Eight California counties are under a state of emergency as an atmospheric river has caused flooding throughout heavily populated areas of southern California. The storm also brought heavy rain and damaging wind to the Bay Area.California Gov. Gavin Newsom issued the state of emergency proclamation Sunday for Los Angeles, Orange, Riverside, San Bernardino, San Diego, San Luis Obispo, Santa Barbara and Ventura counties.Numerous water rescues were reported as motorists were caught in their vehicles as roads flooded.California: this is a serious storm with dangerous and potentially life-threatening impacts. Please pay attention to any emergency orders or alerts from local officials," Newsom said. "California is ready with a record number of emergency assets on the ground to respond to the impacts of this storm.Nearly 5.5 inches of rain fell in Los Angeles on Sunday into early Monday. The National Weather Service expected another 1-2 inches of rain to fall on the city on Monday. The National Weather Service said some areas could end up with 8-14 inches of rain over a multi-day period in California. Nearly 600,000 customers are without power as of early Monday.The National Weather Service defines an atmospheric river as "a long, narrow region in the atmosphere that transports water vapor, like a river, in the sky."SEE MORE: NASA blames fossil fuels, El Nino for 2023 being hottest on recordIn addition to heavy rain, some parts of California experienced damaging wind. Pablo Point, California, recorded a wind gust of 102 mph on Sunday. Further inland, winter storm warnings were issued as heavy snow is expected to inundate areas at or above 6,000 feet of elevation. The National Weather Service said 2-4 feet of snow is expected for areas at 7,000 to 9,000 feet of elevation. Calm weather is not expected to return to the region until the end of the week.