Columbia Valley AVA
At least 90% of Washington wine comes from the Columbia Valley AVA which cuts a large swatch through the middle of the state, even entering partially into Oregon, after all the dividing line between the states is the Columbia River.
When you purchase a Washington wine, it almost always will say it was produced in the Columbia Valley AVA, it's a nice marketing piece for the wineries in the state, but isn't especially helpful for consumers.
It's important to note here that every AVA in Washington is part of Columbia Valley other than Puget Sound, meaning that the dark, cloudy and rainy section of the state which includes Seattle is not included here. Many people don't realize that a large part of the state is sun drenched and if you found yourself there unexpectedly, you might assume that you were sitting somewhere along the California central coast or even somewhere in the Rhone region of France.