Massive wildfires in forest and scrub in western US states have created so much shade from their smoke they have blocked enough sunlight to create cooler more stable conditions, helping the army of firefighters working on them to gain ground, officials say.
Both the National Weather Service and officials with the Oregon Department of Forestry said smoke in the lower atmosphere from California wildfires had floated north over the country's biggest fire, the Oregon Bootleg fire.
The blaze, about 400km south of Portland, was started on 6 July by lightening and has scorched more than 162,000 hectares.
"It's called 'smoke shading' and it's basically put a lid on the lower atmosphere for now, blocking sunlight and creating cooler, more stable surface conditions," said National Weather Service (NWS) meteorologist Eric Schoening.
The phenomenon is unpredictable and the area is still under red-flag warnings this weekend from the NWS, which said the Pacific Northwest may experience high temperatures and wind gusts that can fan the flames and spread hot sparks and embers.
Schoening said the weather would be a "mixed bag" for firefighters.
Oregon forestry department spokesman Marcus Kauffman said the drawback of the "smoke shade" was that it makes it harder to fly planes and helicopters to drop water and chemical fire suppressants, even "while it helps the teams on the ground."
More than 2000 firefighters and support crew had contained about 42 percent of the fire by Saturday, although the fire jumped containment lines the night before, he said.
"We lost 1600 acres [about 650ha] last night," Kauffman said.
The Bootleg fire is one of more than 80 large active wildfires in 13 states that have charred about 526,090ha in recent weeks, an area larger than the state of Delaware, according to the National Interagency Fire Center.
The smoke, even as it provides some help to Oregon firefighters, has recently been carried by the jet stream and other air currents as far as the Northeastern cities of New York and Boston, where some residents have felt the air contamination in their eyes, noses and lungs.