A series of fires burning in California's Santa Cruz and San Mateo counties, called the CZU August Lightning Complex, exploded into a monstrous inferno overnight.

The fire was 25,000 acres late Wednesday, but on Thursday Cal Fire officials said at a 6 a.m. press briefing it had grown to 40,000 acres with no containment. Twenty structures have been destroyed and 8,600 are threatened.

Cal Fire Operations Section Chief Mark Brunton said there was significant fire activity west of Davenport in Santa Cruz County overnight. "We had to do an evacuation of almost last minute due to the rapid approach of the fire," he said.

The fire also made an "extremely active run" from the west side of Empire Ridge Road, spotting over the highway and moving down into the communities of Boulder Creek and Ben Lomond. "Fires continue to threaten those communities with fire moving down into the San Lorenzo Valley," Brunton said.

The blaze is racing through a highly flammable landscape that is parched after a year marked by low rainfall. It hasn't seen a fire in years.

"It's so dry it's something we have not seen historically," said Brunton. "We're seeing fire we've never seen in the coastal area before, in terms of amount and severity.”

In San Mateo County, fire is threatening the communities of Pescadero and La Honda.

Brunton said when he was talking to firefighters on the scene, they all shook their heads and said, "We've never seen anything like this.”

Cal Fire is fighting the CZU Complex with about 10 percent t of the resources it would usually have for a fire of this size and severity. Brunton said due to the high number of fires spread across the state, they "are woefully short of resources." More than 500 firefighters are assigned to the incident.

An arm of the blaze raced through Big Basin Redwoods State Park on Tuesday, and officials said Wednesday night that multiple structures were destroyed, including the park’s headquarters, historic core and campgrounds.

More than 20,000 people are under mandatory evacuation and Brunton said that number is expected to double in the next 30 hours as the fire expands rapidly.

Due to the heavy smoke, Cal Fire has been unable to operate complete aircraft operations, and they don't expect these conditions to improve for 48 to 72 hours. The focus now is on protecting "life and property."

The lack of aircraft over the fire also means that accurate mapping of the blaze is lacking.

The CZU August Lightning Complex is named for the blazes that were sparked amid the rash of thunderstorms that moved across California early this week.