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A weather system known as an atmospheric river hits the West Coast

A MARTÍNEZ, HOST:

While much of the country is still digging out from a historic arctic blast, the U.S. West Coast is facing a different kind of weather challenge, heavy rains. It's part of a weather phenomenon known as an atmospheric river that can drench low lying areas with rain and bring snow to elevated zones, all accompanied by high winds. For more detail, I spoke to Bob Oravec of the National Weather Service and began by asking him exactly what an atmospheric river is.

BOB ORAVEC: It's just a narrow area of high moisture that gets transported away from the tropics towards the higher latitudes. And it's usually ahead of cold fronts. And a lot of times in the winter, it impacts the West Coast and is actually responsible for a good majority of the rainfall during the colder season, which is the season when they get most of their rain.

MARTÍNEZ: So what's been some of the damage and impact from this weather phenomenon on the West Coast?

ORAVEC: Well, a lot of times, it's very beneficial. Obviously, we've all heard the - you know, the prolonged drought along the West - across a good part of the West Coast. So we've had an atmospheric river that just ended recently produce a lot of heavy precipitation, higher elevation snows. And as we go forward over the next week, it looks like we're going to have several more opportunities for additional atmospheric rivers to affect the West Coast, especially the Thursday to Saturday timeframe, when we expect at least some large areas of the Sierra and Northern California to receive over five inches of precipitation.

MARTÍNEZ: But isn't that part of the problem, the drought in that - the ground is so dry that it doesn't actually suck in the moisture, and all of a sudden we've got mudslides and all kinds of awful things?

ORAVEC: Yeah, it's a vicious cycle across the West. A lot of times you get drought, you get fires, you know, you lose a lot of the groundcover that helps retain the moisture. And then when it does rain again, you get the mudslides. So yeah, it's never a nice, gentle rain across the West Coast. A lot of times in the winter, you know, the precipitation events are very heavy. So you do have the threat - when it's not falling as snow, you do have the threat of that rainfall running off and causing some issues with flash flooding.

MARTÍNEZ: But considering that California does need the rain, I mean, should we just kind of take the good with the bad on this?

ORAVEC: Yeah. You almost have to accept that atmospheric rivers are a part of the normal cycle of rainfall. And you definitely need them. You know, we're in a period now where it looks fairly favorable for the next week or so to get, as I said, several atmospheric river events. We may end up with over 15 inches of total precipitation over the next week or so, especially through the Sierra and parts of Northern California. So it's going to make somewhat of a dent in the prolonged drought that they are experiencing.

MARTÍNEZ: Wow, 15 inches. That's a lot of rain.

ORAVEC: Yeah. Well, some of that will be falling as snow at the higher elevations.

MARTÍNEZ: OK.

ORAVEC: But either way, it's very beneficial.

MARTÍNEZ: And one more thing really quick - I mean, this blast has caused major travel disruptions elsewhere in the country. So how badly has the atmospheric river affected holiday travel on the West Coast?

ORAVEC: You know, it hasn't been as impactful as some of the other - you know, we just got out of a very cold pattern across, you know, a huge part of the country. And that was probably more problematic with respect to travel than the atmospheric river. Obviously, any time you get rainfall, high winds that affect any airport, you're going to have issues with travel.

MARTÍNEZ: Yeah.

ORAVEC: But the whole - overall scope of it, it hasn't been as large as what occurred across the central and eastern U.S. region.

MARTÍNEZ: That's Bob Oravec, senior forecaster at the National Weather Service.

(SOUNDBITE OF MOONCAKE'S "CAST THE ROUTE") Transcript provided by NPR, Copyright NPR.