This is likely one of the most understood restrictions in Washington State, the Smokefarm Art festival does not appear to be a restricted event by definition in our RCW's but is a non-profit and likely private property and can restrict firearms if they so desire.
If you are securing your firearm in your vehicle, it should not be an issue as well.
RCW 70.108 Outdoor music festivals.
RCW 70.108.150 Firearms — Penalty.
It shall be unlawful for any person, except law enforcement officers, to carry, transport or convey, or to have in his possession or under his control any firearm while on the site of an outdoor music festival
Any person violating the provisions of this section shall be guilty of a misdemeanor and upon conviction thereof shall be punished by a fine of not less than one hundred dollars and not more than two hundred dollars or by imprisonment in the county jail for not less than ten days and not more than ninety days or by both such fine and imprisonment.
For the purposes of this chapter the following words and phrases shall have the indicated meanings:
(1) "Outdoor music festival" or "music festival" or "festival" means an assembly of persons gathered primarily for outdoor, live or recorded musical entertainment,
where the predicted attendance is two thousand persons or more and where the duration of the program is five hours or longer: PROVIDED, That this definition shall not be applied to any regularly established permanent place of worship, stadium, athletic field, arena, auditorium, coliseum, or other similar permanently established places of assembly for assemblies which do not exceed by more than two hundred fifty people the maximum seating capacity of the structure where the assembly is held: PROVIDED, FURTHER, That this definition shall not apply to government sponsored fairs held on regularly established fairgrounds nor to assemblies required to be licensed under other laws or regulations of the state.