SALEM, Ore. — Hundreds of firefighters battled two large wildfires Friday that threatened to merge near the most populated part of Oregon, including the suburbs of Portland, and the governor said dozens of people are missing in other parts of the state.

The state’s emergency management director, Andrew Phelps, said officials are “preparing for a mass fatality event” and that thousands of structures have been destroyed.

Gov. Kate Brown said more than 40,000 Oregonians have been evacuated and about 500,000 are in different levels of evacuation zones, either having been told to leave or to prepare to do so. She was dialing back on a statement late Thursday issued by the state Office of Emergency Management that said a half-million people had been ordered to evacuate statewide.

Dozens of people are missing in Jackson County in the south and Marion County, where a fire continues to burn east of Salem, Brown told a news conference Friday.

The Oregon Convention Center in Portland was among the buildings being transformed into shelters for evacuees. Portland, shrouded in smoke from the fires, on Friday had the worst air quality of the world’s major cities, according to IQAir.

A change in the weather, with winds dropping and shifting direction and humidity rising, greatly helped firefighters struggling to prevent two fires — one burning southeast of Portland and the other east of Salem, the state capital — from advancing farther west into more-populated areas.

“The wind laid down quite a bit for us yesterday. There also wasn’t that strong eastern wind that was pushing the fire more to the west,” said Stefan Myers of the state’s fire information team.

Winds coming from the Pacific Ocean also neutralized the fires’ advance and even pushed them back, Myers said.

Almost 500 personnel were working on the fires, which were just a few miles (kilometers) apart, with rugged terrain between them that limits boots-on-the-ground efforts to keep them apart, Myers said. If they merge, they could generate such heat that it causes embers to fly thousands of feet into the air, potentially igniting other areas, Myers said.

The high number of fires occurring simultaneously in the span of just a few days in Oregon was fueled by dry conditions, high temperatures and especially strong, swirling winds.

Brown said Thursday that more than 1,400 square miles (3,600 square kilometers) have burned in Oregon over the past three days, nearly double the land that burns in a typical year in the state and an area greater than the size of Rhode Island.

Oregon officials haven’t released an exact death count for the wildfires, but at least four fatalities have been reported in the state. One person was killed in wildfires in Washington.

A Northern California fire that tore through several hamlets in the foothills of the Sierra Nevada this week killed 10 people, making it the deadliest of the year. The North Complex fire near Oroville that exploded in wind-driven flames earlier in the week was advancing more slowly Friday after winds eased and smoke from the blaze shaded the area and lowered the temperature.

In Oregon, one fire approached Molalla, triggering a mandatory evacuation order for the community of about 9,000 located 30 miles (48 kilometers) south of Portland. A police car rolled through the streets with a loudspeaker blaring “evacuate now.”

With the two large fires — called the Beachie Fire and the Riverside Fire — threatening to merge, some firefighters in Clackamas County, which encompasses Molalla, were told to disengage temporarily Thursday because of the danger. Officials tried to reassure residents who abandoned their homes and law enforcement officials said police patrols would be stepped up to prevent looting.

The local fire department said on Twitter: “To be clear, your firefighters are still working hard on the wildfires in Clackamas County. They are taking a ‘tactical pause’ to allow firefighters to reposition, get accountability & evaluate extreme fire conditions.”

“We haven’t abandoned you,” the fire officials said.

The change in weather also aided efforts to contain a fire near Lincoln City, on the Oregon Coast.

“Thank God, we got a wind shift. The wind started coming from the west, pushing the fire back towards the east, and that’s what kept it within its footprint and kept it from growing,” fire spokesperson Ashley Lertora said.

Oregon’s congressional delegation announced Friday that the White House has approved the state’s request for an emergency declaration that will help provide immediate assistance from the Federal Emergency Management Agency.

“As this emergency continues, it’s critical that the federal government do everything in its power to help,” Democratic U.S. Sen. Jeff Merkley said.

In southern Oregon near the California state line, much of the small town of Phoenix was wiped out. A mobile home park, houses and businesses were burned, leaving twisted remains on charred ground.

Many of the residents were immigrants, with few resources to draw on.

Artemio Guterrez stood helplessly next to his pickup, surveying the rubble of his mobile home. His children sat quietly in the truck bed and waited for him to salvage what he could. He found a ceramic pot with a smiley face on it, some charred miniature houses from a Christmas-themed village and a cross that formed when two pieces of glass melted together.

Guterrez, a single father of four, had been at work at a vineyard nearby when he saw thick smoke spreading through Rogue River Valley. He raced home just in time to snatch his kids from the trailer park where they live alongside dozens of other Mexican families. They got out with only the clothes they were wearing.

“I’m going to start all over again. It’s not easy but it’s not impossible either. You have to be a little tough in situations like this,” said Guterrez, who had just returned from his mother’s funeral in Mexico.

Entire mobile home parks with many units occupied by Mexican immigrants who worked in nearby vineyards or doing construction were reduced to ash in Phoenix and nearby Talent.

“We’re kind of like a family. We’ve known each other for years, since we came here or even before then,” Guterrez said of his neighbors at Talent Mobile Estates. “We’re living day by day.”