Tens of thousands of people have been urged to evacuate their homes in the US state of California as a powerful storm batters the state and authorities warned of the potential of mudslides.

Those living near areas which were hit by recent wildfires are most at risk, as there is no vegetation left to bind the soil on hillsides.

Rainfall rates of between 8 and 13 millimetres per hour are predicted in the foothills and the mountains; 13 mm is the threshold for triggering a debris flow.

The Santa Barbara County Fire Department issued a news release stating there was a "high risk for loss of life and property".

Authorities are determined to avoid another disaster like the mudslide that hit Montecito, which killed 21 people in January.

The warning stops short of a mandatory evacuation, although the warning could be upgraded on Thursday.

In the long run, the rain will be very useful, as the state has had an abnormally dry winter, raising fears that the water supply could run low in the coming months.