An ‘atmospheric river’ is causing deadly storms in California. But what exactly is it?

California is being battered by heavy winds and record-breaking rainfall. This ‘firehose of rain’ has so far triggered 130 flooding incidents, 200,000 power outages and 3 deaths. While downtown LA has endured 75% of its annual rainfall already this year. California’s governor has declared a state of emergency in 8 counties. Scientists point to an ‘atmospheric river’ parked over Southern California but what is that?