Areas east and south of Los Angeles, including San Diego, could see up to an inch of rain.
Farther east, parts of Arizona could absorb 2-4 inches of rain, and Phoenix could get 1.5 inches.
This can lead to flood flows across the desert southwest, with more than 10 million people under flash flood watches.
The rain will last until Friday, giving relief to many drought areas. Much of California is still "abnormally dry," according to the US drought supervisor.
Dry conditions, low humidity and windy winds will combine to create an increased risk of fires over the next few days. . and the Midwest.