Officials said crews had carved buffer lines around a trio of fires threatening Spokane, the state's second-most populous city.
The blazes have destroyed hundreds of buildings and forced tens of thousands of people from their homes since Saturday.
However, full containment, in which a wide corridor of ground has been scraped clean of combustible vegetation around a fire's perimeter, had yet to be achieved, authorities said.
"With primary containment lines now in place, crews are focused on strengthening those lines through aggressive mop-up and extinguishing pockets of heat near homes and other structures to reduce the risk of additional damage," the Spokane-area fire command said on Wednesday.
The Spokane fires were ranked as the No.1 firefighting priority in the US as a whole, including the drought-parched Pacific northwest, where dozens of major wildfires have burned for weeks, fouling air quality across a wide region with smoke and soot.
A total of 94 major wildfires were burning in 13 US states, most of them in the west, and more than half of those in Oregon and Washington, according to the National Interagency Fire Center in Boise.
A local resident, Aaron Farinacci, 37, who had served prison time on a manslaughter conviction for killing his father in Arizona, was arrested on Tuesday and charged with arson in connection with one of the three Spokane-area fires.
The cause of the two others remains under investigation.
The Spokane-area fires have scorched more than 4047 hectares on the northern fringe of the city, home to about 230,000 residents.
At the height of the conflagration, stoked by high winds and unusually hot, arid conditions, as many as 64,000 people were placed under evacuation orders.
At least 700 structures went up in flames, the bulk of them homes, and another 400 buildings might have been heavily damaged or destroyed, the fire command said.
No casualties had been reported, and as of Wednesday, all 14 people initially reported as missing had been accounted for as safe.
Cooler conditions and calmer winds on Monday and Tuesday helped ground crews and firefighting aircraft to make significant strides in curtailing the blazes.
"We're cautiously optimistic about how things are looking, with the knowledge that this weekend is going to test things," fire command spokesman Benjamin Cossel told Reuters.
Forecasts call for temperatures to climb above 30C, with low humidity.
Conditions propelling heightened wildfire activity in North America, Europe and elsewhere in recent years are attributed by scientists to climate change, mostly a function of heat-trapping gases emitted from the burning of fossil fuels.
