LOS ANGELES – A fast-growing brush fire has forced thousands of people to evacuate in a mountainous area north of Los Angeles.

The Canyon Fire ignited Thursday afternoon and grew to more than 7.6 square miles (19.7 square kilometers) by 11 p.m., according to the Ventura County Fire Department. At least 400 personnel were battling the blaze along with several planes and helicopters. It remained uncontained late Thursday and was spreading east into Los Angeles County, officials said.

The fire is burning just south of Lake Piru, a reservoir located in the Los Padres National Forest. It’s close by Lake Castaic, a popular recreation area burned by the Hughes Fire in January. That fire burned about 15 square miles (39 square kilometers) in six hours and put 50,000 people under evacuation orders or warnings.

Sunny, hot and dry conditions were expected in the area where the Canyon Fire was burning on Friday, with the daytime high near 100 degrees Fahrenheit (38 degrees Celsius) and minimum humidity in the mid-teens, according to the National Weather Service. Winds were expected to be light in the morning and grow from the south to southwest in the afternoon.

In LA County, around 2,700 residents evacuated with 700 structures under an evacuation order, officials said late Thursday. Another 14,000 residents and 5,000 structures were covered by an evacuation warning. Areas within the Val Verde zone had been reduced from an order to a warning.

The evacuation zones in nearby Ventura County were relatively unpopulated, Ventura County Fire Department spokesperson Andrew Dowd said Thursday. Fifty-six people were evacuated from the Lake Piru recreation area.

Dowd called the blaze a “very dynamic situation” caused by hot, dry weather, steep and rugged terrain and dry fuel.

LA County Supervisor Kathryn Barger, who represents the district, urged residents to evacuate.

“Extreme heat and low humidity in our north county have created dangerous conditions where flames can spread with alarming speed,” Barger said in a statement. “If first responders tell you to leave, go—without hesitation.”

The new blaze comes as a massive wildfire in Central California became the state’s largest blaze of the year, threatening hundreds of homes and burning out of control in the Los Padres National Forest.

The Gifford Fire had spread to 155 square miles (402 square kilometers) by Thursday night with 15% containment. It grew out of at least four smaller fires that erupted Aug. 1 along State Route 166, forcing closures in both directions east of Santa Maria, a city of about 110,000 people. It has injured at least four people. The causes of the fires are under investigation.

Wildfire risk will be elevated through the weekend across much of inland California as a heat wave gripping the area intensifies. August and September are typically the most dangerous months for wildfires in the state.