What is atmospheric river? Satellite imagery shows how it drenched California

In California, millions of people faced dangerous flooding after a storm brought record rains and gusting winds. The US National Weather Service warned of "life-threatening" flash flooding caused by "excessive rainfall," especially in the southern part of the state. 

Governor Gavin Newsom declared a state of emergency for eight counties in Southern California, including Los Angeles, Orange, Riverside, San Bernardino, San Diego and Santa Barbara. He said, "This is a serious storm with dangerous and potentially life-threatening impacts."  

An "atmospheric river" came ashore from the Pacific Ocean. A report by PBS mentioned in one of its reports that an atmospheric river is a narrow corridor or filament of concentrated water vapour transported in the atmosphere. 

It's like a river in the sky that can be 1,000 miles long and on average, atmospheric rivers have about twice the regular flow of the Amazon River. 

The NWS forecast said early Monday, "Increasingly saturated conditions and ongoing flooding will be further exacerbated by this additional rainfall, continuing the threat for life-threatening, locally catastrophic flash, urban, and small stream flooding, as well as a threat for debris flows and mudslides." 

The local NWS office said that downtown Los Angeles saw one of its wettest days ever, with more than four inches of rain. The agency advised residents on social media to "avoid travel if at all possible" due to the "extremely dangerous situation". 

The toll from the atmospheric river poised over California climbed tragically Monday, as three people were killed by falling trees. The storm has spawned flooding and mudslides as it has blazed a damaging trail across the state.

The electricity supply tracker PowerOutage.us said that more than half a million customers were without power by Monday morning, while dozens of flights were delayed or cancelled, according to flightaware.com. 

The massive Pineapple Express storm - the second in recent days - drenched the Los Angeles area with torrential rain, bringing near-hurricane-force wind gusts and raising the threat of flash floods and landslides. Satellite imagery showed the storm over the Pacific Ocean and the west coast of the United States.