The Second Atmospheric River Will Bring Far More Severe Impacts

In act one of this meteorological drama, yesterday and this morning brought heavy rain and flooding to many sections of western Washington.  Well-predicted days in advance.

But the weather prediction models indicated that this would be a two-act event, with the second act being far more serious.   The curtain is about to rise.

Take a look at the precipitation totals of the first act (last two days, below).  Some areas received 7-8 inches.

Many local rivers are now at flood stage, with some at record levels (black dots are record levels, blue are 90th percentile or greater, with normal being 50th percentile)

The media is full of pictures of flooded roads, such as in the Snoqualmie Valley.  A once-in-1-3-year event.

But this is just the meteorological appetizer course.  The real action is about to begin:  a second, stronger event that will throw massive amounts of rain on saturated surfaces.

Consider the total precipitation from the UW ultra-high resolution modeling system for the 72 h start this morning at 4 AM (shown below).

Wow.  There are going to be some happy ducks.  Large areas will get 10 inches of rain or more, particularly in the Olympics and northern/central Cascades.

Serious flooding is inevitable.   

How can I be so sure?  Because of the use of ensembles of many high-resolution forecasts.  If they are all on board with heavy rain, my confidence grows.

Let me demonstrate this to you.   Here is the ensemble of accumulated precipitation at Humptulips on the southwest side of the Olympics.  A huge amount of precipitation (about 6 inches), most over about 12h.   Not much variance in the forecasts.

Stampede Pass in the central Cascades (below)?  A similar soggy story.

Every reservoir in the region will reach normal or above normal levels.  Consider the Seattle Times' favorite drought reservoir system (the Yakima), which they featured in a recent story.

The Yakima River is now at flood stage (see below), and in a few days the entire system will be full to normal levels.

The Torrent Has Begun: The Philippine Connection

The predicted rain has begun.  Rain associated with a potent atmospheric river, with some of the water originating thousands of miles away near the Philippines! 

Consider the situation on Sunday afternoon, as shown by special satellites that observe atmospheric moisture from space (below).  

You can see the plume of atmospheric moisture extending from the western Pacific to the Northwest.

But it is not quite that simple.  There is substantial three-dimensionality to atmospheric rivers.

To prove this to you, below are air trajectories--the three-dimensional motion in space--of the air reaching the Cascades at 10,000, 5000, and 2000 meters above the surface.   At high altitudes, the moisture is coming from the western Pacific, but at lower elevations, the moisture is coming off the Pacific to our south.

Atmospheric rivers are like real rivers, with tributaries contributing moisture along the way.

The big precipitation has started.  Below are the precipitation totals for the past 24h.  

 A number of locations have already received 3-4.5 inches.   And more is on the way.

Our rivers are rising very rapidly.  For example, the levels of the Cowlitz River in SW Washington are zooming upward (see below).   Soon it will be at flood stage.

Remember...this is a two-step event...and this is only the first part!

The Seattle NWS Forecast Office has FINALLY put out flooding warnings over large areas of western WA (see below).  Quite honestly, they waited way too long to do this.  

This is a serious event, and people need time to prepare.