A massive wildfire in Mariposa County nearly doubled in size overnight as flames destroyed structures, threatened power to Yosemite National Park and forced 4,000 people to flee their homes.

The fast-growing Detwiler fire, which is burning west of Yosemite, exploded from 25,000 acres to 45,724 acres, the California Department of Forestry and Fire Protection said Wednesday.

Flames were fueled by tall grass and overgrown shrubs that sprouted along the central Sierra Nevada foothills during the winter rains, said Jordan Motta, a fire captain and Cal Fire spokesman.

The rich fuel source has created fire activity that “we haven’t seen in the last seven or eight years,” he said.

On Tuesday, Gov. Jerry Brown declared a state of emergency in the area after the fire forced thousands to evacuate and damaged power, water and communications infrastructure.

The governor’s order sends additional equipment and fire crews to Mariposa County to help fight the blaze, which is only 7% contained. The declaration also accelerates emergency aid to those affected by the fire.

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As the blaze spread rapidly across thousands of acres Tuesday, the Mariposa County Sheriff’s Office issued evacuation orders to residents in the town of Mariposa, east of Merced.

The fire was burning about three-quarters of a mile from Mariposa, Motta said.

Outside an evacuation center at Oakhurst Evangelical Free Church, Barbara Milazzo, 53, and her daughter Jessica, 15, sat in their minivan in the parking lot Wednesday afternoon, using a quilt hung over the car door to shield them from the sun. Last night, they stayed in a small wooden trailer attached to the back of the van, equipped with a straw mattress, blankets and pillows.

Milazzo spent the day listening to the radio for updates, and texting and calling her friends, some of whom she said have lost property in the fire.

When Milazzo first heard about the fire Monday, she drove to her friend’s home in Hornitos to “fire-sit.” The next day, she saw a plume of black smoke erupt over the hills, and she began packing up her friend’s house, before rushing to her own home in Mariposa, where an evacuation advisory was in effect.

With the power out at her house, Milazzo, a teacher, decided to spend the night in the evacuation center.

“The unknown is the biggest stress,” she said. “It’s a small town’ we all know each other. We just don’t know how their houses are.”

Nearby highways and roads were closed as more than 2,200 firefighters tackled flames and faced “extreme and aggressive fire behavior,” Cal Fire said.

Embers from the blaze sparked spot fires, and entire trees were engulfed in flames, the fire agency reported.

The blaze, which started Sunday east of Lake McClure, has destroyed eight structures, damaged one and is threatening an additional 1,500.

The communities of Hunters Valley, Bear Valley, Catheys Valley, Mormon Bar, the town of Mariposa, Mount Bullion, the Yaqui Gulch/Agua Fria areas and Hornitos continue to be threatened, Cal Fire said. “The fire encroaches on culturally and historically sensitive areas,” the agency said.

To the south of the fire, flames continued to threaten power lines that supply Yosemite, Cal Fire said.

On Tuesday, the blaze knocked out power to Yosemite for several hours, but crews were able to restore service at 11 p.m., said Denny Boyles, spokesman for Pacific Gas & Electric Co. It is unclear whether the fire damaged the park’s power source.

The park’s main source of power comes from a 70,000-volt transmission line, which is not in the fire’s path, Boyles said. But that could change if the fire shifts direction.

Mobile generators were positioned near the park as a precaution.

“It’s a fluid situation with the fire still burning,” Boyles said.

About 8,500 customers along the Sierra foothills were without power as crews worked to repair lines damaged by the fire, he said. Power lines also were de-energized to protect fire crews battling the blaze.

A massive wildfire in Mariposa County nearly doubled in size overnight as flames destroyed structures, threatened power to Yosemite National Park and forced 4,000 people to flee their homes. (KTLA)

Smoke from the fire could be seen from a weather satellite, according to the National Weather Service.

Along with dry, breezy and warm conditions that are dominating Central California this week, forecasters say firefighters also must contend with overgrown vegetation.

“It is the fuels that are extremely flammable right now due to heavy rains this winter with widespread growth and then extended heat waves this summer, which has created a powder keg for fast-burning fuels,” the weather service said in a statement. “Even terrain-driven winds can become stronger depending on fire behavior and fuels.”

As the explosive fire snaked through hills and canyons, conditions on the ground and around the blaze became so unpredictable that a large pyrocumulus cloud has formed. The fire cloud develops when a blaze is so hot that it can create its own environment, said meteorologist Jim Andersen with the weather service in Hanford.

“Anytime you see a fire with a pyrocumulus, you know the fire is really roaring,” he said. “It takes an insane amount of heat.”

Weather conditions below a fire cloud can be impulsive and “very dangerous” as winds shift and swirl around, Andersen said.

As firefighters gain control of the blaze, he said, the cloud eventually will fade.

According to the fire’s incident management team, the blaze is burning in an area where there are many dead trees killed by bark beetle and the drought.

Firefighters were aiming to have the conflagration controlled by July 23, said Motta, the Cal Fire spokesman.

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veronica.rocha@latimes.com

Twitter: VeronicaRochaLA

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UPDATES:

2:45 p.m.: This article was updated with comments from residents and details about a fire cloud over the area.

1:20 p.m.: This article was updated with details about a power outage at Yosemite.

10:30 a.m.: This article was updated with additional details from Motta about the fire.

This article was originally published at 7:45 a.m.