A person walks along a flooded street as a powerful long-duration atmospheric river storm, the second in less than a week, impacts California on February 4, 2024 in Santa Barbara, California. 

A storm of historic proportions unleashed record levels of rain over parts of Los Angeles on Monday, sending mud and boulders down hillsides dotted with multimillion-dollar homes, posing grave dangers for the city's large homeless population and knocking out power for more than a million people in California.

The storm was the second one fueled by an atmospheric river to hit the state over the span of days. About 1.4 million people in the Los Angeles area, including the Hollywood Hills and Beverly Hills, were under a flash flood warning Monday morning. Up to 9 inches (23 centimeters) of rain had already fallen in the area, with more expected, according to the National Weather Service, which called the flash flooding and threat of mudslides "a particularly dangerous situation."