September 28, 2026

A Significant Atmospheric River Hits the Pacific Northwest

The first significant atmospheric river of autumn has arrived.  

The satellite imagery tonight is impressive.

For example, consider the water vapor image from the NOAA GOES-West satellite.  A magnificent moisture plume from the Central Pacific into our region.



Here is another.

The forecast of the transport of water vapor by the European Center model for tonight at 10 PM is shown below.  The atmospheric river is moving huge amounts of water vapor into the Pacific Northwest, with the center of the plume directed to the northern Olympics and southern Vancouver Island.


The plume of moisture will be forced to rise by our regional terrain, producing substantial windward precipitation.

Below is the prediction of the total precipitation through 5 AM Wednesday (from the European Center forecast).  Southern BC and northern WA get hit hard.



The UW, ultra-high-resolution forecast for the same period shows the details (below).

As much as 3-5 inches on the western slopes of the Olympics and central/northern Cascades. Substantial amounts reach the eastern slopes of the Cascades.  Nice rain shadow over central Puget Sound.  

 
I bet I can get away with biking to the UW tomorrow!

If you had refrained from watering your garden... go ahead.  Probably time to find your umbrella as well. September will end up with normal precipitation for most of the region.
________________________

Attend the very interesting weather lecture for the general public on the UW campus. Open to the general public!

Title: What I Have Learned from Flying Through the Clouds
Speaker: Armin Sorooshian, The University of Arizona
Date: Thursday, October 8, 2026
Time: 7:00pm - 8:30pm
Location: Kane Hall 220

To register use this link:


Abstract

What is it like to fly through clouds in the name of science?


And why should we even fly expensive aircraft filled with scientific instruments in and out of clouds?


Aside from their natural beauty and ability to invoke our imagination, clouds play a critical role in regulating Earth's climate, influencing precipitation, and affecting air quality. A necessary ingredient to make clouds is particles (e.g., dust, smoke). It turns out that aircraft are necessary to really dig into how particles and clouds interact.


This talk will describe (with a lot of visuals) what it is like spending hundreds of hours in the back of various types of airplanes flying in and out of clouds to learn about these complex interactions. Stories and examples will highlight the importance of this type of work for reasons ranging from public health, visibility, and climate change. 


6 comments:

  1. Looks like another case of a large rain weather system barely touching the north of Seattle corridor between Shoreline and Lynnwood/Mill Creek. Is there evidence of a larger rain shadow or has the convergence zone which used to exist broken down? In the past we received higher precipitation than Seattle. Now it’s less. Please explain Dr. Mass.

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    1. Just a bit further north in Arlington we received about an inch of rain yesterday, and this year its above average for the year I would say the westerly flow off the pacific has been a bit more south westerly keeping more convergence happening in Northern Snohomish county this year. Growing up near Mill Creek i remember some epic convergence zone conditions back in the 80s and 90s

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  2. Must be a precursor, we don't usually get these conveyor belts this early

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  3. Yesterday I wanted to comment but could not get the prompt to work. Today I expected similar so didn't conjurer a brilliant thought.

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  4. Is this lecture available online? My son is a private pilot living in Japan, and might enjoy it. (He also uses wind tunnels to test various vehicles, so may find it useful.)

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  5. I have read elsewhere that California receives about a third of its annual precipitation from atmospheric rivers. Is there an estimate of how much Washington gets from atmospheric rivers vs. mid latitude cyclones from the Gulf of Alaska?

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