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Ben Rich explains how an atmospheric river forms
California is bracing for "life-threatening" flash flooding just days after much of the state was inundated with heavy rain storms.
Heavy rain and snowfall could cause severe flooding from Santa Barbara to Los Angeles over the weekend.
The "atmospheric river" effect, caused by airborne currents of dense moisture, is expected to bring "the largest storm of the season", officials warned.
More than 22 million people are under flood alerts.
Atmospheric rivers are a phenomena in which water evaporates into the air and is carried along by the wind, forming long currents that flow in the sky like rivers flow on land.
This system is known as a "Pineapple Express" because it brings warm subtropical moisture across the Pacific from near Hawaii.
Experts are warning residents that flooding and travel problems could be exacerbated from remnants of the previous storm, which caused significant damage, prompted water rescues and closed parts of the iconic Pacific Coast Highway.
The Los Angeles National Weather Service (NWS) urged people to "prepare now" for the "damaging, life-threatening flooding" by getting extra supplies, gas and sandbags.
The NWS forecasters say the storm could "likely produce 24 to 36 hours (or more) of continuous rain".
Along with heavy mountain snow and damaging winds, the region could see up to a month's worth of rain - about three to 10 inches (seven to 25cm) - which could cause urban flooding, mudslides and flash flooding, the NWS said.
The first of the two storms halted cable car service in the San Francisco Bay area before drenching Los Angeles and San Diego with torrential rain.
Also in northern California, around San Jose, a 100ft (30m) redwood tree crashed onto a car, trapping a girl inside, before firefighters managed to free her. She suffered minor injuries.
A San Diego resident Ruben Gomez told The Associated Press that firefighters had to rescue his parents after their home flooded with 6ft (2m) of water from the first storm.
The torrential rain earlier this week broke a record amount of daily rainfall - 3.37in fell at Los Angeles International Airport, breaking the 1960 record of 1.55in.
Parts of the Sierra Nevada already reported 12 to 14in (30 to 35.5 cm) of snow and the upcoming storm could bring four feet more of snowfall to the mountains.
Last year, more than 20 people died in the state from similar damaging atmospheric river storms.
It also caused $4.6bn (£3.6bn) in damage, according to CBS News, the BBC's US partner.
English (US)