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The Cascade Mountains divide Washington not only topographically but also climatically. Despite its northerly location, western Washington is as mild as the middle and southeastern Atlantic coast; it is also one of the rainiest regions in the world. Eastern Washington, on the other hand, has a much more continental climate, characterized by cold winters, hot summers, and sparse rainfall. Since the prevailing winds are from the west, the windward (western) slopes of the state's major mountains intercept most of the atmospheric moisture and precipitate it as rain or snow. Certain coastal areas, receiving more than 200 in (500 cm) of rain a year, support dense stands of timber in a temperate rain forest. But in the dry southeastern quadrant, there are sagebrush deserts.

Average January temperatures in western Washington range from a minimum of 20°F (–7°C) on the western slope of the Cascades to a maximum of 48°F (9°C) along the Pacific coast; July temperatures range from a minimum of 44°F (7°C) on the western slope of the Cascades to a maximum of 80°F (27°C) in the foothills. In the east the temperature ranges are much more extreme: in January, from 8°F (–13°C) in the northeastern Cascades to 40°F (4°C) on the southeastern plateau; in July, from 48°F (9°C) on the eastern slope of the Cascades to 92°F (33°C) in the south-central portion of the state. The normal daily mean temperature in Seattle is 52°F (11°C), ranging from 40°F (4°C) in January to 65°F (18°C) in July; Spokane averages 47°F (8°C), ranging from 27°F (–3°C) in January to 69°F (21°C) in July. The lowest temperature ever recorded in the state is –48°F (–44°C), set at Mazama and Winthrop on 30 December 1968; the highest, at Ice Harbor Dam on 5 August 1961, was 118°F (48°C).

In Seattle average annual precipitation (1971–2000) was 37 in (94 cm), falling most heavily from October through March; in the same period, Spokane received an average of only 16.7 in (42.4 cm) annually, more than half of that from November through February. Snowfall in Seattle averages 11.4 in (29 cm) annually; in Spokane, 49.4 in (125.5 cm). Paradise Ranger Station holds the North American record for the most snowfall in one season, when 1,122 in (2,850 cm) of snow fell during the winter of 1971–72. High mountain peaks, such as Mt. Adams, Mt. Baker, and Mt. Rainier, have permanent snowcaps or snowfields of up to 100 ft (30 m) deep.