How the Dual Jet Stream Sparks This Weird Summer Weather

Ninety-degree temperatures in Alaska? Bizarre flooding in Europe? Blame the appearance of a double jet stream.

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What's with the weird weather this month? In case you missed it, McGrath, Alaska, hit 94 degrees on June 17, four degrees warmer than Miami, which sits 4200 miles closer to the equator. On that same day, 1750 miles to the southeast of McGrath in Calgary, Canada, the high temperature barely hit 70. But Calgary and the surrounding area were in the midst of massive downpours and flooding that swamped the city and forced the evacuation of 75,000 people. Meanwhile, on the other side of the Atlantic, parts of Europe have also been coping with extreme flooding. And it's all part of the same strange weather phenomenon.

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"It's the jet stream," says John Nielsen-Gammon, a professor of atmospheric science at Texas A&M University. "Actually, it's due to two jet streams in the Northern Hemisphere when there's normally just one. It's something that we see once a decade or so during this time of year."

The jet stream is that fast moving river of air that rushes west-to-east around the globe, usually between 25,000 and 30,000 feet in altitude. While many theorized about the jet stream's existence at least as far back as the 1800s, it wasn't "discovered" until World War II. In 1944, U.S. B-29 crews flying over Japan reported a current of high-altitude winds, and the name stuck. The jet stream is a year-round feature of our atmosphere, but the double jet stream phenomenon is more common in winter. When it shows up in the summer, watch out.

"Usually at this time of year the jet stream is a single band around the Northern Hemisphere," Nielsen-Gammon says. "But in the last month what we've seen is a smaller jet stream over the Arctic Ocean, and another jet stream in the midlatitudes." Here was the jet stream's location last year, from May 24 to June 23, 2012:

Here's the image for the same period this year. This is where it gets interesting, with the green-shaded band of wind circling the Arctic, and a second stream over the heart of the U.S.

With a typical jet stream, you see colder temperatures north of them, and warmer temperatures south of them. With two streams, the same effect is appearing, but in weird ways. McGrath is located just south of the curious Arctic jet stream—explaining those high temps—and Calgary is located just north of the central jet stream.

"The floods in Calgary come from all the moisture that's being channeled up from the Gulf of Mexico," Nielsen-Gammon says. "In Europe the flooding is being fed from moisture being pulled out of the Mediterranean. It's the same phenomenon." The two jet streams have formed weather barriers that are locking rain and heat in place.

According to Nielsen-Gammon, it's unclear why these double bands appear, though it does not appear to be related to climate change. However, if ocean temperatures increase that would lead to more moisture being drawn into the atmosphere, adding even more moisture fueling storm systems like the on that hovered over Calgary.

What does that mean for the rest of the summer? "The computer models are only good for about eight or nine days out," Nielsen-Gammon says. "But it looks like we'll still have this double jet stream, which means locked-in weather patterns. As long as it lasts, expect to see more of these large variations in temperature and rainfall from place to place."

John Galvin covers natural and man-made disasters for PopMech. Follow him at Twitter: @JohnPGalvin.

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