SAN FRANCISCO—A powerful atmospheric river storm that swept through California set rainfall records and helped douse wildfires. But it remained to be seen how much of a dent it made in the state’s drought. The weather system weakened as it moved south but still dropped enough rain Monday evening to cause mudslides that closed roads in the San Bernardino Mountains northeast of Los Angeles. The atmospheric river storm, a long and wide plume of moisture pulled in from the Pacific, came ashore in Northern California over the weekend. Drenching rains caused widespread flooding and rock slides. Strong winds knocked down trees and toppled two big trucks on the Richmond–San Rafael Bridge near San Francisco. Pacific Gas & Electric reported that 646,000 homes and businesses lost power, though most had it back by Monday. By early Tuesday, light rain and snow were still falling in northern parts of California and the …