Everest Weather Made Up of Roiling Mix of Risks: Page 2

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Forecasting for Everest began after the disastrous 1996 season, which held the record for deadliest spring on the mountain until this year. Since then, forecasts have become pretty accurate at predicting jet stream movements, Fagin said. The best windows usually open up in mid-May.

But even when the jet stream backs off, it’s hard to predict exactly how hard winds will blow. And surprise storms can show up out of nowhere. Models just can’t yet say exactly where or when snow will fall on that small of a scale.

"In general, forecasts have gotten better, but where they tend to do poorly is pinpoint precipitation totals on Everest or any other mountain," Fagin said. "Precipitation forecasts are still not acceptable as far as I'm concerned. That’s still the wildcard."

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Weather can play a role in triggering traditional snow avalanches, which occur in predictable terrains and under certain known conditions, said Karl Birkeland, director of the USDA Forest Service National Avalanche Center in Bozeman, Montana.

But the kind of avalanche that killed 16 Sherpas with tumbling blocks of ice depends on more complicated dynamics, including the continuous downhill movement of glaciers. The Khumbu Icefall, where the accident happened, is notoriously unpredictable and will likely remain that way.

"As glaciers move, certain chunks fall off and roil down the mountainside," Birkeland said. Each chunk can weigh more than 2,000 pounds. "As far as I know, no one has come up with a way to forecast that kind of avalanche."

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