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The science behind the Taku winds

It takes a unique combination of factors to generate this forceful phenomenon

Posted: January 17, 2014 - 1:01am
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High winds blow snow out of the Sheep Creek valley south of downtown Juneau in March 2013.   Michael Penn | Juneau Empire File
Michael Penn | Juneau Empire File
High winds blow snow out of the Sheep Creek valley south of downtown Juneau in March 2013.

Google “mountain wave windstorms” and “Juneau” shows up immediately. Around here, though, people call those windstorms the Taku winds — and Juneau hasn’t experienced much of them this year.

But why not? What are the conditions in which the Taku winds arise?

First and most important, said Tom Ainsworth, Meteorologist in Charge of the National Oceanic and Atmospheric Administration National Weather Service Forecast Office: It needs to be cold. After that, there are three main things that cause the Taku Winds.

First, wind flowing perpendicular to the mountains; down the Taku Inlet and up against Salisbury Ridge, for instance.

Second, there needs to be a stable layer of air a little higher than the mountains, at which the cold air will rise no higher.

Third, something called a “critical level” — a level at which the wind will reverse direction, must also be present.

“If one of them doesn’t happen,” Ainsworth said, “the technical term is a ‘busted forecast.’”

Cold air is dense, and it accumulates on the east side of the mainland mountains. It can’t rise higher than a certain level. When it “piles up,” it spills through gaps in those mountains, flows along the Taku River until a curve perpendicular to the mountain ridge, until it’s accumulated enough to rise over a gap in the mountains and get funneled over Salisbury Ridge. What happens next is comparable to what happens to a river when it encounters a rocky section and turns to whitewater — air currents, too, develop waves, which is why the Taku winds are gusty.

In a stable atmosphere, that cold air gets squeezed between the mountains and the level at which they can go no higher, something Ainsworth compares to a thumb on a garden hose.

Those factors causing that reaction are “fairly large scale and robust,” Ainsworth said.

“The impact to you and me is very localized,” he said. “(But) this is not something that just originates over Mount Roberts. This is a big scale weather pattern.”

The Taku winds are different from normal winds in that they don’t take “the path of least resistance,” Ainsworth said. “It’s really something that’s a really dynamic system and common in mountain areas around the world. Anywhere there are mountains, these things can happen … We know about the Taku because we live here, and we have people that feel it and are impacted by it.”

“It’s a Juneau phenomenon that definitely creates intense conditions,” said Juneau Emergency Programs Manager Tom Mattice.

Maritime weather is generally more stable than inland weather, Ainsworth said, which is why it’s been raining in Juneau for the last three weeks.

“The mountains really complicate a very simple system,” Ainsworth said. “That’s what makes forecasting in a mountainous region such a challenge, and so exciting.”

• Contact Outdoors reporter Mary Catharine Martin at maryc.martin@juneauempire.com.

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