Wash. Rev. Code § 36.33.070
Whenever the county treasurer deems it expedient and for the best interests of the county he or she may invest any moneys in the county current expense fund in outstanding warrants on the county tax refund fund in the following manner: When he or she has determined the amount of moneys in the county current expense fund available for investment, he or she shall call, in the order of their issuance, a sufficient number of warrants drawn on the county tax refund fund as nearly as possible equaling in amount but not exceeding the moneys to be invested, and upon presentation and surrender thereof he or she shall pay to the holders of such warrants the face amount thereof and the accrued interest thereon out of moneys in the county current expense fund.
RCW 36.33.070