People in the Pacific Northwest experienced wet but weirdly warm weather on Thursday as rain generated by an atmospheric river came through. These plumes of moisture are a classic feature of late fall and winter weather on the West Coast, and are also known around here as "The Pineapple Express" because they originate in the tropics near Hawaii. And they can bring in unseasonably warm temperatures.
The region felt a classic example of it on Thursday, says KNKX weather expert Cliff Mass, who teaches atmospheric sciences at the University of Washington.
“We had winds coming from Hawaii. We had an atmospheric river that moved northward into our region. It was startlingly warm, temperatures getting into the low 60s, it was breezy. I mean, you could have been on the big island of Hawaii,” Mass said. “Just fascinating.”
Also noteworthy, Mass says, was the way that the rainfall was much heavier on the western side of the Cascades than in Puget Sound, because of the angle at which the atmospheric river came in – from the west. So Puget Sound was in the rain shadow of the Olympic Mountains.
Rain In Spurts This Weekend
Much of this will continue in the days ahead, says Mass, with residual rain and showers coming in, especially in the convergence zone, north of Seattle.
“We’re going to get some rain over the weekend in spurts," Mass said. "It’s not going to rain the whole weekend.”
Mass says Friday's wet start will fade out during the day.
“So, there’ll be some clouds, but not such a bad day. Temperatures getting up into the upper 50s,” he said.
On Saturday, the day will start out generally dry, Mass says.
“But then a warm front is going to start moving towards us. And I think the rain will move in later in the day,” he says, with temperatures again in the mid-50s.
Rainiest Saturday Night
Then the phenomenon of the atmospheric river comes back into the picture, Mass says. A relatively strong one will hit the region Saturday night and Sunday morning.
“And that’s going to bring a lot of rain on the western side of the Olympics and the western side of the Cascades.”
The reason? This one will again be coming in from the west.
“The air’s going up on the west side of the Olympics, it’s going to descend over us, we’ll be a little bit drier, and then it’s going to go up again on the Cascades, so you’re going to get a lot of rain on the western slopes,” Mass said.
On Sunday, after a wet morning, Mass expects the rain to taper off into showers.
“The plume of moisture will come through and we should be pretty decent," Mass said. "So, it’ll be on and off.”
He says expect those on-and-off showers to continue Monday through Wednesday.
“So we’ve definitely turned the corner. We’re getting some rain and we’re getting these atmospheric rivers that rev up the precipitation,” he said.
To hear the full conversation, including more detail on how atmospheric rivers work and how they’re expected to change with global warming, you can 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.