A powerful atmospheric river storm has swept across California, forcing hundreds to evacuate their homes.
Widespread flash flooding occurred on Christmas Eve in the south of the US state as a result of torrential rain.
In one mountain resort, debris and mud was filmed cascading down roads and fast-moving water was seen rushing through the porch of several homes.
The US National Weather Service said “life-threatening atmospheric river conditions” will continue throughout Christmas Day, with a rare “high risk of excessive rainfall” warning remaining in effect.
California governor Gavin Newsom declared a state of emergency in six counties to allow what he referred to as “emergency authorities and preposition resources to keep our communities safe”.
An estimated 130 evacuation orders were issued to vulnerable homes, Los Angeles County officials said, many of which are in the same area that was scorched by wildfires at the beginning of the year.
Southern California typically gets up to 2.5cm of rain this time of year, but this week many areas could see 10 to 20cm, with even more in the mountains, National Weather Service meteorologist Mike Wofford said.
Atmospheric rivers are long, narrow regions in the atmosphere – like rivers in the sky – that carry water vapour in the atmosphere from tropical regions.
When making landfall, water vapour is released in the form of rain or snow.
Atmospheric rivers occur around the globe but are especially significant on the West Coast of the United States – making up 30% to 50% of annual precipitation.
Not all atmospheric rivers cause destruction and actually play a key part in the global water cycle.
However due to Earth warming up, atmospheric rivers are getting bigger, wetter and more frequent, according to a 2025 study published in the Journal of Climate.
Heavy snow and gusts were expected to create “near white-out conditions” in parts of the Sierra Nevada and make travel “nearly impossible” through mountain passes.
There was also a “considerable” avalanche risk around Lake Tahoe, according to the Sierra Avalanche Centre.
More than 165,000 homes have had power restored after experiencing outages due to the storm, according to the provider Pacific Gas and Electric.
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California is prone to extreme weather events, not helped by the state experiencing wet periods followed rapidly by very dry weather.
The Golden State has previously suffered from drought and wildfires to earthquakes and flooding.
