Possible dry lightning strikes, high temperatures and possible gusty winds were all in the forecast as crews fought the blaze for a sixth day in bone-dry conditions.
Meanwhile, many residents who live on the south and east sides of the 35-square mile blaze near Cle Elum returned home to find out whether their homes were spared.
The so-called Taylor Bridge Fire has burned out of control for much of the week, destroying dozens of residential properties and hundreds of other structures on the east side of the Cascades.
"People are finding a little bit of everything. Some homes were damaged, some homes were destroyed and some homes weren't even touched," Fred Slyfield, emergency management specialist for Kittitas County, said Saturday morning.
About 900 firefighters and eight helicopters were still building a line around the fire, which started Monday at a bridge construction project and exploded through dry grass, brush and trees. More than 400 people fled their homes. About 30 people are in local shelters, Slyfield said.
On Saturday, crews were mostly focusing their efforts on strengthening the northern flank of the fire between Highway 97 and the intersection of Highway 970 and Highway 10.
The blaze was about 40 percent contained on Saturday morning, and crews hoped to have it completely contained by Monday, officials said.
Fire danger remained high in the area, with hot, dry weather and a chance for storms and lightning expected Saturday evening.
"We're kind of on edge about that," said Mick Mueller, a spokesman at the fire command center.
Fire officials are on alert and crews are ready to respond quickly should lightning and winds trigger new fires in the area late Saturday, fire spokesman Matt Comisky said Saturday morning.
A spot fire that flared up outside the fire lines Friday was brought under control by two structure protection strike teams using helicopters and aircraft with flame retardant. That blaze was confined to approximately 100 acres.
Meanwhile, many residents were in the area were under orders to be ready to evacuate at a moment's notice.
A Red Flag Warning continues Saturday with very hot, dry and unstable conditions. With temperatures reaching 90 to 100 degrees, fire activity is expected to increase and significant threat to structures remains a concern.