November 29, 2021

The last atmospheric river in the series. Will Seattle break the November precipitation record?

The third atmospheric river in a sequence will be directed towards our region tomorrow.

Examining the situation, I was impressed by the rate at which this atmospheric river will be moving moisture into our region.  

Specifically, the integrated water vapor transport (IVT), the product of water vapor and wind speed, is predicted to reach extraordinary levels tomorrow morning off our coast (see figure below).  Blue is high.  Purple levels are quite unusual, as is the great north-south extent of this moisture plume.   

It is also coming almost from due west, rather than from the southwest as was true of most of the moisture plumes this fall.

And the moisture/water vapor "river" of this event extends THOUSANDS of miles across the Pacific (see below).  Essentially from the Philippines to our door.

This event will bring substantial precipitation to the regional terrain of Washington and Brtish Columbia, and with the moisture plume's west-east orientation, will bring one of the most extreme rain shadow situations you have ever seen.   I will show you.

The precipitation for the 24 h ending 4 AM Wednesday is predominantly over southwest BC and Northwest Washington, with huge contrasts (see below).  

Virtually nothing over the lowlands from Olympia to the San Juans, as air descends the coastal terrain.  There is one exception...a narrow Puget Sound Convergence Zone feature over Seattle.  Precipitation increases over southwest Washington where there is less high terrain to the west...and thus less rain shadowing.

3-7 inches perhaps on the western side of the mountains of Vancouver Island and the Cascades of southern BC.  The North Cascades and Olympics get a piece of it.

But nothing east of the Cascade crest until the Rockies and Blue Mountains.

Super rain shadow event, while the potent atmospheric river is hitting southwest British Columbia quite hard.  There will be a dry break on Wednesday.

This has been a wet November, but we won't beat any records in Seattle.  So far we have 10.14 inches....very respectable, but 2006 had 15.63 inches.  We won't come close to that.

But we ARE winning the first-place laurels for September through November rainfall, for which we are slightly beating 2006 and some other years (see below).  No wonder lawns are full of moss.


  1. "No wonder lawns are full of moss" -- almost perfect for playing lawn bowls. Shame we need wellies, a long coat and a sou'wester.

  2. I remember the 2006 was dreary.

  3. We have not had this number of atmospheric rivers in such a short time period hitting into the coast,” said Rachel White, a professor at the University of British Columbia who studies how large-scale atmospheric patterns contribute to extreme weather. “The scary possibility is that climate change is making those more likely and more frequent.”

    1. I emailed her about that. She told me she never said that. There is no evidence for that statement...cliff

  4. I will take anything, but long power outages.

  5. Pretty much floating in Birch Bay 😢

  6. Marysville/Everett didn't get the memo that it was supposed to be in a rain shadow today.

  7. All this rain and only 20" of snow up at Paradise Rainier.

  8. Any comments on UV energy levels? Have we been close to breaking records on those as we hit the shortest days of the season?

  9. Any UV data? Have we been close or broken any records as we enter the lowest angle light of the season?

  10. WSU AgWeatherNet station NOOKSACK reports 17.96" for November. Seems about right with CoCoRaHS stations in Whatcom ranging from 14.05-22.08" for November. The prior November record in Whatcom County at the longstanding station CLEARBROOK (CLBW1) was 14.89" in 1909.

  11. What do you think of lowland snow chances after the 10th ensemble models seem to be looking favorable for lowland snow?

  12. waiting for cold weather how long?

  13. It has been very wet, especially north of Seattle. Just for grins, here are some precipitation time-series traces of data from Seattle-Tacoma Int’l. Airport since the start of the 2021-22 water year (October 1). Also included are traces from the past 20 years or so, similar traces at Bellingham (airport) this year, and from several spots in Vancouver.


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

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