Wash. Rev. Code § 80.01.035
When a commissioner has heard all or a substantial part of an adjudicative proceeding and leaves office before entry of a final order in the proceeding, at the request of the remaining commissioners the commissioner leaving office may be appointed by the governor as commissioner pro tempore to complete the proceeding. A proceeding is completed when the commission enters a final order purporting to resolve all contested issues therein, from which no party seeks clarification or reconsideration, or upon entry of an order on clarification or reconsideration, even though the order is subject to a petition for judicial review. A commissioner pro tempore shall receive a reasonable compensation to be fixed by the remaining members of the commission.
RCW 80.01.035