring relatively cool air and a predictably dry season.

Dryland farming caused a large dust storm in arid parts of eastern Washington on October 4, 2009. Courtesy: NASA/GSFC, MODIS Rapid Response.^[13]

In the autumn and winter, a low-pressure cyclone system takes over in the north Pacific Ocean, with air spiraling inward in a counter-clockwise fashion. This causes Washington's prevailing winds, the Chinooks, to come from the southwest, bringing relatively warm and moist air masses and a predictably wet season. The term "Pineapple Express" is used colloquially to describe the extreme form of the wet-season Chinook winds.^[14]
Despite western Washington's having a marine climate similar to those of many coastal cities of Europe, there are exceptions such as the "Big Snow" events of 1880, 1881, 1893 and 1916 and the "deep freeze" winters of 1883–84, 1915–16, 1949–50 and 1955–56, among others. During these events western Washington experienced up to 6 feet (1.8 m) of snow, sub-zero (−18 °C) temperatures, three months with snow on the ground, and lakes and rivers frozen over for weeks.^[15] Seattle's lowest officially recorded temperature is 0 °F (−18 °C) set on January 31, 1950, but low-altitude areas approximately three hours away from Seattle have recorded lows as cold as −48 °F (−44 °C).^[16]
Weather during the cold season is greatly influenced by the Southern Oscillation. During the El Niño phase, the jet stream enters the U.S. farther south through California, therefore late fall and winter are drier than normal with less snowpack. The La Niña phase reinforces the jet stream through the Pacific Northwest, causing Washington to have even more rain and snow than average.
In 2006, the Climate Impacts Group at the University of Washington published The Impacts of Climate change in Washington's Economy, a preliminary assessment on the risks and opportunities presented given the possibility of a rise in global temperatures and their effects on Washington state.^[17]

Rain shadow effects

Main article: Rain shadow

Washington experiences extensive variation in rainfall.

Rainfall in Washington varies dramatically going from east to west. The western side of the Olympic Peninsula receives as much as 160 inches (4,100 mm) of precipitation annually, making it the wettest area of the 48 conterminous states and a temperate rainforest. Weeks may pass without a clear day. The western slopes of the Cascade Range receive some of the heaviest annual snowfall (in some places more than 200 inches or 5,100 millimetres water equivalent) in the country. In the rain shadow area east of the Cascades, the annual precipitation is only 6 inches (150 mm). Precipitation then increases again eastward toward the Rocky Mountains.
The Olympic mountains and Cascades compound this climatic pattern by causing orographic lift of the air masses blown inland from the Pacific Ocean, resulting in the windward side of the mountains receiving high levels of precipitation and the leeward side receiving low levels. This occurs most dramatically around the Olympic Mountains and the Cascade Range. In both cases the windward slopes facing southwest receive high precipitation and mild, cool temperatures. While the Puget Sound lowlands are known for clouds and rain in the winter, the western slopes of the Cascades receive larger amounts of precipitation, often falling as snow at higher elevations. (Mount Baker, near the state's northern border, is one of the snowiest places in the world: in 1999, it set the world record for snowfall in a single season: 1,140 inches (95 ft; 29 m).)^[18]
East of the Cascades, a large region experiences strong rain shadow effects. Semi-arid conditions occur in much of eastern Washington with the strongest rain shadow effects at the relatively low elevations of the central Columbia Plateau—especially the region just east of the Columbia River from about the Snake River to the Okanagan Highland. Thus instead of rain forests much of eastern Washington is covered with grassland and shrub-steppe.