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.
For Washington this means prevailing winds from the northwest bring 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.
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.