Most of our moisture comes from water evaporating from the Pacific Ocean.
As this moist air moves into the area, it encounters the mountain barrier
to our southwest and is forced to move upward along the western slopes
of the Olympic Mountains. As the air moves upward, it begins to cool and
condense, forming clouds. As it continues to rise, the moisture condenses
even more until precipitation begins to fall. By the time the air reaches
the top of the mountains, much of the moisture has already been squeezed
out of it, before it ever begins its descent into the San Juan Islands .
That's the reason there are vast rain forests on the southwestern
side of the Olympics. They receive over 200 inches of rain a year.
To add to this effect, as the air moves down the North Eastern side
of the Olympic Mountains, it warms up and dries out even more .
By the time this air reaches the San Juan Islands, it is usually so warm
and dry that we seldom get the rain and snow that our neighbors to the
southwest enjoy, leaving us in the Rain Shadow.
The Olympic Mountains cover a relatively small area
so there is only a small area that gets the better weather.