What is an Atmospheric River?

You may have recently heard the term "Atmospheric River" and wondered what exactly is meant by that.  I won't get too technical so I don't bore you.  But I thought a short blog would help some curious readers.

First, an Atmospheric River (or AR) is a relatively new term in the field of meteorology.  Research aircraft flying through Pacific storms off the West Coast helped to identify this phenomena.  And actually, it's rather common.

Here's an excellent satellite image of the Pacific Ocean.  This image doesn't show clouds like most satellite pictures, but rather it shows moisture in the atmosphere.

A couple of things to point out in this image.  The vast majority of moisture in the atmosphere is found over the tropics near the equator.  This is indicated by the orange colors.  But you'll also note strips of moisture emanating from the tropics up into the middle latitudes (light blue and yellow colors).  These are the atmospheric rivers.  As storms come off of the Asian continent, the southerly winds ahead of the storm pull moisture up from the tropics in a narrow plume configuration.  In the image above, you can see three ARs in the Pacific and one in the western Atlantic.

As I said, these are rather common.  But the use (or overuse) of the term by the media has come to imply that ARs are bad.  Some of the stronger ARs do cause flooding.  But the weaker ARs are actually beneficial and bring needed rain to the west coast of North America.  Scientists have actually come up with a ranking of ARs to better denote the beneficial from the hazardous.

Our upcoming rain event is an example of a moderate-strong AR (4 on a scale of 1 to 5).  Below is an image from a computer forecast model that helps to visualize this AR.  The center of the storm is in the Gulf of Alaska.  The AR extends from the tropics into the Pacific Northwest.

Because this moisture is coming from the tropics, it's also warm.  As such, it brings warmer temperatures into our area.  Snow changes to rain and the snow levels rise up to or above our mountain tops.  The duration of the event is usually key.  A short event (1-2 days) often doesn't cause much flooding in the Inland NW (like this event).  A long duration event (3-5 days) often causes the majority of our flooding events.

It still looks like it's going to rain Monday evening through Wednesday evening when a cold front will sweep through the region.  2-3 inches of rain are expected for northeast Washington.

After that, we could see a little snow on Thursday.  There's a better chance of snow on Saturday, with 1-2" possible.  Next week in general has a low chance of rain or snow each day.