Open carry is not expressly forbidden, but it is unclear. The law states that you cannot carry in a way that warrants alarm. Now if there was ever a law open to interpretation, that is one. Wether or not you will be harassed depends on where in the state you are. In rural areas you will probably fair better. In Seattle and the other cities, not so well.
On the other hand, Washington has a very fortunate law that says you DO NOT NEED a permit to carry concealed if you are in route to, in route from, or actively participating in a legitimate outdoor activity. Not too bad. Getting your permit is easy though, and allows you to buy handguns without waiting the usual week.
Hope that helps. Welcome to Washington!