RIVERSIDE (CNS) - A winter storm will continue to affect Riverside County and other parts of the Southland today — bringing showers and a chance of thunderstorms, as well as periods of heavy snow in the mountains, the National Weather Service said. The storm system, which landed in the area on Wednesday, is expected to continue into at least Thursday and possibly into Friday. A winter weather advisory was in effect until 10 a.m. Thursday for Riverside valleys and Inland Empire above 2,500 feet. The forecast in those areas called for mostly cloudy skies with a chance of showers and a slight chance of thunder showers Thursday and into Friday night before becoming sunny Saturday. Dense fog was also expected. In the mountains, a winter storm warning remained in effect until 10 p.