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'Super rain shadow' and tornadoes off the coast showcase special Northwest weather

Tim Durkan
Tim Durkan Photography
A sunset and cloud formation over the Olympics, as seen from Seattle, Nov 1, 2019.

Washington’s mountain ranges not only produce spectacular vistas and recreational activities year-round. Their location near the Pacific Ocean also creates a variety of unique weather features that add to the special atmosphere in the Northwest.

“You have to love the meteorology of this region. I certainly do,” says KNKX weather expert Cliff Mass, who teaches atmospheric sciences at the University of Washington and wrote a popular book on the subject.

Mass says this week, the Olympic Mountains created a "super rain shadow" – as a weather system came in off the Pacific and pelted areas west of the Olympic Crest, while those mountains sheltered people on the eastern side.

“We had this huge amount of rain on the southwest side – the windward side of the Olympics. Some places getting as much as 9 or 10 inches of rain over the last few days,” Mass says. “But then, on the other side, on the lee side, the northeast side of the Olympics, there’s been hardly any rain at all.”

The contrast is amazing.

“Some places, such as south of Port Townsend, have only had a few hundredths of an inch of rain, while there’s a thousand times more rain on the southwest side of the Olympics,” he says.

Mass says this also happened in the Cascades, though a bit less dramatically – with 4-6 inches of rain on the western side of those mountains and much lighter rain in Eastern Washington.


Along with all the winter rain, we do get small tornadoes – as we did this week.  Mass says our proximity to the ocean plays a big role in producing twisters here.

Over water, they’re called water spouts. There was one reported this week near Manzanita, Oregon. And at about the same time, Mass says, the weather service radar picked up a circulation approaching the Washington Coast that was a potential tornado.  

“This time of the year, the ocean is still relatively warm – the Pacific is in the upper 40s, around 50,” Mass says. “And when a slug of cold air comes off of Canada or Alaska or Asia and it goes over that warm water, that causes a large change in temperature with height, which makes the atmosphere very unstable.”

Instability produces thunderstorms and it can produce water spout or tornadoes when the conditions are right. Mass says that happens a few times a year, when there’s a strong interaction with terrain, such as the Puget Sound Convergence Zone or “some feature that helps spin things up,” he says.

“But we tend to not get the really strong ones because our thunderstorms are not very strong,” he says. “But we can get the weak ones, the F-0s or the F-1s.”


More rain. “Every 24 hours something is coming in ... One system after the other," he says. "With temperatures getting up to around 50 every day.” Winds along the coast reaching 40-50 mph Saturday, with lighter gusts inland.

This weekend, these systems will tend to be heavier in the morning, with some clearing in the afternoons, both Saturday and Sunday.  

Next week: more of the same.  

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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