Rainshadow Effect

Mount Olympus and Sequim are only 35 miles apart, but Mount Olympus typically receives well over 200 inches of precipitation whereas Sequim receives less than 20 inches, the driest area of western Washington.

This “rainshadow effect” is a dominant feature of the climate of the northeast corner of the Olympic Peninsula, extending across to the San Juans and Whidbey Islands.  Large Pacific weather systems typically move onshore from the west-southwest, dropping copious amounts of rain and snow as they rise and cool over the Olympics, then warm up and actually evaporate moisture as they descend in the lee of the mountains.  This makes the Sequim area much drier, allowing different plant communities and species to occur here.   

Average yearly precipitation for local sites clearly shows these differences:

Sequim: 16.3”                                             

Port Angeles: 25.3”                                    

Elwha Ranger Stn: 55.8”

Forks: 108”

Quilcene (northern Hood Canal): 54.2”

Cushman Dam (above Hoodsport): 94.5”

Aberdeen (Grays Harbor): 79.5”

East Orcas Island (San Juans): 28.7”

Seattle: 37.2”