AILSA CHANG, HOST:
A powerful winter storm known as an atmospheric river is sweeping into California this week. Heavy rainfall is expected tomorrow into Friday and also risks of mudslides and floods. Alejandra Borunda from NPR's climate desk is here to tell us more about all of this. Hi, Alejandra.
ALEJANDRA BORUNDA, BYLINE: Hi, Ailsa.
CHANG: OK. So just remind everyone, what exactly is an atmospheric river? - which is a really cool name.
BORUNDA: Right? That's what I think, too. I'll let atmospheric scientist Christine Shields, from the National Center for Atmospheric Research, describe them.
CHRISTINE SHIELDS: If you look at them from a satellite, they actually look a little bit like a river, where they're sort of long and narrow and they meander.
BORUNDA: Yeah, so they're these rivers in the sky, and they can bring a ton of water from the hot, humid tropics up towards the midlatitudes, like California. And the - I think the thing to remember is that they're pretty common. We usually get a few every winter, except for during really intense drought years. And they deliver a really high percentage of the precipitation for the entire year for the state sometimes. So they're really important for those states' water resources.
CHANG: OK, so they're really important. They're really common. So why are people really worried about this week's storm?
BORUNDA: Great question. So usually the concern with atmospheric rivers is flooding because the rain can come down so fast and hard, the ground just can't absorb it or infrastructure gets overwhelmed. But right now, what most people are worried about is potential debris flows or mudslides in the burned areas around Los Angeles.
CHANG: Right.
BORUNDA: The - right. That's because fire can make these soils actually extra bad at absorbing water. And after the burns, there's not many trees or shrubs or whatever holding the ground in place. So these big intense bursts of rain - they can trigger slides. And people are really worried because they've actually seen this happen. More than 20 people died in debris flows after the Thomas fire a few years ago. It was a real wake-up call.
CHANG: Absolutely. OK, well, since you are on the climate desk, I have to ask the ever-present question, and that is, how much is climate change affecting atmospheric rivers like this one?
BORUNDA: Yeah, it's a good question. And the short answer is that statistically, scientists don't yet see a signal. But climate scientists like Daniel Swain from UCLA are very confident that that will change soon.
DANIEL SWAIN: There's very strong evidence that climate change will increase the intensity of atmospheric rivers and the rain and, therefore, flood risks that they produce.
BORUNDA: So Swain says scientists don't yet know if climate change will affect the number of atmospheric rivers or where they go. But it's pretty simple to figure out that they will probably be more intense.
CHANG: Pretty simple maybe for an atmospheric scientist, but how would that work?
BORUNDA: I think we can understand it. The logic is tied to very well understood thermodynamics, and it's basically this. Warm air holds more moisture. So a hotter atmosphere can soak up more water like a sponge. That extra water can then get squeezed out when the atmospheric river hits land, and that means more water comes out of the sky. And Swain points out that there's, what he calls, anecdata (ph) that some of the state's heaviest rain ever was in just the past few years.
SWAIN: That, I think, is telling us something important, even though the long-term trend is not necessarily statistically significant. The anecdata is in alignment with what the science says should be happening.
BORUNDA: So we know that there's going to be some of this super intense rain in this atmospheric river. So Swain says be really careful and listen to those emergency alerts in the coming days.
CHANG: That is NPR's Alejandra Borunda. Thank you, Alejandra.
BORUNDA: Yeah, thanks so much. Transcript provided by NPR, Copyright NPR.
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