Amazing Murk, Very Wet on the Cascade Slopes, and an Unusual Atmospheric River

The murky, wet conditions during the past few days are some of the worst I have seen in years for February in our region.  Unbelievably dark and wet.  For example, below is the view from the SpaceNeedle PanoCam around 9:30 AM today (left) compared to four days ago.   It was like the city was not there.

And, of course, there is the heavy rainfall along the western slopes of the Cascades that is producing flooding and landslides.

The origin of all this water is a moderate atmospheric river that is coming from an unusual direction.

An atmospheric river is a narrow current of warm, moist air from the tropics and subtropics that extends northward into the midlatitudes.    A NOAA satellite two days ago, shows the tongue of moisture passing northwest of Hawaii and then bending eastward toward the Northwest.

A model simulation of the water vapor added up in the vertical (known as column-integrated water vapor) for 4 PM Wednesday shows the tongue of moisture reaching our coast from a west-northwest direction.

This is somewhat unusual, with most atmospheric rivers in our region coming from the south or southwest.

Such an orientation to the atmospheric river (roughly east-west) has major implications for us:  it tends to put bountiful precipitation over the western side of the Cascades. 

In fact, the last time we had a major atmospheric river from this direction was in January 2009, an event that almost took out the Howard Hanson Dam.

The Howard Hanson Dam located east of Tacoma

Why is a west-east or northwest-southeast atmospheric river orientation so potent for producing heavy rain on the windward (western) side of the Cascades?   

Because it is associated with westerly wind (wind from the west) that moves rapidly up the mountains, cooling and releasing the large amounts of water vapor in the atmospheric river.

Consider the diagram below.  If an atmospheric river is from the south, with attendant southerly winds, there is not much upslope flow since the air is running parallel to the terrain.  But a westerly atmospheric river has westerly winds that move directly up to terrain slope, condensing massive amount of water vapor into liquid water.

To put it another way, imagine you are on a hike.  If you walk straight up hill, you work hard and gain lots of elevation.  But turn 90 degrees and walk parallel to to crest line, it is no effort at all (assuming you don't bang into a tree, of course).

With a westerly atmospheric river, there is not only LOTS of precipitation on the western side of the Cascades, but a rain shadow downstream of the Olympics and on the eastern side of the Cascades.

Want proof?  Here is the National Weather Service precipitation analysis for the 24-h ending 4 AM this morning.  Huge precipitation contrasts with a half foot on the western side of the Cascades.

The western slopes of the Cascades have been getting hammered with a large volume of precipitation.  A number of rivers are flooding now, with water over nearby roadways.  Be careful.

The good news:  a major change is in store, with dry conditions on Sunday.