A massive, fast-moving wildfire has destroyed at least 24 homes and threatens another 5,000 in a drought-stricken area about 100 miles north of San Francisco, authorities said Sunday.

The so-called Rocky Fire was only about 5% contained by midday Sunday. It was just one of 21 large wildfires burning statewide, fire officials said.

Fueled by triple-digit temperatures, single-digit humidity and erratic winds, the Rocky Fire more than doubled in size overnight, from just over 20,000 acres on Saturday to 47,000 acres on Sunday, said the California Department of Forestry and Fire Protection, known as CalFire.

“The grass, the brush, the trees, they are tinder-dry,” said CalFire spokesman Daniel Berlant. More than 12,000 people had been evacuated from the area around Clearlake, Calif., located about 100 miles north of San Francisco and 100 miles northwest of Sacramento. The fire originated Wednesday afternoon. By Sunday, fire crews were assisted by four C-130 air tankers based in Colorado.

California Gov. Jerry Brown declared a state of emergency and activated the California National Guard to help with disaster recovery.

Many of the California blazes were sparked by lightning, and more than 8,000 firefighters were working to battle them. One firefighter was killed on Thursday at the scene of a fire 100 miles south of the Oregon border. Engine Capt. David Ruhl, a South Dakota firefighter, was killed battling a fast-moving blaze that broke out in the Modoc National Forest. He was in a vehicle looking for ways to fight the blaze when officials lost contact with him. His body was recovered Friday.

Part of a Black Hills National Forest firefighting team, Ruhl had been helping California firefighters since June.

Contributing: KXTV-TV, Sacramento; The Associated Press