The Most Impressive Rain Shadow in the Nation!

When air approaches a mountain barrier, it is forced to rise and, in doing so, cools.  As the air cools, it has less ability to "hold" water vapor, and eventually the air becomes saturated, producing clouds and precipitation.   

On the lee side of the barrier, the air sinks and warms by compression, causing the relative humidity to drop, causing clouds to evaporate and ending any chance of precipitation

This dry area on the downstream side of terrain is called a rain shadow.

 Washington State is "blessed" with world-class rain shadows...one on the downslope (eastern) side of the Olympics and the other to the west of the Cascades.

According to all the AI queries I made, Washington has the most profound rain shadows around (see sample below).

This week was a great example.   

On Monday, there was a potent atmospheric river of massive amounts of water vapor  approaching Washington from the west.

As a result, the western side of local terrain experienced heavy precipitation, but the mountains produced a profound rain shadow downstream (see the precipitation totals over the past 72 h below).

3.5 inches on the windward (western) slopes of the Olympics and 0.04 inches on the lee (northeast) side of the barrier.  Wow.

That's roughly 85 times more on one side than the other.  Mama Mia!  That is a spicy rain shadow.

Here is a close-in look at the rain shadow action.    No wonder some folks like living in Port Townsend and Sequim! 

On Tuesday morning, the rain shadow was very apparent in the visible satellite image:

And the weather radars showed the profound impacts on rainfall.  Very nice rain shadow.

Some precipitation is coming in on Friday, but most of the precipitation and rain shadow action during the next few days will be over British Columbia:

Be proud.... one online source event even provided us with a medal!

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You are invited to attend a 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).

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

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.