Tropical storm Hilary was barreling along the Baja California peninsula in Mexico on Sunday and bearing down on the southwestern US, where it was expected to cause heavy rain, dangerous flooding and possibly tornadoes.

The storm was downgraded from a hurricane shortly before 8am Pacific Time, weakening as it moved north, but the National Weather Service’s National Hurricane Centre warned that it was likely to cause “catastrophic and life-threatening flooding” in regions unaccustomed to heavy rain.

Strong winds and waves buffeted a large swath of the Baja California peninsula on Sunday morning as the centre of the storm moved up off the Mexican coast, and officials there expected it to make landfall later in the morning.

Mexican authorities said that one person had died after a family’s vehicle was swept away in Mulegé, Baja California Sur, on Saturday night. Rescuers were able to save four of the five passengers, a local official said. Nearly 1,500 people were in shelters, officials said.

In California, a light rain was falling in Los Angeles just after daybreak and forecasters warned that winds would intensify during the morning. Officials urged residents of some communities to evacuate, with several inches of rain expected there and in southern Nevada.

Governor Gavin Newsom of California declared a state of emergency in 11 counties, including Los Angeles, San Diego and Orange. Across the state, officials cancelled events, closed parks and beaches, and deployed more than 7,500 emergency responders.