An atmospheric river has been pounding California. When will the rain end?

MICHEL MARTIN, HOST:

In Southern California, days of rain have turned hills into rivers of mud.

LEILA FADEL, HOST:

At least three people have been killed by a dangerous storm in Northern California from hurricane winds. And back in Southern California, some homes have been washed away, and firefighters have been busy rescuing people from rushing waters and stranded vehicles. President Joe Biden called to offer federal assistance to LA Mayor Karen Bass.

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PRESIDENT JOE BIDEN: We'll get any help on the way as soon as you guys request it, so just let me know. That's why I'm calling.

KAREN BASS: OK. Well, thank you so much, Mr. President.

MARTIN: NPR's Nathan Rott has been covering this storm from Ventura, on the California coast between LA and Santa Barbara, and he is with us now. Good morning, Nate.

NATHAN ROTT, BYLINE: Hey. Good morning, Michel.

MARTIN: So I understand that the president's call came during the mayor's press briefing last night, where she was describing what she saw. Can you just tell us what else she said and what she did see?

ROTT: Yeah. So she had spent the day basically traveling around different parts of LA and looking at various homes and various parts of the city that have been impacted by mudflows. And when I say mudflows, I mean earth that had gotten so saturated with water that it basically turned into this thick, destructive soup. Our colleague Liz Baker visited a home in Culver City - which is its own city in LA - that's backyard is just now totally mud. Here's the home's owner, Ivo Panayotov.

IVO PANAYOTOV: Grass turned upside down. There is actually one tree that is fallen from the mudslide. It looks like a hurricane went through it.

ROTT: So LA Mayor Karen Bass and city officials said that more than 120 debris flows have been reported as of last night just in the city of Los Angeles alone, and we know that there have been many others in the broader region.

MARTIN: Do we have any sense of how much longer these mudslides are going to be a risk for people?

ROTT: I mean, as long as it keeps raining and probably a little bit after, 'cause when I say, like, the ground is saturated here, Michel, I mean, it is wet. You know, downtown Los Angeles got more than 6 inches of rain in less than 24 hours. Parts of the Santa Monica Mountains just north of the city had upwards of 10 inches. So any more rain on those areas could trigger additional debris flows.

MARTIN: And do we have a sense of how much longer it's going to be raining?

ROTT: Yeah, the question everybody wants to answer, right? I talked to a meteorologist at the National Weather Service here last night who said this river of transported tropical moisture, this atmospheric river is weakening, but it's definitely not over yet. It's raining right now outside of my house. And we should continue to see scattered showers and thunderstorms through Wednesday.

But, you know, where I am, about an hour's drive north of Los Angeles, even with the rain, people are definitely feeling like the worst of this is over. I visited a part of Ventura yesterday that had been flooded earlier this week and talked to the owner of my favorite surf shop, which had water standing up to its entrance on Sunday, and they were able to keep it out with sandbags. The store's owner, Bill Hubina, says unless flood infrastructure gets improved in this area, in his opinion, he doesn't think they're going to continue to be so lucky in the future.

BILL HUBINA: This will continue getting worse every year till we really do flood eventually. But it's just Mother Nature, and tides are rising and global warming, basically.

MARTIN: So I heard him say it's global warming, Nate, but is that what's really driving these rains?

ROTT: So this is a very complicated answer, Michel. Look; we know sea levels are rising. We know the coastlines are changing. With atmospheric rivers, it's a little more complicated. Basically, scientists know that human-caused climate change is making precipitation events more likely. Warm air holds more moisture. What - they have not detected a signal of that, though, in the data so far.

MARTIN: That is NPR's Nathan Rott in Ventura, Calif. Nate, take care of yourself.

ROTT: Thank you. Transcript provided by NPR, Copyright NPR.