By Steve Gorman
LOS ANGELES (Reuters) - The latest spate of harsh wintry weather churning from the Pacific bore down on storm-weary California on Tuesday, with forecasters warning of wind gusts reaching hurricane force, accompanied by torrential rain and heavy mountain snow.
Southern and central California, still sodden from storms that have swept ashore one after another since late December - one just days ago - were expected to be hardest hit by the newest onslaught, arriving early on the second official day of spring.
High-wind warnings and advisories were posted for a vast stretch of California from the Mexico border through Los Angeles to the San Francisco Bay area. Winter storm warnings were in effect for high mountains, where several feet of snowfall was possible.
The National Weather Service (NWS) also issued an excessive- rainfall notice for much of the Southern California coast, warning of an at least a 40% chance of showers exceeding flash-flood conditions.
A region that is home to more than 17 million people, extending well into central Arizona and including most of the greater Los Angeles area, was placed under flood watches.
Tuesday's storm was expected to spread beyond California across the Southwest and into the central Great Basin and Rockies by late on Tuesday, carrying with it a mix of heavy showers, snow and high winds.
The storm marks the 12th so-called atmospheric river to hit the U.S. West Coast since December, the product of an immense airborne current of dense water vapor carried aloft from the ocean and funneled overland in bouts of heavy rain and snow.
The storm was driven by a Pacific cyclone swirling around an intense low-pressure system, drawing up vast quantities of moisture and steering it at the coast, according to meteorologist William Churchill of the NWS Weather Prediction Center.
Unlike many of the earlier atmospheric rivers, the latest storm is packing a cooler load of moisture, meaning more of it will fall as snow in the higher coastal mountains and Sierra Nevada range.
As much as 2 to 5 feet (0.6-1.5 meters) were forecast at elevations above 6,000 feet, where some alpine areas remain blanketed in snow left by winter storms last month. The weather service warned that the additional snowfall would heighten avalanche hazards in those areas.
Up to 3 inches (7.6 centimeters) of rain were expected in coastal regions and valleys of Southern California, and as much as 6 inches in lower mountains and foothills, the NWS said.
Forecasts also called for howling winds across much of the state, with gusts expected to reach nearly 75 miles per hour (121 kilometers per hour) in Southern California's mountains and deserts, and 55 mph along coasts and through inland valleys.
Sustained winds blowing at or near gale force were predicted throughout the storm zone, Churchill said.
(Reporting by Steve Gorman in Los Angeles. Editing by Gerry Doyle)