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, 18 and the "deep freeze" winters of 1883–84, 1915–16, 1949––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.It borders Idaho to the east, bounded mostly by the meridian running north from the confluence of the Snake River and Clearwater River (about 116°57' west), except for the southernmost section where the border follows the Snake River.Oregon is to the south, with the Columbia River forming the western part and the 46th parallel forming the eastern part of the Oregon-Washington border. Its northern border lies mostly along the 49th parallel, and then via marine boundaries through the Strait of Georgia, Haro Strait and Strait of Juan de Fuca, with the Canadian province of British Columbia to the north.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.
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.
It includes large areas of semiarid steppe and a few truly arid deserts lying in the rain shadow of the Cascades; the Hanford reservation receives an average annual precipitation of 6 to 7 inches (150 to 180 mm).
Farther east, the climate becomes less arid, with annual rainfall increasing as one goes east to 21.2 inches (540 mm) in Pullman, near the Washington-Idaho border.
Washington is part of a region known as the Pacific Northwest, a term which always includes Washington and Oregon and may or may not include some or all of the following, depending on the user's intent: Idaho, western Montana, northern California, British Columbia, and Alaska.
The high mountains of the Cascade Range run north-south, bisecting the state.
For Washington this means prevailing winds from the northwest bring relatively cool air and a predictably dry season.