At its peak, the storm reached a Category 4 on the five-step Saffir-Simpson scale, but is expected to weaken to a tropical storm before reaching southern California on Sunday afternoon, an advisory from the US National Hurricane Center (NHC) said.
It was still packing nearly 85 miles per hour (140 kilometers per hour) sustained winds with stronger gusts as it blew across the Baja California Peninsula early Sunday morning, the NHC said.
"Heavy rains from Hilary spreading northward over the Baja Peninsula and the southwestern United States," the US government agency said. "Catastrophic and life-threatening flooding likely."
One person died in Mexico after a vehicle was swept away by a rising stream, Mexico's Civil Protection agency said in a statement on Saturday, while warning of landslides and road closures on the Baja California Peninsula.
Hilary is expected to track inland and north over the next day or two, depositing up to 10 inches of rain on parts of Mexico, California and Nevada, according to the NHC.
Tornadoes were also on the cards from mid-morning through Sunday evening in parts of the Colorado River Valley, Mojave Desert and Imperial Valley.
Despite slowing down, the storm remains treacherous, with millions of people urged to take precautions.
The US Federal Emergency Management Agency (FEMA) has deployed teams to areas in Hilary's path, while California Governor Gavin Newsom declared a state of emergency for much of the state's southern area.
Nancy Ward, director of the California Governor's Office of Emergency Services, said Hilary could be one of the worst storms to hit the state in more than a decade.
"Make no mistake," she told a press conference Saturday. "This is a very, very dangerous and significant storm."
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