To the west are the Olympic Mountains and the Willapa Hills. To the east are the Okanogan Highlands, the western foothills of the Rocky Mountains, and the Blue Mountains which extend south into Oregon. The mountains and the sea give Washington two climates. Western Washington has mild summers and winters. The temperature seldom exceeds 85 degrees in the summer and rarely drops below freezing in the winter except at higher elevations.
Winter storms drop most of their moisture as they cross the Cascade Mountains.
Rainfall averages 40 inches per year, ranging from 150 inches in the Olympic rain forest to 15 inches on the east side of the Cascade Mountains. East of the Cascades the major runoff occurs during the snowmelt period from May through July. Streamflows gradually rise over a period of a month or more and usually reach peak discharge during June. Fluctuations in streamflows are mostly caused by variations in solar radiation and air temperatures. At times, rainfall adds significantly to the runoff.