LOS ANGELES — With strong El Niño conditions now established in the Pacific Ocean, Southern California has been getting a taste of what might be in store this winter: lots of desperately needed rain, but also dangerous mudslides and flash flooding.

After several hours of heavy rain, mud and debris cascaded down hillsides north of Los Angeles on Thursday, blocking two freeways, including Interstate 5, the critical artery linking this city with San Francisco, Sacramento and Seattle. Mud, sometimes up to windshields, swallowed hundreds of vehicles across the region. Traffic coming south toward Los Angeles all but stopped, stranding travelers far from home.

Near Tehachapi, north of here, the hills beside State Route 58 suddenly caved in. Jessica Rose, who was driving to her grandfather’s funeral in Los Angeles, pulled off the highway and onto an overpass just before it hit.

“I saw the mudslide — it looked like a brown waterfall coming down the mountain,” Ms. Rose, 22, said. “It was like Niagara Falls but brown, like the chocolate waterfall from Willy Wonka. Cars were all crashing into each other because there was 15 feet of water. Semi trucks were stuck in it. It was the most serious thing I’ve ever seen.”