San Francisco, California - Millions of people faced dangerous flooding in California Monday after a storm brought record rains and gusting winds, leaving at least one person dead.

The US National Weather Service warned of "life-threatening" flash flooding caused by "excessive rainfall," especially in the southern part of the state, as a so-called "atmospheric river" came ashore from the Pacific Ocean.

"This is a serious storm with dangerous and potentially life-threatening impacts," said Governor Gavin Newsom as he declared the state of emergency for eight counties in Southern California.

Areas under the emergency order include Los Angeles, Orange, Riverside, San Bernardino, San Diego, and Santa Barbara.

According to the NWS, another five to eight inches of rain were expected to fall around the Los Angeles area Monday, bringing the total amount of rain in the last 48 hours to up to eight to 14 inches in some areas.

"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 NWS forecast said early Monday.

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

In northern California, a man was killed when a one of the state's iconic redwood trees fell on him in his backyard, according to police in the city of Yuba, near Sacramento.

San Francisco and the surrounding Bay Area registered wind gusts as high as 102 miles per hour Sunday, the local NWS said. Wind warnings remained in effect until at least 8 AM PT Monday.

That region was also at risk of "minor flooding," the NWS said.

More than half a million customers were without power by Monday morning, according to the electricity supply tracker PowerOutage.us, while dozens of flights were delayed or canceled, according to flightaware.com.

High snowfall totals were expected in the mountains – up to "several feet" in some spots – while marine weather and high surf warnings were in effect along the coast.

The storm is part of a Pineapple Express, a weather phenomenon born out of tropical moisture near Hawaii. The NWS described it as "the largest storm of the season."

While wet weather is not unusual during California's winters, scientists say human-caused climate change is altering the planet's weather patterns.

This makes storms wetter, more violent and unpredictable, while causing dry periods to be hotter and longer.