When you first get your deck boards, you should let them sit for several weeks before installing them. This process allows the moisture content of the wood to equalize with that of the surrounding air, and is called acclimation.
When acclimating your decking, store the lumber in an open area similar to the final installation site. Unless you plan on keeping your Washington deck under a tarp 24/7, do not cover the boards. Stack them outdoors, with spacers between each layer so that the air can flow freely along every side of every board.
Failing to properly acclimate your decking will cause the boards to warp, crack, and buckle after installation. These problems are caused when the wood absorbs moisture from or releases moisture into the air, causing the wood to shrink or expand. By letting the deck boards acclimate and get used to your local climate, you can avoid all of these issues.