Wash. Rev. Code § 84.56.230
On the first day of each month the county treasurer shall distribute pro rata to those taxing districts for which the county treasurer also serves as the district treasurer, according to the rate of levy for each fund, the amount collected as consolidated tax during the preceding month: PROVIDED, HOWEVER, That the county treasurer, at his or her option, may distribute the total amount of such taxes collected according to the ratio that the levy of taxes made for each taxing district in the county bears to such total amount collected. On or before the tenth day of each month the county treasurer shall remit to the respective city treasurers and all other taxing districts for which the county treasurer does not serve as district treasurer, their pro rata share of all taxes collected for the previous month as provided for in RCW 36.29.110.
RCW 84.56.230