Second, stronger storm expected to hit California with potentially deadly floods

By Steve Gorman

LOS ANGELES (Reuters) - A second, more 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 300-mile 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 3-6 inches (7-15 cm) was forecast for most of the region's coastal and valley areas, with 6-12 inches 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 15 inches (38 cm) in mountainous parts of Santa Barbara 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, mud flows and landslides.

"Everyone needs to be prepared for significant impacts," Ariel Cohen, the chief meteorologist for the NWS Los Angeles-Oxnard branch.

In Los Angeles, the height of the encroaching storm is expected to coincide with the music industry's Grammy Awards show on Sunday, prompting organizers to set up large tents for the pre-ceremony red carpet procession of the stars.

Elsewhere, crews were busy filling and stacking sandbags and clearing storm drains and culverts.

Ski areas on the other hand were looking forward to a bonanza, as snowfalls measuring 2-4 feet (61-122 cm) are expected in the higher-elevation mountains, the NWS said.

Frozen precipitation from the storm will also benefit the region's snowpack, helping to rebuild a key source of freshwater that has lagged below normal despite last year's winter's record-breaking storms.

Much of the region on Friday was still cleaning up from an earlier bout of heavy rains that swept northern and southern portions of the state on Wednesday and Thursday, triggering scattered street flooding, rock slides and mud flows.

Both storms formed from vast airborne currents of dense moisture called atmospheric rivers. They also fit the definition of a storm system known as a "Pineapple Express," drawing on especially warm, subtropical waters around the Hawaiian islands.

A series of about a dozen atmospheric river storms lashed California in rapid succession last winter, causing mass evacuations, power outages, levee breaches and road closures in a state long preoccupied with drought and wildfires. At least 20 people perished in those storms, which nevertheless helped break the grip of a years-long drought in California.

(Reporting by Steve Gorman in Los Angeles; Editing by Jacqueline Wong)