Cars lie partially submerged in water, as the first in a pair of Pacific storms floods parts of Southern California [Source: Reuters]

A second, more powerful atmospheric river storm was headed for Southern California this weekend, threatening to unleash 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 300-mile (480-km) 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 flash-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 3-6 inches (7-15 cm) was forecast for most of the region’s coastal and valley areas through Monday, with 6-12 inches expected in the foothills and lower-elevation mountains.

With soil already saturated and streams running high from the 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 NWS said there was a good chance of rainfall totals as high as 15 inches (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 receive the heaviest downpours, leaving them most vulnerable to potential flash floods, mud flows and landslides. Hillsides and canyons scarred by recent wildfires are particularly prone to washouts.