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Rain relents, and weather expert shares Puget Sound convergence zone primer

A photographer looks out over the Puget Sound and Mount Baker, some 70 miles distant, Friday, March 13, 2015, at the marina in Edmonds, Wash.
Elaine Thompson
The Associated Press
A photographer looks out over the Puget Sound and Mount Baker, some 70 miles distant, Friday, March 13, 2015, at the marina in Edmonds, Wash.

If you’re tired of all the precipitation that has been raining down on the Northwest since early April, take heart. KNKX weather expert Cliff Mass promises this weekend will be dry and even a bit sunny.

“This weekend we have some really good weather for Passover and Easter," Mass said. "High pressure will be building in, so I don’t expect it will be raining at all over the weekend and temperatures will zoom up into the low to mid-60s. And there will even be some sun. So, a really nice weekend.”

But he warns it won’t last long. Early next week, Mass says moist, southwesterly flow will return, bringing back the much wetter pattern.

“I expect a lot of rain next week, on and off — a lot of springtime weather systems coming through.”  


Mass has some standard advice for people who want to avoid getting wet in the rain: check the radar, because it may be a front or some other system moving through. If you can wait until it has passed, you might stay dry.

And there’s another thing that can be especially useful to know about, especially at this time of year.

“The number one local weather feature here in Western Washington is the Puget Sound convergence zone,” Mass said.


Credit Courtesy Cliff Mass
The Puget Sound Convergence Zone

It’s a phenomenon that produces a narrow band of often intense rain, caused by winds coming off the ocean from the north to northwest. When they hit the barrier of the Olympic Mountains, the air splits and peels around the mountains, then comes together — or converges — on the other side, over Puget Sound.

“When the air comes together, it’s forced to rise," Mass said. "You can’t just pile up air, some of it is forced to rise. And that produces a band of clouds and precipitation, somewhere generally between downtown Seattle and South Everett.”

Springtime is high season for the Puget Sound convergence zone, because this is the season when those winds from the north and northwest are most common. And the band is generally only five to 10 miles wide, with extremely dry conditions on either side of it, produced by the rain shadow effect of the mountains.

“It can be raining and cool in North Seattle," Mass said. "But the amazing thing is it’s generally clear to the south and north."

“So Sea-Tac Airport could be clear, temperatures could be 5-10 degrees warmer than here in North Seattle. And the same thing is true up at Everett and north of there.”

Exactly where the zone sets up depends on the angle that the winds hit the Olympics.

Sometimes, the band of rain extends all the way into the Cascades, producing even heavier precipitation, especially if it reaches the Cascade Crest.

“Over the mountains, it has an extra uplift, from air being forced to go up the slopes of the mountains,” Mass said. “And during the last several weeks, we’ve had a number of convergence zones occurring. And they’ve been getting massive amounts of rain on the western side of the Cascades in the central part of the barrier. Some places, like the Chester Morris Reservoir that Seattle depends upon, they’ve gotten about 10 inches of rain the last few weeks there.”

Mass says knowing about the Puget Sound convergence zone is almost a right of passage here.

“It happens probably 10-20 times a year or so. It’s really good to know that you get this rain band,” he said. “If it’s raining where you are in North Seattle and a convergence zone is going on, just go north and south and you could be in much better conditions.”

You can listen to the full conversation above to learn more about the Puget Sound convergence zone and how long it’s been a known weather feature of Western Washington.

To hear the full conversation, click on the "play" icon at the top of this post.

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.


Mass also helps organize the Northwest Weather Workshop, an annual gathering to talk about Northwest weather, climate and major meteorological events. This year, it takes place May 3-4 at the NOAA Sand Point facility. The event will include special sessions on the meteorology of Northwest wildfires and the cold/snow period of last winter. More information, the agenda and registration information is found here

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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