A giant Douglas-fir tree is seen atop a downed power line in Felton, Calif., on Tuesday. Roads are closed, power is out, and some schools have closed in Santa Cruz county as a weeklong winter storm continues. (Marcio Jose Sanchez/Associated Press)

Over 600 residents in the Los Angeles area were ordered to leave their homes on Wednesday morning ahead of a powerful Pacific storm expected to arrive midday.

The U.S. National Weather Service said the new storm would bring winds of close to 100 km/h along the Southern California coast and drop rain at a rate of up to 38 millimetres per hour. As much as 125 millimetres of rain was forecast for some mountain areas in the region.

The storm follows on the heels of weather systems that have already flooded coastal communities in the region, spawned a tornado and killed at least one person.

Weather warnings were in effect across the state, but the concern was greatest in Southern California, where an area more than 650 square kilometres scorched by wildfires in the past two years has lost vast amounts of vegetation that would normally prevent soil runoff.

Debris basins already full

Officials say the threat of mudslides is real as basins designed to capture debris-laden runoff were full after two days of storms this week.

County Fire Chief P. Michael Freeman begged residents to listen to the evacuation orders and said safety officials are racing against Mother Nature.

"And if she wins the race there's no way we can assure that firefighters, as well-equipped and as trained as they are, will be able to get into your neighbourhood and make rescues," Freeman said.

Residents were told to be prepared to be away from their homes for as many as five days.

An earlier storm from Tuesday did serious damage, crushing a woman to death with a fallen tree, flooding coastal neighbourhoods and leaving thousands without power as lightning and tornados surged ashore.

With files from The Associated Press