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 floes 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 climatic 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.
