WATCH: Stunning Time-Lapse Video Shows Furious Wildfire Outside Los Angeles

A stunning time-lapse video showed the furious growth of a wildfire raging northwest of Los Angeles as nearly 20,000 people were told Sunday they'll have to flee if it spreads further south.

A huge plume of thick smoke spewed from the hilly rural terrain in images captured over the course of seven hours on Saturday, according to timestamps on the 1 1/2 minute clip posted on X, formerly Twitter.

Director and Fire Chief Joe Tyler of the California Department of Forestry and Fire Protection called it a "stark reminder" of why officials work to prevent wildfires across the Golden State.

Some 19,000 people living in Castaic, about 40 miles from downtown LA, were warned to be "ready to to evacuate" after Los Angeles County's first major wildfire of the year covered more than 16 square miles, the Associated Press reported.

"If you're in a warning area, be prepared with a 'go bag,' with overnight clothes and your cellphone, your medicines, your glasses. Have your car fueled up," said Kenichi Haskett, a section chief for the LA County Fire Department.

Firefighters battled sweltering temperatures and steep terrain to put out spot fires that erupted when unpredictable winds blew embers ahead of the wildfire's leading edge, Haskett said.

The weather also made it hard for aircraft crews to drop water and fire retardant.

"When it's windy, it just sprays the water everywhere we don't need it. So that's a challenge," Haskett said.

The fire was only 2% contained late Sunday afternoon and the cause remained unknown.

The blaze, dubbed the Post Fire, started near Interstate 5 in Gorman, about 60 miles northwest of Los Angeles.

It forced the evacuation of at least 1,200 campers, hikers and off-roaders from the Hungry Valley state recreation area, where two structures were incinerated.

Flames were moving toward Pyramid Lake, a popular boating area that was closed for Father's Day as a precaution.