The climate in Washington varies greatly from west to east. A mild oceanic climate (also called "marine west coast climate") predominates in western Washington, and a much drier 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 bringing relatively cool air and a predictably dry season. 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 to come from the southwest, bringing relatively warm and moist air masses and a predictably wet season. The term Pineapple Express is used to describe the extreme form of this wet season pattern.
The coastal 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 to receive high levels of precipitation and the leeward side to receive 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. In contrast, the leeward slopes facing northeast experience a rain shadow effect, with low precipitation and warmer temperatures. As a result, there are temperate rain forests on the southwest side of the Olympic Mountains while the northeast side has a sub-mediterranean climate. The San Juan Islands and the city of Sequim are known for their dry climate compared to the rest of the coastal region. The Olympic rain shadow extends into Canada. Victoria is known for its mediterranean climate.
The Cascade Range forms a larger barrier than the Olympics and a correspondingly strongly orographic effect. 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. 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 River 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 grassland and shrub-steppe.
The average annual temperature ranges from 51° F (10.6° C) on the Pacific coast to 40° F (4.4° C) in the northeast. The recorded temperature in the state has ranged from -48° F (-44.4° C) to 118° F (47.8° C) with both records set east of the Cascades. Western Washington is known for its mild climate, considerable fog, frequent cloud cover and long-lasting drizzles in the winter, and sunny and dry summers. The western region occasionally experiences extreme climate. Arctic cold fronts in the winter and heat waves in the summer are not uncommon. The western side of the Olympic Peninsula receives as much as 160 inches (4064 mm) of precipitation annually, making it the wettest area of the 48 conterminous states. Weeks or even months 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/5080 mm) in the country. In the rain shadow area east of the Cascades, the annual precipitation is only 6 inches (152 mm). Precipitation increases eastward toward the Rocky Mountains.