The dry Pacific Northwest in July and August is normal for work, but this year has been particularly arid.
Several media outlets have declared this the driest summer for Seattle or the region on record. Is that true?
Let’s find out together.
At Seattle’s SeaTac Airport, This Summer (June 21-September 21) was the driest on a record from the late 1940s (0.50 inches).. 2017 is right behind (.52 inches).
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What about a nearby station with a longer record (Kent), going back to 1912? That is shown below with a trend line. 2022 was the driest summer on record but some came close in the 1925-1945 period. The trend shows a slight decrease (maybe half an inch) over 110 years. Very small long term trend will become important.
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What about east of the Cascades? This is important because of the agriculture there and the frequent wildfires. A different story from the west.
Spokane (going back to the late 1800s) was arid but the other six years were drier.
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And Kennewick, also with a record from the late 1800s, is not arid at all, with 29 more arid years.
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So what can we conclude? Western Washington this summer has been particularly dry. The driest was recorded for many stations. But there is only a minimal tendency to be drier west of the Cascades. Eastern Washington has had a dry year, but it was no exception.
Let me stress, there is only a small trend of drier summers in our region.
That’s important, because if climate change is to blame for the dryness, you’ll see a long-term trend. If an extremely dry summer like this one is due to climate change, the long-term trend will be much drier summers. Not available.
What do climate models suggest? At UW, we are running high resolution regional climate models based on a set of global climate models. This is the gold standard of such work.
Here is the summer precipitation (in this case, June-July-August) projections for Seattle and Pasco driven by a VERY strong increase in greenhouse gases (RCP scenario 8.5) for 1970 -2100. Climate models show VERY little change in summer rainfall through 2022 for both stations. For Seattle, there will be a slight decrease in rainfall by 2100. Almost no change in the Columbia Basin.
So what should you conclude from all of this?
We had a very dry summer in western Washington. But there is little overall expansion, but there is a lot of variation that is probably of natural origin. This is supported by climate model simulations that show little change in summer rainfall even with large increases in greenhouse gases.
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Atmospheric Science 101
Like last year, I’m teaching atmospheric science 101: a general introduction to weather and climate, this fall. You can learn more about the class on class website. I talk about everything from atmospheric basics to weather prediction, thunderstorms, storms, and local weather to climate and global warming.
I will be teaching the class in person at UW, but will also be delivering it via zoom. So people can get it remotely.
If you are over 60 years old, you can join the class via Access Program for a very small fee (about $15). Last year, I had over 125 people do so.
So, if you are a UW student looking to learn about the weather or a student who is not interested in the subject, I welcome you to join me this fall. My first class was on September 29.
