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:

https://forms.gle/S5xWHUWhrqzexKtH9

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. 

How Unusual Was this Summer's Weather?

Now that we have entered fall, it is time to evaluate whether our summer weather was unusual.

Are summers getting warmer or drier in Washington State?

Starting with the overall view for the entire state, below is the July 1-Sept 26 precipitation difference from normal.  

A complex pattern.  The western slope of the Cascades (where there was little fire) was drier than normal, while the eastern slopes (with more fire than normal) were WETTER than normal.  

The majority of the state was near normal for temperature (light green or yellow below), with a few areas a bit above normal 

In short, not an exceptional summer period for temperature and precipitation.

What about the long-term trends in summer temperatures?   

Average daily temperatures have risen over the past 70 years at Hoquiam, Olympia, Yakima, and Spokane by about 2-3.5 F over the period.

In contrast, the trend in daily high (max) temperatures has been very small west of the Cascades, and about 4F at Yakima and 1°F at Spokane.

Finally, for precipitation, there is modest summer drying in the west and a small downward trend east of the Cascades.   But there is very little precipitation during the summer in any case.

Fortunately, there is no overall downward trend in annual precipitation, so the region should be ok with its water supply.

The bottom line of this blog.  

This summer was not that unusual for temperature and precipitation in Washington State (but there was an unusual lightning event in July).  

The region is slowly warming, although high temperature extremes are hardly changing.  Annual precipitation is steady, but our dry summers are getting slightly drier.