(Bloomberg) -- Watching Siberian snowfall in October is usually a good way to predict how cold the coming winter will be in New York and other mid-latitude regions. This year, not so much.
If there's lots of snow across Eurasia, expect bitter cold. If snowfall is well below normal, the season will probably be mild. This year, the snowfall is close to the average, which doesn't offer a clue either way.
“I don't think it is turning out to be that great an indicator for this coming winter,” said Judah Cohen, director of seasonal forecasting at Atmospheric & Environmental Research, a Verisk Analytics Inc. business in Lexington, Massachusetts. “There is no really strong signal right now.”
Cohen is the father of the Siberian snowfall theory. It goes like this: If snow advances across Eurasia during October, covering more ground than normal, then there's a good chance the polar vortex -- the winds that corral arctic cold -- will break down during the winter. That would free waves of bitter cold to advance southward.
The phrase “polar vortex” has become shorthand in the U.S. for any particularly nasty blast of frigid winter temperatures, but technically it refers to a weather pattern that hovers around the North Pole and can keep the arctic chill bottled up. While the U.S. Climate Prediction Center hasn't embraced the theory, many commercial forecasters use it.
Without a clear sign, Cohen will look more closely at other indicators over the next few months, such as blocking patterns over Scandinavia, air temperatures over Siberia and the potential impact of a predicted El Nino.
For now, he's leaning toward a mild winter across the U.S., though there's a good chance he'll be revising his outlook in the coming weeks.
“I don't think it will be adjusted warmer, but I think it could be adjusted cooler,” he said.
To contact the reporter on this story: Brian K. Sullivan in Boston at bsullivan10@bloomberg.net
To contact the editors responsible for this story: Lynn Doan at ldoan6@bloomberg.net, Will Wade, Margot Habiby
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