The Mills Canyon Fire, burning near Entiat (EHN'-tee-aht), was 25 percent contained on Sunday, said spokesman Vladimir Steblina of the Pacific Northwest Incident Management Team.
Residents of several dozen homes have been told to evacuate. People living in another 500 homes have been warned to be prepared to leave, if the fire gets closer.
The National Weather Service says thunderstorms with dry lightning were expected in the area around 8 p.m.
"When the winds start swirling around, you get really erratic winds," Steblina said. "We might end up with extreme fire growth around the thunder cells."
About 781 firefighters, assisted by eight helicopters were battling the fire on Sunday, concentrating on the north and south lines.
"The day has gone as planned," said fire spokeswoman Laurie Dowie on Sunday afternoon. Fire lines on both the south and north ends of the fire were strengthened.
No people have been injured in the fire, but three small sheds or outbuildings have been burned, Steblina said.
In addition to the blaze, firefighters are battling extreme heat. The high in nearby Wenatchee reached 106 degrees on Saturday. Fire officials were expecting temperatures above 90 again on Sunday.
"People are paying a lot of attention to being hydrated," Steblina said, noting that the only medical issues on the fire have been a few cases of poison ivy.