That Arctic blast hitting the U.S.? There's a wavy jet stream to blame.

Though some seasonal variation is natural with the jet stream, there are some indications that climate change is affecting the waviness of these air currents.

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By Denise Chow

An unusually wavy jet stream — a ribbon of fast-moving air that flows over the Northern Hemisphere — is to blame for an Arctic onslaught that is blasting most of the United States with snow and record-low temperatures this week.

About two-thirds of the country is being affected by the deep chill, with the National Weather Service forecasting that an uncommonly large area could see record-low temperatures Wednesday, including most of the East Coast and from the Midwest south to the upper Texas coast.

This week’s wintry blast, like all weather events, is the result of a complex interplay between atmospheric conditions and the movement of air around the planet. The frigid conditions across much of the U.S. are being caused by a ripple in the jet stream that pushed an Arctic air mass into Canada that is now shifting across the central and eastern United States, according to Marc Chenard, a meteorologist and forecaster with the weather service’s Weather Prediction Center in College Park, Maryland.

The jet stream, which flows west to east in the upper atmosphere over the Northern Hemisphere, is driven by temperature differences between the cooler polar region to the north and warmer air masses to the south.

As this river of air circles the planet, it can undulate and create larger-than-normal peaks and troughs that impact weather systems. When the jet stream dips south and creates a trough, this allows cold, polar air to funnel southward. Conversely, when the jet stream swells into a peak, warmer air rushes in to fill that void and drier conditions typically prevail.

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“What we have now is a wavy pattern where the peak of the wave is over the western U.S. and the downward portion of the wave is coming over the eastern U.S.,” Chenard said.

The ridge over the western part of the country is responsible for the dry conditions and above-average temperatures that have fueled devastating fires in California in recent weeks, said Jeff Masters, co-founder and director of meteorology for Weather Underground, a commercial weather service headquartered in San Francisco.

“The two are correlated — where you get one extreme, it gets balanced by the opposite extreme,” Masters said.

According to the weather service, record-cold conditions are expected across the Plains, stretching to the East Coast this week. Though some records may fall in the coming days, Chenard said what’s more unusual is how much of the country will be blanketed by the Arctic air mass.

“It’s not uncommon to get record lows for a specific date, but it’s not too common for it to span this large of a geographic area,” he said.

And though some seasonal variation is natural with the jet stream, there are some indications that climate change is affecting the waviness of these air currents, and their speed. A 2015 study published in the journal Environmental Research Letters found a link between rising concentrations of greenhouse gases in the atmosphere and wavier jet stream patterns, which can intensify and stretch out periods of rain, extreme heat or drought.

“We had a near-record year for Arctic sea ice melt this year, and that has probably increased the odds of this sort of behavior in the jet stream,” Masters said.

He added that he is especially interested to see if any record-lows are set for the entire month of November.

“Daily lows aren’t that hard to set — there are around 100 cities that could do that this week,” he said, “But if we set an all-time low for November this early in the season, that would be something. That would speak to the unusualness of all this.”