A major storm pummeled Northern California with rain and snow on Wednesday night and threatened to cause flash flooding and rockslides in the latest wave of damaging weather to wash over the West Coast. The National Weather Service extended a flood watch into Saturday for areas north of San Francisco as the strongest atmospheric river — a large plume of moisture flowing onshore — that California and the Pacific Northwest has seen this season inundated the region. The storm system unleashed winds the night before that left two people dead and hundreds of thousands without power in Washington state.