The Spokane Area Fire, which contains three separate blazes, was threatening the areas under evacuation orders, Spokane Fire Chief Tom Williams told Reuters.
The fires had burned 5,390 acres (2,180ha) by Sunday, according to a federal incident management team.
Williams said the fires had yet to be contained.
No injuries or deaths had been reported, officials said at a Sunday afternoon news conference.
Spokane Mayor Lisa Brown called it "the worst natural disaster our region has faced".
Avista Utilities chief executive officer Heather Rosentrader said around 10,000 electricity customers in the region had no power as of Sunday afternoon.
State officials have said that more than 101,171ha are burning across Washington State.
Washington Governor Bob Ferguson said he had spoken with US President Donald Trump on Sunday and requested assistance from the Federal Emergency Management Agency. The governor has declared a statewide emergency.