December 09, 2010

Atmospheric River on the Way

Friday Afternoon Visible Satellite Photo---you can see the Pineapple Express already forming!

We have had quite a bit of heavy shower activity over the lowlands as well as snow in the mountains, but a period of heavier rain, particularly in the mountains, is on tap for later this weekend.

The first part of Saturday will actually be quite decent, but the action starts later that day as an unusually strong and well formed warm front pushes north across the region (see graphic).

Behind the warm front there will be a shift to strong, warm ,moist southwesterly flow that is connected to the subtropics--known as an atmospheric river...or as we call it here, a pineapple express.

This figure shows the amount of moisture in the vertical for Sunday at 4 AM. You can clearly view the narrow plume of moisture associated with the atmospheric river.

This strong plume of moisture will intersect our regional mountains, dumping substantial amounts of precipitation, particularly over the Olympics and N. Cascades.
Here is the 24-h rainfall for the period ending 4 PM Saturday....when the plume of moisture is mainly south of Washington. For those living in Washington State, if you want to do something outside over the it before 3 PM on Saturday!

And here is the next 24-h. Serious precipitation over the Olympics and N. Cascades, with some amounts reaching 5-10 inches. A clear flood threat.

At this point, this event is no record-breaker, but it will bring serious rain and much warmer temperatures. Rain will fall on all that nice fresh snow in the mountains, SERIOUSLY contributing to a significant avalanche threat later in the weekend. Backcountry travel should be avoided then.


  1. Cliff, I hope you will weigh in on the significance, or not, of the fact we've had such a wet calendar year. I see at KSEA we're 6.5 inches past normal precip and that we passed last year's total some time ago.

  2. Thanks for the heads up Cliff! I need to go get some wellies,,,

  3. Ah, those of us who farm directly on a river (Nooksack, in my case) can always see this coming: November cold temps & snow bring on a pineapple express and rising waters in December. Or swap out the months for Dec/Jan, Jan/Feb ~ whatever!

    Love this blog, btw, and your book as well.

  4. My take on this winter is get used to this atmospheric river showing up.

  5. The Hawaiian Islands are just get inundated with heavy convection, the start of this river. You can see it well on Satellite.

  6. I was going to go flying Saturday afternoon, but now I think I'll just sit tight. Your weekend forecasts are far more interesting and insightful than any of the official weather products put forth by official aviation channels. (though I always check those as well)

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  8. We are getting the rain here in Hawai'i. The system is strong in Kona, but a nice light constant rain here in dry Ka'u.

  9. So the real question here, Cliff, is should I risk going out to Duvall (Snoqualmie River area) for my birthday with family when I have a final in Seattle Monday afternoon? :)


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