What exactly is an atmospheric river?

You may have seen the phrase "atmospheric river" thrown around a lot recently about floods in the Northwest, but what does it really mean?

Transcript

00:00 Now we've been talking a lot about atmospheric rivers, but what exactly does that mean?

00:04 Let's head to Bernie Rayno to find out more.

00:06 Well in tonight's WeatherWise segment we want to talk about what's occurring in the northwest,

00:12 what's called an atmospheric river. Now listen, it seems like a lot of things are called atmospheric

00:18 rivers these days, but this is a bona fide active atmospheric river that we're getting in the

00:23 northwest tonight. It begins with the surge of moisture. Now we have it showing in the Pacific,

00:27 you get atmospheric rivers all over the planet, where you have a moisture being transported from

00:33 one location to another, in which case we're showing you in the western U.S. Oftentimes you

00:39 get these surges of tropical moisture coming in from the tropical Pacific all the way into the

00:44 inland area. It's a wind flow over the ocean that can span thousands of miles, by the way,

00:51 and when you can get that wind flow focused, not a large area, but a focused area of moisture,

00:59 those bands of extreme high moisture are called atmospheric rivers. The stronger the winds,

01:04 the stronger the atmospheric river. Now here's the other thing going on, especially in the west,

01:08 whenever you can get this moisture being forced inland, what ends up happening is when you have

01:13 terrain, it is forced to rise, what we call orographic lift, along the western side of the

01:18 mountains, and with that you get upward motion and you get the intense precipitation to become

01:24 even more intense. Here's the other key, most atmospheric rivers, almost all of them,

01:28 are also accompanied by a surge of tropical moisture. So snow levels, while you do get

01:33 snow in the mountains, they are typically sky high and flooding is always a concern. If you think

01:39 about it this way, in an atmospheric river, the moisture isn't spread out, it's consolidated

01:44 into a single band. Case in point, take a look at our current atmospheric river. You notice it is a

01:51 long area of precipitation, but it's not all that wide and all of this moisture is focused

01:59 into the northwest and that's why we're worried about flooding.