In Grays Harbor and Pacific Counties the wet season begins in October, reaching a peak in winter, then gradually decreasing in the spring. December and January are the wettest months, with rainfall often recorded on 20 to 25 days or more days each month. While heavier rainfall intensities occur along the windward slopes of the Olympic Mountains and Willapa Hills, in most of our area rainfall is usually of light to moderate intensity and continuous over a period of time, rather than sudden, heavy downpours experienced at the higher elevations.
During the spring and summer months dry, cool and stable air moves into the Pacific Northwest, becoming warmer and drier as it moves inland from the Pacific Ocean. This results in a dry season beginning in the late spring and reaching a peak in mid-summer. During July and August, the driest months, it is not unusual for two to four weeks to pass with only a few showers.
The “rainforest” area along the southwestern and western slopes of the Olympic Mountains receives the heaviest precipitation in the continental United States. Annual precipitation ranges from 70 to 100 inches over the Coastal Plains to 150 inches or more along the windward slopes of the mountains. The greatest annual precipitation recorded in the “rainforest” area is 184 inches at Wynoochee Oxbow, elevation 600 feet. The heaviest rainfall during a single storm was 12 inches in 24 hours; 23.5 inches in 48 hours; 28.6 inches in 72 hours; and 35 inches in four days recorded at Quinault Ranger Station, January 21-24, 1935. On Blue Glacier, elevation 6,900 feet and near the summit of Mt. Olympus, 149 inches of precipitation were recorded between August 1957 and July 1958. The total snowfall for this period was 542 inches.
Winter season snowfall ranges from 10 to 30 inches in the lower elevations and between 250 to 500 inches in the higher mountains. In the lower elevations, snow melts rather quickly and depths seldom exceed six to 15 inches. In midwinter, the snowline in the Olympic Mountains and the Willapa Hills is between 1,500 and 3,000 feet above sea level. The higher ridges are covered with snow from November until June.
There are two periods of high flow in the streams of western Washington. One occurs during the winter months, coinciding with the periods of maximum precipitation, and the other in the spring or early summer caused by the seasonal rise in temperature with the resultant melting of snow accumulations in the higher elevations augmented at times by rainfall. During the periods of high water flow and heavy rains, rivers and streams may overflow their banks and inundate low-lying areas, and ground water levels may rise, saturating the soil and endangering water-sensitive plants. The following table provides historic rainfall information for some of the communities in Grays Harbor and Pacific Counties.
Average Precipitation from 1931 to 2006 (in.)