LOS ANGELES—Heavy overnight rains in Northern California left two people dead in a submerged car as authorities on Thursday urged residents of several Southern California mountain and canyon communities to voluntarily leave their homes because of possible mud and debris flows. Firefighters in Millbrae, just south of San Francisco, were able to rescue a driver who had climbed atop his vehicle at a flooded underpass. But they were not able to reach people in another car, said San Mateo County Sheriff’s Detective Javier Acosta. Acosta told the Mercury News in San Jose there was “overwhelming rain” rising rapidly in the underpass when firefighters responded to a call for help shortly before 6 a.m. It took several hours to drain the area, which is when authorities discovered the bodies. An investigation is underway. In the Sierra Nevada, an evacuation warning was issued downstream of Twain Harte Lake Dam after cracks were …