Just when you thought it was safe to go outside again, the polar vortex is back, blasting the Midwest and eastern half of the U.S with very cold weather. While this will undoubtedly be unpleasant, there is an upside.

You might remember the polar vortex from January and later in January, when it brought extremely low temperatures to a good deal of North America. Starting next week the atmospheric phenomenon, usually confined to the Arctic regions of our planet, will be dipping down once again into many states.

Models are “very confident that it’ll be significantly colder than average” in much of the eastern two-thirds of the nation, said Mike Halpert, acting director at NOAA’s Climate Prediction Center. During the worst parts, temperatures could be as much as 20 to 35 degrees below average. The most affected areas will likely be places that have already felt the freeze this year, such as Minnesota, Wisconsin, and the Dakotas.

Those states are currently feeling a little relief as the weather has momentarily cleared up in the Midwest, leading to warmer temperatures in the 50s and 60s and heavy rain instead of snow. Though it might be a nice break from the freezing temperatures, unfortunately, this is actually a bad thing.

According to Weather Underground, there is so much snowpack on the frozen ground in the central and northeastern U.S. that warm weather and rain could lead to flash floods. Ice flows breaking up in rivers could also get carried downstream and jam up the flow, leading to spillover. It seems that the expected arrival of the polar vortex next week may be a blessing: The return of freezing temperatures could save the region from the worst of this.

“This week’s thaw will be short-lived, preventing the kind of major flooding that would result if all of the snowpack were to melt in a week,” wrote meteorologist Jeff Masters at Weather Underground.

The polar vortex originates in the far north, where sunlight has disappeared during the winter season, creating the Northern Hemisphere’s coldest air. Moving southward, this air gradually warms, until it reaches a place where the warming occurs very quickly. A swift-moving river of air moves west to east here, marking the typical southern edge of the polar vortex.

Another climactic phenomenon in play is known as the Arctic Oscillation, where atmospheric mass moves back and forth over many years between the Arctic and the middle latitudes. During a positive Arctic Oscillation, pressure is lower than normal over the Arctic but higher than normal over the mid-latitudes. Because air moves from high to low pressure, the polar vortex is pushed upward, nearer to the pole, creating warm weather in the Arctic Circle.

During a negative phase, conditions are reversed, with high pressure in the Arctic and low pressure in the mid-latitudes. This is the time when the polar vortex can develop waves or kinks that bring freezing air southward. Interestingly, this year’s Arctic Oscillation was not largely negative. This could help explain why the polar vortex only came down in North America and eastern Siberia. Other locations around and within the Arctic Circle such as Alaska, Scandinavia, Europe, and western Russia had much balmier than normal temperatures. While this year’s Arctic Oscillation wasn’t very negative, scientists have noticed a trend in recent decades toward more negative phases. Some blame loss of sea ice and other effects from climate change, though the true cause remains unclear.

Though many folks like to think this perpetually dark and frozen winter they are suffering through is especially miserable, it’s actually not been a particularly severe one when taking a long-term view of the entire country.

“People are saying this winter’s been really cold,” said Halpert. “When looking at the last three months, yeah, we’ll be a little on the cold side compared to average. But it’s certainly nothing historic.”

Just how cold it is, of course, depends on where you are. While some states, like Wisconsin, are experiencing what may be in the top five or 10 coldest winters on record, California is in the middle of a warm and dry drought. But a lot of the U.S. hasn’t been having anything really out of the ordinary weather-wise.

As I write these words, temperatures across half the U.S. are plummeting like a rock.Extreme lows are forecast by tonight: -32 degrees Fahrenheit in Fargo, N.D.; -21 degrees F in Madison, Wisc.; -15 degrees F in Chicago and Indianapolis, according to the National Weather Service. Wind chills will reach a bizarre 60 degrees below zero F in some places, freezing exposed skin within one minute. That number is more typical for Mars—at night, according to the Curiosity rover NASA has free-wheeling over there.

