An atmospheric river is coming to California. It could bring risks of debris flows

A rain-filled storm called an atmospheric river is sweeping into California this week. The weather phenomenon could bring heavy rain to many parts of the state—including the areas around Los Angeles that recently burned. Experts are warning that intense rains could create risks of debris flows and landslides in and around the burned areas.

What are atmospheric rivers?

Atmospheric rivers (ARs) are a normal part of winter weather along the West Coast. They are narrow bands of water vapor that wind sinuously through the atmosphere, like a river; they can transport vast quantities of water across the ocean, generally from the warm tropics toward the cooler mid-latitudes, like California.

One common flavor of AR, explains Christine Shields, an atmospheric scientist at the National Center for Atmospheric Research, is the colloquially termed “Pineapple Express.” Those ARs originate in the tropics near Hawaii and “essentially just move water from lower latitude to a higher latitude, like a river on earth,” she says.

When the ARs collide with the topography along the coast, they are forced upward higher in the atmosphere. That upward motion causes the water vapor they carry to turn to liquid; it falls out as rain or snow. Sometimes, the atmospheric river can also run up against other storms, adding to the atmospheric churn and often intensifying the precipitation. That may happen during this particular AR, says UCLA climate scientist Daniel Swain.

Several ARs usually arrive in California every winter, delivering as much as half the total annual precipitation to some parts of the state. This storm, forecasts project, could add 4 feet of snow to the Sierra Nevada snowpack. That snow can stay frozen until late in the summer, melting slowly and providing water downstream well into the second half of the year.

“You often hear about them when they are more extreme, and so people assume that they are hazardous,” says Julie Kalansky, director of the Center for Western Weather and Water Extremes, a group at Scripps Institution of Oceanography that studies atmospheric rivers. “But a lot of atmospheric rivers can be extremely beneficial because they bring much needed rain to our region,” she says.

Scientists at the Scripps Institution of Oceanography created a scale for ranking atmospheric rivers. It takes the duration of the potential precipitation, as well as the intensity, into account. ARs ranked 1 or 2 are “generally more beneficial,” says Kalansky. Those ranked 4 or 5 have the potential to be more dangerous, she says.

What are the concerns and risks from this atmospheric river?

This AR is currently forecast as a 1 or 2 in Southern California.

But the forecasts indicate that there could be short bursts of extra-intense rainfall, Kalansky says— similar to intense rainfall that caused flooding in San Diego in 2024.

Researchers and emergency managers are also particularly worried about severe rain falling on areas that recently burned, like in parts of Los Angeles. High-intensity fire can make soils less able to absorb water, and many of the places that burned in Southern California’s January fires have been denuded of vegetation and structures that help hold steep hills in place. A serious blast of rain, says Swain, could destabilize slopes—and potentially trigger debris flows or mudslides.

After the 2017-2018 Thomas Fire near Santa Barbara, an atmospheric river hit newly burned slopes. The total amount of rain was large, but not unprecedented—but some short bursts of rain dropped more than 3/4 inches in just 15 minutes. The intense rain triggered massive debris flows that killed more than 20 people.

Emergency managers and the National Weather Service are warning of the risks of debris flows during this atmospheric river. They are asking people to pay close attention to emergency alerts and evacuations.

Is there a climate connection?

California’s climate can shift dramatically from year to year. The big swings make seeing the fingerprint of human-caused climate change on weather events like atmospheric rivers challenging, Shields says. So far, a clear link hasn’t emerged, statistically.

But climate scientists expect that to change as the planet warms further.

“There’s very strong evidence that climate change will increase the intensity of atmospheric rivers and the rain and therefore flood risk that they produce,” says Swain.

The reason is based on well-established thermodynamics, he explains.

Human-driven climate change has primed the atmosphere to hold more of that water. Atmospheric temperatures have risen about 2 degrees Fahrenheit (just over 1 degree Celsius) since the late 1800s, when people started burning massive volumes of fossil fuels. The atmosphere can hold about 4% more water for every degree Fahrenheit warmer it gets. When that moist air hits mountains on the California coast and gets pushed upwards — or if it encounters another storm that churns the air upward into colder parts of the atmosphere — the air cools and its water gets squeezed out, like from a sponge.

It’s less clear whether, or how, climate change could influence the overall number of atmospheric rivers or the tracks they take, says Kalansky.

Swain points out that though statistical analyses don’t yet show a climate influence, some of the heaviest and most intense rainfall ever recorded at many different sites across the state have occurred in just the past few years. He calls that “anec-data—” not yet enough to provide statistical evidence, but enough to provide common-sense signals.

“That, I think, is telling us something important. Even though the long-term trend is not necessarily statistically significant, the anec-data is in alignment with what the science says should be happening.”