A powerful atmospheric river storm was headed for Southern California this weekend, threatening to unleash potentially life-threatening floods and landslides, forecasters warned on Friday, even as much of the state was drying out from an earlier deluge.

Gradually intensifying rain was expected to begin dousing California on Saturday, with the most intense downpours soaking a 500-kilometer stretch of coast on Sunday and Monday as the storm spreads from San Luis Obispo and Santa Barbara south through Los Angeles and San Diego counties.

The National Weather Service (NWS) posted flood watches for the entire region in anticipation of staggering amounts of precipitation likely to fall over a 36-hour period, accompanied by strong gusty winds.

Rainfall averaging 7-15 cm was forecast for most of the region's coastal and valley areas, with 15-30 cm expected in the foothills and lower-elevation mountains.

With soils already saturated and streams running high from an earlier storm that drenched the region on Thursday, the flood potential from the coming onslaught is even higher than it would be otherwise, forecasters said.

"The amount of water that will be hitting the ground will generate significant, widespread and possibly life-threatening" floods, the weather service said in a forecast discussion posted online.

The NWS said there was a good chance of rainfall totals as high as 38 cm in mountainous parts of Santa Barbara and Ventura counties where the storm would probably hit hardest.

Communities on the south-facing slopes of mountains and foothills are expected to be hit with the heaviest downpours and thus most vulnerable to potential flash floods, mudflows and landslides.