Sunday's official estimate put the wildfire burning in north-central Washington at more than 370 square miles, compared with 260 square miles on Friday.
The forecast for Monday and Tuesday calls for lighter winds and temperatures, said Spokane-based National Weather Service meteorologist Greg Koch.
"Overall, it looks like the weather scenario is improving," Koch said.
Then on Wednesday a "vigorous" front is expected to cover Washington, bringing rain to much of the state. But it will also bring lighting, he added.
"The benefits of the system are still up in the air," Koch said. "We may get some rain where we need it, but we may also experience some lighting that could cause some new ignitions."
Okanogan County Sheriff Frank Rogers estimates that 150 homes have been destroyed, but suspects that number could be higher. His deputies haven't been able to search parts of the county where homes are spread miles apart. No serious injuries have been reported, Rogers said.
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There are nearly 1,400 firefighters battling the flames, assisted by more than 100 fire engines, helicopters dropping buckets of water and planes spreading flame retardant.
On Sunday, Rogers was driving to the town of Twisp to survey the damage.
"It's the first time in four days I've seen blue sky," he said. "Every day, when you got up, it was nothing but smoke. All we've seen of the sun is a red ball."
Although the weather is improving, the towns in the scenic Methow Valley remain without power and have limited landline and cellphone service. Okanogan County Public Utility District officials told KREM that fully restoring power to the area could take weeks.
Gov. Jay Inslee said Friday about 50 fires were burning in Washington, which has been wracked by hot, dry weather, gusting winds and lightning. About 2,000 firefighters were working in the eastern part of the state, with about a dozen helicopters from the Department of Natural Resources and the National Guard, along with a Washington State Patrol spotter plane.
Karina Shagren, spokeswoman for the state's Military Department, said 100 National Guard troops were on standby, and up to 1,000 more in Yakima could receive additional fire training. Active-duty military could be called in as well, Inslee said.
The Washington state Department of Natural Resources announced Saturday evening that firefighters from New Mexico, Utah and Wyoming are coming to the state to help.
Early Saturday, authorities announced that they are bringing in two military air tankers from Wyoming to help fight wildfires in the Pacific Northwest and elsewhere. Officials at the National Interagency Fire Center in Boise, Idaho, said the tankers were activated to ensure that firefighters had adequate air tanker capability in the region.