The National Weather Service downgraded the former hurricane to a tropical depression but not before California Governor Gavin Newsom declared a state of emergency for much of Southern California, with flash flood warnings until at least 3 a.m. (1000 GMT) on Monday in an area more accustomed to drought.
Mountain and desert areas could get 5 to 10 inches (12 to 25 cm) of rain rain, as much as the deserts typically see in a year, forecasters said.
The storm passed northward through Mexico's Baja California peninsula, where Pacific hurricanes are expected. It killed at least one person in Mexico amid reports of flash flooding in the peninsula, sweeping away some roads.
Images on social media showed raging torrents gushing down city streets that turned into rivers.
It crossed the border Sunday afternoon, hitting San Diego county with its first tropical storm ever recorded and becoming the first to pelt Los Angeles county since 1939, triggering serious flooding.
San Bernardino county, to the east of Los Angeles, ordered evacuations of towns in the mountains and valleys where social media images showed torrents of water, mud, rock and trees.
In more populated Ventura county northwest of Los Angeles, the National Weather Service warned of life-threatening flooding as up to 2 inches (5 cm) of rain fell within two hours.
US President Joe Biden ordered federal agencies to move personnel and supplies into the region after local officials prepared for days.
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