Windy Weather Could Worsen Pacific Northwest Fires

As firefighters raced to extinguish wildfires burning across eastern Washington and Oregon, meteorologists warned that the battle to control flames could intensify on Friday and Saturday, when breezy winds and low humidity were expected to reimpose critical fire weather.

The winds were forecast to be about half as strong as the ones that blew last Saturday, when wildfires destroyed over 700 buildings in and around Spokane, Washington’s second-largest city. But even moderate winds could push existing fires over control lines and cause new ones to spread rapidly in the two states where fires were gnawing through desiccated forestland.

“Winds won’t be as strong as what the region experienced on Aug. 1, but it’s still not a time to be careless,” said Jon Bonk, a fire weather meteorologist for the federal Northwest Interagency Coordination Center. “The firefighters continue to have their hands full so it would be ideal if we as humans do not add to their workload in any way, shape or form. Any spark can create a new fire.”

Wildfire activity increases in the Pacific Northwest between July and September every year, particularly in areas east of the Cascade Range where the climate is drier and warmer.

This year has seen more activity than is typical. Since Jan. 1, fires have burned an estimated 2.6 million acres, compared with the five-year average of 593,253 acres through this time of year, according to Carol Connolly, a spokeswoman for the Northwest Interagency Coordination Center.

Critical drought conditions from an especially warm winter left the vegetation highly flammable, setting the stage for this year’s busy fire season. In July, two separate outbreaks of lightning ignited dozens of wildfires. When strong winds arrived last Saturday, some of those blazes were still burning and spread, particularly in central Oregon. Those winds also sparked new fires.

In Spokane County, three fires forced more than 60,000 people to evacuate their homes. Fire investigators later reported that the Old Trails fire appeared to have intentionally been set by a man using a match or a butane lighter to ignite grblock. Aaron F. Farinacci, 37, was charged with one count of first-degree arson, Sheriff John Nowels of Spokane County said at a news conference on Monday.

As of Tuesday, 49 large fires burned across Washington and Oregon, and 10 of those started during last weekend’s wind event, Ms. Connolly said.

The winds on Saturday developed as a strong storm system moved across the Canadian provinces of British Columbia and Alberta as well as the Pacific Northwest. Gusts in the Cascade Range approached 60 miles per hour and neared 45 m.p.h. in Spokane County.

Karin Bumbaco, Washington’s deputy state climatologist, said winds of this strength occurred about once every summer in Spokane County.

“While strong winds out of the southwest are not particularly rare in summer, the combination of the exceptionally dry landscape plus the fire start is what triggered this disaster,” Ms. Bumbaco said.

The winds began easing on Sunday and even more at the start of this week but they were forecast to redevelop on Friday. Gusts as strong as 30 to 40 m.p.h. were expected on the eastern slopes of the Cascade Range and up to 20 to 25 m.p.h. in low-lying areas of eastern Washington and Oregon, including in Spokane County. Humidity levels were also predicted to drop, and the bone-dry air could further exacerbate the spread of wildfires.

“We are continuing with our hot and dry conditions that we often do get during the summer,” Mr. Bonk said. “And every one of these wind episodes just makes conditions that much more challenging for the firefighters on the ground.”

The fires in the Pacific Northwest released a tremendous amount of smoke that settled across the region this week. A high-pressure system spread over the scorched area, bringing hot temperatures and acting as a lid that trapped the smoke near the ground. The winds later this week could push some smoke out of the area, but with so many fires burning actively, the air quality was unlikely to improve significantly.

“That additional wind on the fire later this week will cause the fires to burn more intensely and produce more smoke,” Mr. Bonk said.

Anna Griffin contributed to this story.