The western half of the state is close to the ocean with warm ocean currents, so you get a lot of moisture. However the Cascade Mountains split the state N and S. Windward side is Seattle side, so all the rain stays there.
Eastern Washington then is in the rain shadow. Actually I think it's high desert - there's very little annual cloud cover.
The Cascades go all the way to California so this pattern applies to Washington, Oregon, and maybe Nevada.
In the context of my original comment, I can only speak from an experiential sense. I think it's the strength or frequency of the sunlight, maybe the UV, is what makes the difference.
We have this same pattern in Vancouver and a good chunk of British Columbia as well, though this winter which just ended was the mildest of the 11 years I've been here
Eastern Washington then is in the rain shadow. Actually I think it's high desert - there's very little annual cloud cover.
The Cascades go all the way to California so this pattern applies to Washington, Oregon, and maybe Nevada.
In the context of my original comment, I can only speak from an experiential sense. I think it's the strength or frequency of the sunlight, maybe the UV, is what makes the difference.