"Atmospheric River" Soaking Pacific Northwest

Satellite view from yesterdayof a long narrow band of clouds
stretching from the Philippines to the Pacific Northwest. 
That's the atmospheric river dumping torrential rains and
causing flooding and landslides today in 
Washington and Oregon 

 As many of us know, it's really rainy during the winter in the Pacific Northwest. 

The first half of this week, though, is a super soaker out there due to an atmospheric river off the Pacific.

An atmospheric river is a relatively narrow plume of deep moisture that crosses large sections of oceans, and dump heavy rainfall wherever they make landfall.  They're called atmospheric rivers because they act like real, land based rivers.

The amount of moisture in these atmospheric rivers is often equivalent to the water flow near the mouth of the Mississippi River.

These atmospheric rivers usually have their origins in the tropics. The one hitting the Pacific Northwest has its origins near the Philippines. 

The more intense atmospheric rivers can cause catastrophic flooding.  There was a nasty one that caused record rainfall and dangerous, fatal landslides in southeastern Alaska earlier this winter, for instance. 

The atmospheric river now hitting the Pacific Northwest seems pretty impressive, too, so it is or will cause I  plenty of problems with flooding. And landslides. 

Remember I said that atmospheric rivers have their origins in the tropics.  Which means these are often quite warm.  In this case, snow levels in the Cascade Mountains will rise to as high as 8,000 feet above sea level. This will create melting snow, which will feed runoff into already flooding rivers.

Higher elevations are also prone to avalanches, as rain or heavy wet snow falling on top of drier snow layers can start the snow sliding down steep mountainsides. 

Even more dangerous is the risk of landslides which is often a problem in the Pacific Northwest. It's already been a rather rainy winter up there, even by their standards. Soils are saturated and heavy, and the torrential rains with this atmospheric river will make things worse. 

Most of the moisture from the atmospheric river will get wrung out in the Cascade Mountains and in the Rocky Mountains of central and northern Idaho. 

 But the strong jet stream that organized this atmospheric river will continue eastward, contributing to expected high winds today and especially tomorrow in most of Montana, much of Wyoming and the western Dakotas.

Winds could gust to 70 mph or more in some of these areas. 

The jet stream will buckle over the eastern Pacific Ocean in the next couple of days, shutting off the atmospheric river and turning the rain much lighter and more showery in the Northwest starting tomorrow.