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 east 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:
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.
Yep- Central north sound got shafted on rain. Could you please explain why sometimes south Snohomish County gets a convergence zone with MORE rain than the surrounding area but not this time? Seems to me that an atmospheric river coming from the WSW is about right for a north Seattle-to-Everett convergence zone.
ReplyDeleteThank you Dr. Mass. Fascinating phenomenon with the rain shadowing. With two sets of high mountains (Olympics and Cascades) Seattle and central Puget Sound are boxed in on both sides. The winds appear to be the determinant of the rain shadowing vs. convergence zone. A direct Westerly flow largely blocks rain from reaching Seattle to Everett. Explains the “misty” rain we are famous for here.
ReplyDeleteThat was the trend most of this Summer. The WSW and WSE flow that we get in winter must setup the north end convergence zone that is right along the King-Snohomish county line and now further north in the county. Also need a wraparound northerly flow to really slow down the fronts where it dumps rain or snow.
And I spoke too soon yesterday.
Rain picked up in the afternoon for several hours. Lynnwood and Mukilteo received over 0.50 inches in 24 hours (0.7 inches where I live). Very low rainfall Seattle, southward with more north sound. Great post!
I read about the oncoming "atmospheric river"; but experienced almost zero rain here at my home in the Phinney Ridge area. I wondered what happened and figured it was some kind of "Super Rain Shadow" Much as I despise all these newfangled terms to describe weather - I suppose someone is going to come up with something like the "atmospheric river", "convergence zone" and other terms that the scientists have come up with to describe weather in our area.
ReplyDelete"Everwet Everett?" While we have the most impressive rain shadow, do we also have the most impressive convergence zone...
ReplyDeleteYour article made me wonder how global warming might affect our rain shadows. Based on your explanation, I assumed less rain would fall on the western side because the air is a couple degrees warmer. Interestingly I found a research letter, based on models, that indicates warming will increase precipitation on the lee side (e.g. eastern Washington). Link: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023GL107298
ReplyDelete