As each hour passes, more and more television and radio reporters are attributing the insane cold to a “polar vortex” up in northern Canada. Vortex, yes, but upper Canada? Not exactly. One forecaster called the beast a hurricane in the Arctic, which is dramatic but wrong. So what is this mysterious marvel and why is it invading America?

The polar vortex is a prevailing wind pattern that circles the Arctic, flowing from west to east all the way around the Earth. It normally keeps extremely cold air bottled up toward the North Pole. Occasionally, though, the vortex weakens, allowing the cold air to pour down across Canada into the U.S., or down into other regions such Eastern Europe. In addition to bringing cold, the air mass can push the jet stream—the band of wind that typically flows from the Pacific Ocean across the U.S.—much further south as well. If the jet stream puts up a fight, the moisture it carries can fall out as heavy snow, which atmospheric scientists say is the circumstance that caused the February 2010 “snowmageddon” storm that shut down Washington, D.C.

But why does the vortex weaken? Now it gets interesting. More and more Arctic sea ice is melting during summer months. The more ice that melts, the more the Arctic Ocean warms. The ocean radiates much of that excess heat back to the atmosphere in winter, which disrupts the polar vortex. Data taken over the past decade indicate that when a lot of Arctic sea ice disappears in the summer, the vortex has a tendency to weaken over the subsequent winter, if related atmospheric conditions prevail over the northern Atlantic Ocean. The situation looks something like that shown in the graphic below. (For a full explanation, see the Scientific American article that accompanies the graphic.)

Although the extent of summer sea ice in the Arctic varies year to year, overall it has been disappearing to a notable degree since 2007 and it is forecast to continue to vanish even further. That could mean more trouble for the polar vortex, and more frigid outbreaks—a seeming contradiction to “global warming,” perhaps, but not for “global weirding,” also known as climate change.

The bitter chill gripping North America is a result of Arctic air that has spilled southwards, and global warming may be a cause, an expert has said.
Arctic air is normally penned in at the roof of the world by a powerful circular wind called the polar vortex, said Dim Coumou, a senior scientist at the Potsdam Institute for Climate Impact Research (PIK) near Berlin.

When the vortex weakens, the air starts heading southwards, bringing exceptional snow and chill to middle latitudes.

The weather shift is also helped by changes in a high altitude wind called the jet stream.

This convection, which usually encircles the northern hemisphere in a robust and predictable fashion, starts to zigzag, creating loops of extremely cold weather or unseasonably mild weather, depending on the location.

“We’ve seen a strong meandering of the jetstream, and the cold air associated with the polar vortex has been moving southwards, and in this case over the eastern parts of Canada and the United States, bringing this extreme cold weather,” said Coumou.

The phenomenon has occurred repeatedly in recent winters, he noted.

What drives the polar vortex is the difference in temperature between the Arctic and mid latitudes, said Coumou.

Once sharp, this differential has blurred in recent years as the Arctic — where temperatures are rising at about twice the global average — warms up, he said.

“We’ve seen this type of cold spell more often lately in recent winters, in Europe but also in the US,” Coumou said in a phone interview.

“The reason why we see these strong meanderings is still not fully settled, but it’s clear that the Arctic has been warming very rapidly. We have good data on this. Arctic temperatures have risen much more than other parts of the globe.”

Last month, European scientists reported that the volume of sea ice in November was around 50 percent greater compared with a year earlier, following a recovery in the Arctic summer.

Despite this bounce-back, sea ice remains at near-record documented lows and its overall trend is one of retreat, they said.

Coumou cautioned that Arctic sea ice “is just one of the important factors” behind disruption of the polar vortex”.

“Other factors include snow cover, stratospheric warming events or other short-lived phenomena,” he said.

Other specialists said the link between warming and the spillover of Arctic air was still debated.

“There is no consensus,” said Francois Gourand, a forecaster at Meteo France, the French national meteorological agency.

“The melting of sea ice can have an impact on atmospheric circulation but these effects are complex and hard to pin down,” he said.

“The overall trend of the sea ice is downwards, yet in Europe we can have mild winters sometimes, or cold winters — there doesn’t seem to be a clear link.”