A winter storm system is heading to California, and is expected to bring the first significant snow to the northern Sierra Nevada since November, but spare Southern California the threat of mudslides. On Thursday, rain is forecast for the San Francisco Bay Area, while the Sierra is expected to see heavy snowfall by Thursday afternoon into Friday morning. The highest elevations of the Sierra could get 1½ feet to 2 feet of snow, while the elevation at Lake Tahoe could get perhaps half a foot of snow, said meteorologist Mark Faucette of the National Weather Service’s Reno office.