The most powerful in a series of winter storms lashed Northern California and Nevada on Sunday with heavy rains and strong winds, causing widespread flooding, downing trees and unleashing mudslides. Hundreds of people were evacuated from their homes as rivers overflowed their banks. Several key highways including Interstate 80, Interstate 280, U.S. 395 and U.S. 101 were closed for periods because of hazardous conditions. Authorities were trying to determine whether the deaths of three people in the Bay Area — one killed by a falling tree, two others by car accidents — were related to the storm.