Stock up on hot chocolate and spare shovels, because it looks like much of the world, including the eastern United States, is headed for a frigid, snowy winter.

Heavy Siberian snowfall in October may have set off a slow-motion chain of atmospheric events that in coming weeks will disrupt the polar jet stream, the vast river of air that flows around the northern hemisphere and shapes the course of its winters.

"Normally the jet stream flows fast from west-to-east, keeping the cold air locked up over high latitudes and the milder air in the middle latitutdes," said Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research, a weather risk management company.

As the jet stream shifts, "the cold air drives south. The warm air flows northward. The strong east-to-west flow breaks down, and you get a mixing of the air masses," Cohen continued.

Cohen plays an unusual role in the climate science community. Unlike meteorologists reluctant to forecast more than a few days into the future, or climatologists who work on a scale of years and decades, Cohen makes seasonal predictions.

His specialty is winter, and in particular snowfall: In the late 1990s he noted a close correlation between autumnal snows in Siberia, the crown of the Eurasian landmass, and subsequently cold winters in eastern North America, western Europe and East Asia.

Correlation isn't causation, but subsequent research fleshed out a two-part mechanism. First, a mass of cold air forms over snow-covered Siberia, deflecting the jet stream's flows like a boulder in a river.

Thus diverted, the jet stream meanders north and south, and cold air typically sealed into the Arctic flows out. Then the mechanism's second part kicks in: Energy generated by the jet stream's undulations is transferred upwards into the stratosphere, eventually gathering above the Arctic and warming its air.

A warming Arctic weakens the jet stream, which gathers strength by flowing down the atmospheric pressure gradient between warm mid-latitudes and the Arctic. As the jet stream slows, it again tends to meander, looping south with deliveries of cold air.

This type of pattern is formally known as a negative Arctic Oscillation, and it's especially conducive to storm formation, says climatologist David Robinson of Rutgers University.

"You can have some pretty significant coastal storms spawned by the cold air coming south and joining forces with the warmth of the Gulf Stream coming north," Robinson said. The so-called Snowmageddons of winter 2009-2010 and 2010-2011 came out of this dynamic.

Climatologist James Overland of the National Oceanographic and Atmospheric Administration said Cohen's proposed connection is entirely plausible, though there's an element of randomness that makes exact predictions difficult.

The Siberian cold air bubble and warming Arctic atmosphere almost certainly influence the jet stream, "and this increased forcing might increase the chance of a mid-latitude effect in winter," Overland said, "but that process is also confounded with the random chaos of atmospheric circulation."

According to Cohen, it takes several months for the effects of Siberian fall snows to propagate through Earth's atmosphere. They won't fully be felt until January and February, though an early ripple might be seen in jet stream anomalies producing snowstorms predicted for the holidays.

In the near-term, however, another Arctic source of jet stream disturbance could also make winter even colder than usual. Arctic sea ice melted to record lows during the past summer, exposing vast expanses of dark, sunlight-absorbing water. Subsequently heated air may have slowed the jet stream, a phenomenon tentatively linked to the summer drought and perhaps early winter cold.

This link hasn't been proven, but "if we see a lot of extremes this winter that are connected to high-amplitude jet stream patterns, it will be another piece of evidence that we can put on the pile," said climatologist Jen Francis of Rutgers University. "It won't be, 'That proved it!' But it would certainly be consistent with what we're expecting to happen."

Better understood than its jet stream connection is what melting sea ice does to the Arctic's climate. Air holds more moisture as it warms, and exposed water evaporates faster than frozen. The result is a relatively moisture-laden atmosphere, with some of the water carried south and some across the Arctic ocean to Siberia, where it may have fueled the October snowstorms.

Cohen normally keeps these explanations separate from discussions of climate change, but here they intersect. It's estimated that human-generated greenhouse gas pollution is already responsible for a warming of about 2 degrees Fahrenheit in the Arctic.

"With a more open Arctic, you get deeper snows in Siberia in fall and early winter, just because there's more moisture available. Even if it warms up a bit, it's still cold enough to snow," said Robinson, who co-authored a recent Journal of Geophysical Research paper describing exactly that trend.

If Cohen is right about Siberian snowfall and the jet stream, then global warming will deliver, at least sometimes, paradoxically cold winters.

"What people didn't anticipate is that the warmer atmosphere could hold more moisture, and that more snowfall in Siberia nudges the Arctic Oscillation into a negative phase," Cohen said. "Nature delivered completely the opposite trajectory than expected."