A huge wildfire near Yosemite National Park swelled to more than 70,000 acres overnight as flames destroyed at least 45 homes and forced two towns to evacuate.

Burning east of Merced, the Detwiler fire in Mariposa County grew from 48,000 acres to 70,096 acres Thursday morning, said Nancy Phillipe, spokeswoman for the California Department Forestry and Fire Protection.

The aggressive blaze, which was 10% contained, has forced more than 4,000 residents to flee their homes and triggered evacuations for the historic Gold Rush towns of Mariposa and Coulterville. The massive fire has destroyed 45 homes, according to Cal Fire Incident Commander Todd Derum, and firefighters expect to find even more gutted houses in the coming days. At least 1,500 more residences remain threatened.

More than 3,100 firefighters tackled 2- to 4-foot flames, and observed some flares up to 25 feet tall, said Jeff Marshall, a fire behavior analyst with the Cal Fire Incident Management.

As the conflagration moves east, he said, flames are encroaching on areas where trees have been killed off by bark beetles.

Weather conditions in the fire zone will do little to assist firefighters. Winds driven by the rugged, steep terrain and low humidity “will create favorable conditions for continued fire growth,” the National Weather Service in Hanford, Calif., said in a statement.

Weather satellites spotted “explosive fire behavior” overnight near Lake McClure, the weather service said.

Smoke from the wildfire, which could be detected as far away as Idaho, continued to affect the Sierra Nevada, Yosemite and foothill communities, according to the weather service.

When the inferno erupted Sunday afternoon in the Hunters Valley area, it “established itself very rapidly” and immediately threatened residents there, said Mariposa County Sheriff Doug Binnewies.

Access to the small, tightknit community was limited because the only road in and out is “recognized to be hazardous for the residents,” the sheriff told a town meeting Wednesday night.

As the rapid flames moved through the area, residents were evacuated, and several homes were destroyed, said Binnewies, who also is the county’s interim fire chief.

“It’s no question that if the property owners had not reacted to the evacuation order, they and their loved ones would have been in harm’s way,” he said.

With little time to gather their belongings, homeowners were forced to leave behind pets and livestock. Binnewies said he had a list of hundreds of horses, donkeys, pigs, cats, dogs, exotic birds and other animals that have been found.

“This was for life protection and those are hard decisions, and you have to grab what’s around you,” he said.

Heidi Falany, 31, of Fresno, told The Times that her husband’s grandparents’ home along rural Hunters Valley Road in Mariposa appeared to have been one of the first houses destroyed by the flames.

She said her husband’s late grandfather, Joe “Papa Joe” Falany — a World War II Navy veteran who survived his ship being hit by kamikaze pilots in the Philippines — built the hacienda-style house with a red-tiled roof on the family’s Casa de Falany Ranch in the 1970s.

His 90-year-old widow still lived in the home. Heidi Falany said the family had not broken the news about the house to her.

“My husband was just in shock, just like me,” Falany said. “But we’re both Christians, and we believe it’s just possessions and we have our family still; we’re thankful that it was just the house.”

People in the area are accustomed to wildfires, she said. Residents watch out for flames and regularly say they’re praying for each other. But this year, things were different.

“We’ve had such significant rain this year,” she said. “All the brush that grew up from the rain has created this perfect storm for the fire.”

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Fueled by overgrown vegetation and dry, breezy and hot conditions, the blaze progressed into neighboring communities, said Binnewies, the sheriff.

“The fire was very deliberate continuing toward Mariposa town proper,” he said.

In his 30 years’ experience, Binnewies said, it is the first time he has seen the entire town evacuated.

Not everyone has left, however.

Mariposa County Sheriff's spokeswoman Kristie Mitchell said officials decided that inmates at the county jail were in an area considered safe enough to remain and shelter in place. The small jail sits across the Cal Fire headquarters near Frank Wilson Road and Highway 140.

Lt. Codie Hart, who oversees the jail facility, said a bulldozer cut a line in the ground to prevent the fire — which is about 50 yards away — from approaching the facility.

On Thursday morning, a firetruck was parked near the driveway. Firefighters said they were standing by before heading back to base camp.

Officials also decided not to evacuate John C. Fremont hospital, a long, white-and-green building with a helipad, just off Smith Road. The one-story facility is the only hospital in Mariposa County and the only one with an emergency department.

“They said we have a defensible space that they felt we could shelter in place,” said Theresa Loya, the chief nursing officer for the hospital.

Loya said fire helicopters were constantly making water drops, defending not only the hospital but the rest of Mariposa County as well. Some three dozen nurses remained at the hospital. Even when the facility lost power, generators kept electricity flowing.

In downtown Mariposa, a haze of smoke settled over the town Thursday. Business owners were allowed past roadblocks Thursday to briefly check on their stores, said Officer Michael Flores of the California Highway Patrol.

Some businesses remained open despite evacuation orders.

Brian Redding, 31, a cashier at Grizzly Gas’ mini market, said he hadn’t seen much business that morning, except for a few firefighters who had stopped by to grab snacks.

“It’s pretty slow. Kind of like a ghost town,” he said.

The fire has triggered multiple road closures along highways 49 and 140, which fire officials said have impacted access to Yosemite. The park has remained open as the firefight continues near Mariposa.

Denny Boyles, spokesman for Pacific Gas & Electric Co., said the blaze briefly knocked out power to Yosemite on Tuesday.

Thousands in the region remained without power Thursday as crews worked to replace 29 transmission poles and more than 200 distribution poles that were damaged by the fire.

The cause of the fire is still under investigation.

For many area residents, the Detwiler blaze has stirred memories of the explosive Telegraph fire, which scorched more than 34,000 acres and destroyed 30 homes in 2008. That fire was sparked by target shooting near the area of Sherlock and Telegraph roads in the small town of Midpines, according to Cal Fire.

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veronica.rocha@latimes.com

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UPDATES:

2:20 p.m.: This article was updated with comments from Cal Fire’s Todd Derum.

11:55 a.m.: This article was updated with comments from Mariposa residents.

8:55 a.m.: This article was updated with additional details about the Telegraph fire.

This article was originally published at 8:10 a.m.