OROVILLE — Firefighters lined roads to keep flames from reaching homes as helicopters dropped water on a growing wildfire Wednesday in Northern California that has forced at least 13,000 people to evacuate, with another day of extreme heat expected.

The Thompson fire broke out before noon Tuesday about 70 miles (110 kilometers) north of Sacramento, in and around the city of Oroville, in Butte County. It sent up a huge plume of smoke and grew to nearly 4.7 square miles (12.1 square kilometers) by Wednesday morning. It was zero percent contained.

A state of emergency was declared Tuesday night by the city of about 20,000 people along the Feather River at the foot of the Sierra Nevada. Evacuation centers were set up at a church and the county fairgrounds.

There was no immediate official report on property losses. An Associated Press photographer saw fire burn three adjacent suburban-style homes in Oroville.

Related Story: PG&E Warns of Possible Outages in Extreme Fresno Heat Wave

Fire Ignites Grass and Whips Up Flags

The fire ignited sprigs of grass poking from the concrete edges of Lake Oroville as gusty winds whipped up American flags lining a bend of the state’s second largest reservoir and the nation’s tallest dam.

Residents stood on hillsides in the night, watching the orange glow, as aircraft made water drops to keep the fire from spreading. A crew of more than a dozen firefighters saved one home as goats and other farm animals ran to find safety.

The fire’s cause is being investigated. Red flag warnings for critical fire weather conditions, including gusty northerly winds and low humidity levels, were in effect when it erupted.

The warnings were expected to remain in effect until 8 p.m. Wednesday, said Garret Sjolund, the Butte County unit chief for the California Department of Forestry and Fire Protection.

“The conditions out there that are in our county this summer are much different than we’ve experienced the last two summers,” Sjolund said in an online briefing. “The fuels are very dense, brush is dry. And as you can see, any wind will, move a fire out very quickly.”

Related Story: Record-Setting Heat Sets Its Dangerous Aim on Fresno This Week

High Temperatures and Legal Consequences

More high temperatures above 100 degrees (37.8 Celsius) were forecast Wednesday, the National Weather Service said. Hot conditions were expected to continue into next week.

Authorities warned of full legal consequences for any illegal use of fireworks during the Fourth of July holiday.

“Don’t be an idiot, cause a fire and create more problems for us,” said Butte County Sheriff Kory L. Honea. “No one in the community is going to want that. And we certainly don’t want this.”

The governor’s office announced late Tuesday that federal funding had been approved to help with firefighting efforts. Gov. Gavin Newsom this week activated the State Operations Center to coordinate California’s response, dispatch mutual aid and support communities as they respond to threats of wildfire and excessive heat.

More than a dozen other fires, most of them small, were active in California, according to Cal Fire. The largest, the Basin Fire, covered nearly 22 square miles (57 square kilometers) of the Sierra National Forest in eastern Fresno County and was 26% contained.