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By Joe Edwards
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An atmospheric river has been forecast to hit California this week, with the National Weather Service (NWS) warning of the potential for flash flooding, debris flow, and significant mountain snow.
Why It Matters
Atmospheric rivers vary in size and intensity, but those with the highest water vapor content and strongest winds can bring extreme rainfall and flooding, according to the National Oceanic and Atmospheric Administration (NOAA).
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What To Know
An atmospheric river is expected to arrive Wednesday night, bringing "heavy and impactful" rainfall to central and Southern California through Thursday night.
A forecast map from the NWS indicated that there was a slight risk of excessive rainfall from the coastal ranges into the central valley and the Sierra Nevada foothills.
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Burn scar areas in Southern California are at significant risk of flash flooding and debris flows during the event, the service said.
Additionally, heavy snowfall is expected over the Sierra Nevada and northern California terrain from Thursday through Saturday. While snow levels will rise throughout the event, the intense snowfall is likely to create hazardous travel conditions on many mountain passes due to snow-covered roads and reduced visibility, the NWS said.
The NOAA says atmospheric rivers are long, narrow bands in the atmosphere that transport large amounts of water vapor outside the tropics. Their size and strength can vary significantly, but, on average, they carry a volume of water vapor comparable to the flow of the Mississippi River at its mouth.
Exceptionally strong atmospheric rivers can transport up to 15 times that amount. When they make landfall, they typically release this moisture as rain or snow.
What People Are Saying
AccuWeather senior meteorologist John Feerick told Newsweek: "Most valley locations across the state are expected to see 1 to 2 inches of rain, but the heaviest rain will fall over the foothills and into the mountains; especially over Southern California. Some of those locations are likely to see 4 to 8 inches of rain, and this will lead to widespread flooding, mudslides and road washouts.
"This is especially expected to be a problem in the areas of burn scars from the recent wildfires. While the Sierra Nevada mountains will see some very heavy snow, snow levels will be quite high and will actually climb above pass level for a time on Thursday."
Meteorologist Jerry Steffen said in a sequence of posts on X: "Today's highs will be slightly cooler than yesterday with those midday numbers peaking in the lower 70s.
"This low will help guide an Atmospheric River of moisture over Southern California providing a good chance of precipitation between Wednesday and Friday.
"The Coachella Valley may see a few light showers Wednesday night followed by more widespread activity Thursday afternoon into Friday morning. The Tram may see up to 10 inches of snow by Friday afternoon."
The NWS said in a post on its website: "Periods of moderate to heavy rain is expected Thursday to Friday with a slight chain of flood impacts across the Central and Southern Sacramento Valley, Delta, and Northern San Joaquin Valley."
What Happens Next
Forecasts are subject to change. The NWS issues regular updates on its website and on social media.
Update, 2/12/25, 12:58 p.m. ET: This article was updated with original comment from AccuWeather senior meteorologist John Feerick.
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