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How extreme heat in Eastern Washington can cause cooler temperatures in the west

A view of Seattle's ship canal on July 10, 2020.
Tim Durkan
Tim Durkan Photography
A view of Seattle's ship canal on July 10, 2020.

The first week of August is generally the hottest time of the year in Washington. This year, people in the Puget Sound region already have experienced some record temperatures, with highs topping 90 degrees for the first time in 2020 on Monday. Olympia reached an eye-popping 98 degrees. Seattle made it to 94.

But a cooling trend that will continue through Monday has started, says KNKX weather expert Cliff Mass.

"Temperatures are going to drop back down to the lower to mid-70s,” said Mass, adding that the weather disturbance causing this may also bring a few sprinkles along with partly cloudy skies. “So you’ll really know the difference on Monday. And then later in the week, we'll slowly get back up into the mid to upper 70s.” 

Still, for as hot as it has been around Puget Sound this past week, it’s truly mild compared to what is happening in Eastern Washington right now. That’s due to the influence of marine air on the west side of the Cascade Mountains.

“Exposure to the ocean and Puget Sound keeps the temperatures down here in Western Washington. But that's not true in Eastern Washington,” Mass said. “During the last several days, they've had temperatures that have gotten up above 90 degrees — in fact, on Thursday they got up to around 105 in a number of locations.”

Mass says the reason they get so warm is usually the combination of high pressure aloft and sinking air. The air compresses and warms as it falls.


“And so what they develop is something called a ‘heat low’ or a ‘heat trough,’” Mass said. “This is a narrow area of low pressure that's found on the east side of the Cascades."

He says these small patches can have very big impacts.

“Not only is it associated with warm temperatures, it also has very low humidity,” he said. “And then at the periphery of this heat low, you can have strong winds because there's big differences in temperature. This can be quite dangerous if you have the combination of warm air, low relative humidity and strong winds that can really encourage wildfires."


Ironically, having warm temperatures and low pressure east of us can actually cool us down, Mass says:

“Winds tend to be pushed around by pressure. And so when you have high pressure over the ocean — as we generally have the East Pacific High — and then we have low pressure in Eastern Washington, that tends to push marine air into Western Washington."

And in fact, this has happened several times this summer already and it's going to happen again over the weekend, as that marine air comes into us, Mass says.


The influence of marine air also explains some of the pretty amazing temperature variations you can get on the west side, from one locale to another.

“We have startling differences in temperature. So if you're a little bit hot and you're in Seattle,” Mass said, “go to the water.”

He says on very warm days, you can experience a drop of as much as 10 or 15 degrees.

And it’s not just the ocean or Puget Sound. Even Lake Washington will be a cooler place on a hot day, because Northerly winds known as the "Sound Breeze" develop there in the afternoons.

“And if you're near the water by Lake Washington, you can feel that cooler air coming down,” Mass said.   

Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX environment reporter Bellamy Pailthorp. Cliff Mass is a University of Washington professor of atmospheric sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.

Bellamy Pailthorp covers the environment beat for KNKX, where she has worked since 1999. From 2000-2012, she covered the business and labor beat. Bellamy has a deep interest in Indigenous affairs and the Salish Sea. She has a masters in journalism from Columbia University.
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