The Atmospheric River Event is Ending--- And ANOTHER One is Predicted

The heavy rainfall is over, but some rivers are still rising.   The most impactful atmospheric river event of the past few decades.

Consider the 5-day rainfall totals below, and be prepared to be impressed.  

Many of the windward slopes of the Cascades and Olympics received more than 10 inches, with some experiencing 15-17 inches.

You will also notice extreme rain shadow areas in the lee (east) of major barriers, which received less than an inch.

Numerical weather prediction models were STUNNINGLY good, as shown by the predicted 5-day precipitation totals by the UW WRF model made last Sunday (see below).  

Close correspondence between predicted and observed rainfall.

The weather prediction world has changed dramatically during the past decades, with virtually every major extreme weather event predicted days before.

The problem is that governments, local agencies, and others are not sufficiently taking advantage of these skillful forecasts to save lives and property.  The media do not understand the profound impact of improved weather prediction technology.  

This problem is compounded by the National Weather Service, whose forecasts and warnings have declined substantially compared to the state of the art.  

 I can provide a half-dozen cases of the NWS failing to provide timely warnings even when model forecasts are excellent, such as this event and the Seattle ice storm of December 2022.

I hate to reveal this, but  ANOTHER significant atmospheric river/precipitation event is in the forecast.

Consider the predicted 72-h precipitation total ending 4 AM on Thursday.  Yikes...pretty wet, with some mountain areas getting 10 inches.  This is problematic, falling on saturated soils and rivers already well above normal.

To address the reliability of this forecast, one should consult ensembles of many forecasts.  Below are plots of forecast precipitation at Stampede Pass in the central Cascades from many forecasts.

All are going for a major precipitation event starting in the middle of next week.  You can take this to the bank.

All of this is a reminder that the most impactful severe weather of the Pacific Northwest is flooding.  Not heatwaves, not wildfires, not windstorms.

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.