The Mills Canyon fire remained at about 35 square miles and about 25 percent contained, fire officials said.
Temperatures climbed above 100 degrees in central Washington on Monday. But winds were mild, and firefighters kept the blaze from expanding.
"The acreage has not changed much, but we are not out of the woods yet," fire spokesman Vladimir Steblina said.
Crews have strengthened containment lines around the fire near Entiat, but officials are worried about the potential for dry lightning and wind to kick up the blaze.
The National Weather Service said more thunderstorms were expected in the area later Monday.
Residents of three dozen homes have been told to evacuate. Officials notified residents of about 500 other homes to watch fire updates and be prepared to leave if necessary.
Officials said crews did good work protecting more than 100 houses along Highway 97A. On Saturday, they were working to protect 14 homes along Roaring Creek, near the fire's northern edge.
Another priority was digging fire lines along the western edge to contain the blaze.
Crews are using about 10 aircraft to dump fire retardant or buckets of water. More than 1,000 firefighters are working on the blaze.
Monday's focus was on the southwest and northwest corners of the fire.
"They're taking advantage of the light winds we've had to go to a direct attack on the fire," fire spokesman Danny O'Connor said.
That means they're getting closer to the flames and going after hot spots, he said.
The Mills Canyon fire was the largest of a half-dozen fires burning in central and eastern Washington. The others were all less than 3 square miles, and most were contained or nearing full containment.