November 25, 2021

Atmospheric River Update

The first of three plumes of moisture (atmospheric rivers) is upon us and moderate rain is currently observed along the coast (see radar at 8:30 AM).  If you are living in the interior of western Washington, NOW is the time to head out for some Thanksgiving exercise.  Rain will move in later this afternoon.  If you are in Oregon, you have the whole day.

This first plume of moisture is associated with the first "atmospheric river" in a sequence of three. You can see the "river" in the satellite-based atmospheric water vapor image shown below. Lots of water vapor over the tropics, with thin tendrils of moisture heading into the midlatitudes, including one approaching our region. As I have explained previously, when such moisture is forced upwards by our terrain, moderate to heavy rain results.

The media has recently discovered this term (atmospheric river), often hinting (or stating) that they are deadly or unusual.    

Let me clarify things.  Atmospheric rivers are normal features of the earth's atmosphere that have been known for a very long time, but were previously called other names (such as the warm sector jet or water sector moisture plume).   The image above has six atmospheric rivers, and a similar image over the Atlantic (below) shows two.
Like anything else, atmospheric rivers vary in strength, location, and longevity.  And if even a modest one gets stuck over a region for a period of time, unusually heavy precipitation totals can occur (such as happened earlier this month over Northwest Washington and southwestern British Columbia.

Let's examine the latest forecasts for our region.  The predicted accumulated precipitation over the 24-h ending 4 AM Friday....the total of the first atmospheric river...... is shown below.

The moisture is coming in from the southwest, so the western slopes of the Olympics, Cascades, and Vancouver Island mountains get quite wet, with totals reaching 2-3 inches.  But Puget Sound and Portland are rain shadowed, generally getting less than an inch.   But if you really want to be dry, head to the lower eastern slopes of the Cascades, where the descending flow will prevent any rain.  

Yes, you can stroll around Vantage or Yakima without a worry about getting wet.

A closer view reveals the profound rainshadow to the northeast of the Olympics.   Wow.  A spot south of Port Townsend should be essentially dry for the period.  No wonder so many folks retire there.  Palm Springs in western Washington.

Friday afternoon and Saturday morning should be dry over the region.  An opportunity to work off some of Thanksgiving excesses.

The next front/atmospheric river will spread precipitation over the region Saturday night and Sunday.  

To illustrate, here is the 24-h precipitation ending 4 PM Sunday.   More precipitation than the Thanksgiving storm, with 5-8 inches at some mountain locations.   But again, profound rain shadowing, particularly over Yakima and Portland.   And unfortunately, little precipitation in California where is it acutely needed.

What about the real rivers?

Below is the latest flooding forecast from the NOAA/NWS river forecast center.  The biggest predicted problem will be the Skagit River, where a moderate flood is predicted, with a minor flood on the Nooksack.

To give you more insight into the situation, below is the river stage level and discharge rate at the Skagit River near Mt. Vernon.  Time is on the x-axis and the current time is shown by the vertical line.  You will notice three peaks in the river level, the first on Saturday from today's atmospheric river and then a larger peak on Monday from the weekend event.   The red horizontal line shows the major flood level..which is barely reached.   We are not going to be close to the November13-16th event, which is also evident in the plot 

The same is true of the Nooksack River, which caused so much trouble recently.  The second event will bring things up to a major flood level, but not nearly as high November 16-17.  Can be thankful for that.

Enjoy Thanksgiving.  I have to start cooking!


  1. I mean, they're not wrong that atmospheric rivers can be deadly. At least 5 people died in this November's.

  2. Yeah 5 killed in mudslides north of the border here. Vancouver has been essentially cut off by road and rail from the rest of Canada. Glad to hear this next series of heavy rainfall events will be closer to a normal soggy late November!

  3. didn't we use the term pineapple express before atmospheric river?

    1. Yes, but it offended the pineapples so the name changed.

    2. They still use it sometimes, especially if it actually comes out of Hawaii. As I understand it, Any moisture plume from beyond Hawaii is an atmospheric River, when it actually originates from Hawaii, it's a Pineapple Express.

      I think it's done to differentiate the two. Also, I think the main difference is, a Pineapple Express will also be mild, like reaching up into the 60's during the event.

    3. Seriously, I think "Pineapple Express," linking Hawaii to the PNW, is a special case of the more generic (and scientifically correct) term "Atmospheric River."

  4. Cliff,
    I truly appreciate your weather analyses and agree with your post-mortem about the Abbotsford (Sumas Prairie)flooding. Dyking and then draining low-lying land is asking for trouble.
    The suffering that the recent extreme weather events cause is getting politicians attention:

  5. Professor Mass, you've talked about varying forecasts and uncertainty, but here you point to a forecast of river levels as if it's fact.

    The truth of the matter is that the forecasts of the river levels is based on an educated guess, and for the most part is correct, but in the last event the forecast for skagit river level at mt vernon ranged from minor flood to 41', which would have been nearly 4 vertical feet over the previous record. It ended up at nearly 37 feet, inches from the record.

    Summary? Take the river level forecasts as a general estimate of where the water level will be, but recognize that there is an error bar, and the real level can be quite a bit over or under the estimate.

    as a farmer whenever I see "minor flood" on my local rivers I do the things I do for a major flood, and about 1/3rd of the time i'm glad I did.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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