Tropical storm Hilary made landfall in Mexico's northern Baja California peninsula on Sunday and was accelerating towards the south-western US, where it was forecast to bring heavy rains, destructive wind and flooding.
Experts expected the eye of storm to land in San Diego county between 3pm and 6pm local time on Sunday.
Alex Tardy, a National Weather Service meteorologist, said the area will see unprecedented rain, wind and flooding.
"We've never had that in the history of San Diego," Mr Tardy told Fox 5 San Diego.
"The rain is going to be a concern because there's going to be too much, and too much at once."
The US National Weather Service’s National Hurricane Centre earlier said the storm had weakened from a Category 1 hurricane as it moved north, but warned that it was still likely to cause “catastrophic and life-threatening flooding".
Heavy rain associated with the storm is expected to last until Monday morning, officials said.
The Mexican government said that one person had died on Saturday after their car was swept away in Baja California Sur.
Officials said that 37 temporary shelters had been opened in Baja California Sur, and more than 1,400 people were being housed.
Californian Governor Gavin Newsom on Saturday declared a state of emergency for 11 counties in Southern California, including in Los Angeles and San Diego, and local officials urged residents in high-risk areas to evacuate their homes.
More than 7,500 emergency responders were sent.
“California has thousands of people on the ground working hand-in-hand with federal and local personnel to support communities in Hurricane Hilary’s path with resources, equipment and expertise,” Mr Newsom said in a statement.
“We’re mobilising all of government as we prepare and respond to this unprecedented storm.”
Across the state, events were cancelled, while several parks and beaches were closed.
Karen Bass, the Mayor of Los Angeles, said the city was well positioned to deal with the storm and urged residents to stay indoors.
“This is an unprecedented weather event but Los Angeles has deep experience responding to crisis,” she said referring to earthquakes, fires and other emergencies that have hit the city in the past.
US President Joe Biden on Friday said the White House is closely monitoring the developments of the storm and that the Federal Emergency Management Agency is ready to respond as needed.
“I urge everyone, everyone in the path of the storm to take precautions and listen to the guidance of state and local officials,” Mr Biden said.