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Region will be ‘stuck in perfect weather’ as record-breaking jet stream loses strength

"Cloudy and wet" has been the weather mantra so far this July. But KNKX weather expert Cliff Mass say all that is going to change this weekend.

Seattle residents got doused with a half-inch of rain on Thursday, Mass says. It came in the form of a Puget Sound convergence zone, a narrow band of heavy rainfall that forms after currents of moist air from the west are split by the Olympic Mountains, and then come back together again. The zone set up from Seattle northward and dumped so much rain that it nearly amounted to the typical average for an entire month this time of year, Mass says.

“It was really amazing, but that’s going to fade now,” he said. “Everything’s going to change."


Mass says high pressure will develop over the region for the next day or two. That will warm things up and dry them out. He expects the showers to be over by the end of the day on Friday. With temperatures rising into the mid to upper 70s.

Saturday will be even warmer, with highs around 80. There will be some clouds in the morning, but plenty of sun by the afternoon. Mass says the forecast will be similar on Sunday, with temperatures inching up a bit more, to 81 or 82.

“So, definitely an improvement,” Mass said. “And then next week, there’s just no rain in sight. We’re going to be stuck in perfect weather.”

Mass says expect temperatures in the upper 70s with plenty of sun.

“Not too hot at all — no heat waves,” he said. “Really quite nice weather.”


Mass adds that — meteorologically speaking, based on past records — we are entering the driest time of the year, as we head toward what is known to be the driest day of the year: July 29.

“And the driest time of the year is the last two weeks of July and the first two weeks of August,” Mass said. 

Mass says some of the wet weather experienced on the West Coast, such as that convergence zone and heavy rain in Seattle and North Seattle Thursday night, came at least in part from an extraordinary weather feature high above us.


A jet stream that shattered speed records has been moving wet air aloft from west to east over the past few days. Mass says this amazing anomaly at 25,000-35,000 feet enabled some flights from Asia to land as much as an hour early at Seattle-Tacoma International Airport. He says this is the strongest jet stream in observational history for the summer months — by a lot.

“The jet stream on Thursday morning was 134 knots — that is extraordinarily strong,” Mass said. “The record for the date was 110 knots.”

Thursday's record translates to 154 miles per hour (the previous one translates to about 127 mph).

Mass says 134 knots is the strongest jet stream every recorded over the past 50 years (when the record began) for all dates from April 1 to mid-October.

“It was startling,” Mass said, adding that in addition to the faster flight times, it affected our weather as well as weather in other places, making it warmer than normal in California and colder than normal over British Columbia and Alaska.

“Also, the snow level came down in the Rockies, there was some snow in some higher peaks," Mass said. "And of course, some of the precipitation we’ve seen — that was associated with the jet stream as well.

“So, everything is interconnected; you have very active weather aloft, that affects things down at low levels.”  

As for the cause, Mass says the strong jet stream appears to be associated with a pattern known as the Madden-Julian oscillation, which is a cyclical area of thunderstorms and convection in the tropics that can throw strong winds our way.

Mass says the jet stream is fading though as a high-pressure system moves in that promises to bring mild and sunny weather to Western Washington for the next week or more.

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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