SCOTTS VALLEY – The CZU Lightning Complex Fire burning in the mountains of Santa Cruz and San Mateo counties swelled to 48,000 acres Thursday, leading authorities to completely evacuate the communities of Felton, Zayante and Scotts Valley.

“It’s an historic event,” said Ian Larkin, chief of CalFire’s San Mateo-Santa Cruz Unit. “We have not seen fires burn like this in either one of these counties for many, many years. And those fires were much smaller than what we have in front of us today.”

Crews saw a growth of 700 to 1,000 acres an hour in heavy timber, said CalFire incident commander Billy See.

“That’s a dangerous rate of spread for our firefighters and for all those residents out there,” See said.

“We need to be cognizant that we’re in historic times in this fire that we’re dealing with here on the coastline,” he continued. “All the residents here in San Mateo County and Santa Cruz County need to be aware of that, pay attention to the social media and pay attention to the orders and warnings coming out. It’s a dangerous situation for everyone involved.”

Just shy of 1,000 personnel have been assigned to the fire, but “it’s still not enough,” See said.

As of 6 p.m., the fire was zero percent contained. The fire is threatening 20,952 structures and has destroyed at least 50 structures. Nearly 50,000 people have been evacuated.

“We’re trying to put the few resources we have in the most advantageous, strategic areas as possible to suppress those fires and protect those structures,” said CalFire Operations Chief Mark Brunton.

Authorities urged residents to stay out of the evacuation zones.

“I want to make sure the citizens understand that in order for us to do our job we need to make sure they are safe,” Larkin said. “Get out of the area. … We’ll do our best to protect your property, but most importantly we want to make sure you are protected.”