The emergency message crackled over the car radio, warning listeners of looming flash floods.
Wayne Foss of Lancaster dismissed it without a second thought and continued on his way to an afternoon business meeting in nearby Lake Hughes.
But Foss found himself caught in a raging storm, with golf-ball-size hail falling and the road home submerged by fast-moving rain, mud and debris.
"I've never seen anything like that before, and I've lived in Southern California since I was 11," he said. "It got really scary really quick."
Northern Los Angeles County was pummeled Thursday by a series of torrential downpours that caused mudslides and flash floods that inundated roads, trapped drivers and forced the closure of nearly 40 miles of Interstate 5, cutting off California's main north-south artery.
Officials said there was no timetable for Interstate 5 to reopen because the freeway was engulfed by mud and debris, and dozens of stranded vehicles will have to be cleared.
Dozens of firefighters responded to the Lake Hughes area, where mudslides flowed across and along roadways, engulfing some cars.
Firefighters had to use a helicopter to rescue four stranded people and their dogs. In the Elizabeth Lake area, mud surrounded homes.
No injuries had been reported by late Thursday, but authorities said rescue crews would continue to search affected areas through the night.
The Leona Valley was the hardest hit, with rain reportedly falling at a rate of 4 to 5 inches an hour and winds gusting to 60 mph. That storm eventually diminished, but as it moved eastward it brought record daily rainfall to Palmdale.
The strong showers and thunderstorms are expected to continue through Saturday, said Joe Sirard, a meteorologist with the National Weather Service in Oxnard.
Interested in the stories shaping California? Sign up for the free Essential California newsletter >>
A rare confluence of factors led to the heavy, sustained rainfall, he said.
Southern California is experiencing unusually high humidity, which joined forces with a low-pressure system bringing in tropical airflow from the south.
That, combined with additional moisture in the atmosphere from high ocean temperatures, created a very warm, humid air mass over the region.
This exacerbated the thunderstorms, causing
the torrential rain, Sirard said.
"These storms were moving very slowly and dumping tremendous amounts of rain," he said. "Very unusual factors have played a role in the flooding today — lots of humidity and just the right amount of instability in the atmosphere."
As Michael Henderson left his job in Lancaster for his evening drive home to the Leona Valley, he was shocked to see that the roads had seemingly become rivers. Emergency vehicles zoomed past him in every direction.
"It was just panic," Henderson said. "I'm in an intersection with gushing water and I knew I couldn't stop, but I couldn't keep going because I couldn't see."
He turned off the road and took shelter under an awning at a roadside store until it was safe to continue. He made it home after two hours, with the sun setting on mud and debris on the roadways.
Henderson and his family moved to the Leona Valley less than a year ago and hadn't seen much in the way of weather until now.
"This is quite an introduction," he said.
For more breaking news, follow @sjceasar