Get Ready For More Rain As Huge Atmospheric River Is Set To Hit Puget Sound Region, Again

Oct 20, 2017

Stormy weather that took out power for tens of thousands of residents in the Pacific Northwest this week made national headlines as The Washington Post commented on the fascinating “river in the sky” that has been dumping rain on the region.

The heavy precipitation comes courtesy of an unusually strong atmospheric river that extends all the way to Asia. It showed up so clearly on satellite pictures that one meteorologist with the Seattle office of the National Weather Service nicknamed it “the big dark.”

More of that is in store this weekend. Expect lots of rain, especially from Seattle southward and especially on Saturday, says KNKX weather expert Cliff Mass, who teaches atmospheric sciences at the University of Washington.

Chance Of Thunderstorms Friday

Mass says the remnants of a strong front that went through the region Thursday would keep things pretty showery on Friday.

“Now we’re in the unstable air, where we have showers and maybe even a few sun breaks,” Mass said. “But some of the showers could be heavy and there could even be a few thunderstorms scattered in."

Friday’s temps will only reach the low to mid-50s, he says. “So, cooler than we’ve had.”

Big Rain Saturday

Expect a big change in the weather on Saturday, when that atmospheric river again lands on us. Mass says this is the unusually long one and it will dump lots of rain.

“There’s going to be a plume of moisture that stretches all the way from Asia to the Northwest,” Mass said.

He says Saturday morning might not be very wet. “But you’ll notice the temperatures will start warming up a bit,” he said. And then sometime before noon, the rain from that atmospheric river will start.

“Temperatures will warm as this moist, warm airflow from the subtropics comes in,” Mass said.

The precipitation will be fairly heavy, especially in the mountains and south of Seattle.

“Seattle will get plenty – we’ll probably get an inch out of it. But some places in the mountains could get several inches, and the rivers are going to come up,” he said. The National Weather Service had a flood watch in effect for the weekend.

“By the end of Saturday, there could well be three or four inches in the mountains and half an inch (or) an inch here in Seattle,” Mass said. “So, it’s going to be pretty wet. And again, this is from an atmospheric river – this current of warmth and moisture that’s coming across the Pacific.”

He says that current will slowly start to shift southward late on Saturday night into Sunday morning, allowing for lighter weather on Sunday.

Tapering Off Sunday

“Maybe some light rain on Sunday morning, but that’s going to back off so I think maybe a few scattered showers.  But I don’t think it’s going to be anything like Saturday,” Mass said.

He says temperatures will be relatively warm, in the upper 50s or even near 60.

Monday and Tuesday, Mass says a ridge of high pressure will build up, allowing for sun and continuing the warm temps in the high 50s.

“So it’ll be really quite nice Monday or Tuesday before another weather system comes in late Tuesday into Wednesday,” Mass said.   

To hear the full conversation, including a discussion of different kinds of atmospheric rivers that hit here and also how the rapid change from sunny and clear to very wet and often windy is typical this time of year in the Northwest, 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.