“I don’t think the rain is going to stop for the next 24 to 36 hours,” Dr. Swain said earlier on Sunday.
On Sunday, meteorologists and officials urged residents of Los Angeles to brace for flooded streets in the valleys and mudslides in the mountains. The entire county, home to almost 10 million people, was under a flash flood warning until midnight.
As of Sunday night, more than four inches of rain had fallen on the Santa Monica Mountains and the totals were rising at rates of more than half an inch per hour, according to Joe Sirard, a meteorologist with the National Weather Service in Oxnard, north of Los Angeles.
And when rain falls in large quantities on the mountains, it rushes downhill, sometimes taking the saturated land with it, and collects in low-lying areas, such as the vast sprawl of the San Fernando Valley, leaving intersections and streets under water. Rivers and streams could swell, overtopping their banks and flooding the neighborhoods surrounding them.
“Many, many hours of rain adds up,” Mr. Sirard said.
Soumya Karlamangla contributed reporting.