A Polar Vortex in 2015?

When temperatures plunged into the sub-zero range for much of the country in early 2014, the term “polar vortex” became a household expression. The Polar Vortex is not a new phenomenon. The polar vortex was first described as early as the mid-nineteenth century.

The polar vortex is an area of low pressure—a wide expanse of swirling cold air—that is parked in polar regions. For North America, the one in the Arctic has the greatest impact on weather patterns. The air within the Northern Hemisphere polar vortex is referred to as polar air or Arctic air, depending on temperature, with the Arctic air being the coldest. South of the polar vortex, the atmosphere is much warmer, representing sub-tropical air, and, further south, tropical air. The boundary between the cold air within the vortex and the warmer air to the south is a region of sharp horizontal gradients in temperature, with temperatures decreasing to the north. These sharp temperature gradients give rise to the polar front jet stream, a fairly narrow ribbon of especially fast-moving air, flowing broadly from west to east. Hence, the southern boundary of the polar vortex essentially corresponds to the location of the polar front jet stream.

Sometimes this low-pressure system, full of cold Arctic air, strays a little bit too far from home. Part of it can break off and migrate southward, bringing all of that cold air with it. Just like that, areas as far south as Florida get to experience their own little taste of life in the Arctic.

MILWAUKEE, WI – JANUARY 7: A man braves the cold and walks along the shore of Lake Michigan as temperatures remain in the negative digits on January 7, 2014 in Milwaukee, Wisconsin. A ‘polar vortex’ of frigid air centered on the North Pole dropped temperatures to the negative double digits at its worst. (Photo by Darren Hauck/Getty Images)

We actually want a strong polar vortex to stay warm?

The breaking off of part of the vortex is what defines a polar vortex event. But it actually occurs when the vortex is weaker, not stronger. That might sound weird—but it actually makes sense. Normally, when the vortex is strong and healthy, it helps keep a current of air known as the jet stream traveling around the globe in a pretty circular path. This current keeps the cold air up north and the warm air down south.

Polar Vortex

This pair of images compares the boundaries of the polar vortex. The left image plots the height of the 500 millibar surface, about halfway up in the atmosphere, averaged for all Januarys over the period 1981-2010. The right image maps the 500 millibar height surface averaged for 1-5 January 2014. The shape of the polar vortex is quite different. The polar vortex is defined by the blue and purple colors poleward of the yellow colors, separating the polar and Arctic air and from warmer air to the south. The boundary of the polar vortex is where the yellow transitions to the green, and the tightly packed lines indicate a strong horizontal temperature gradient and define the jet stream. The air within in the jet stream, and within the vortex as a whole, rotates from west to east like a big whirlpool. Note the meanders in the vortex—these are the longwave ridges and troughs. This period saw a very strong Arctic outbreak over the central United States linked to a strong longwave trough moving into the area. Credit: M. Serreze, NSIDC

But without that strong low-pressure system, the jet stream doesn’t have much to keep it in line. It becomes wavy and rambling. Put a couple of areas of high-pressure systems in its way, and all of a sudden you have a river of cold air being pushed down south along with the rest of the polar vortex system.

That’s what happened in early 2014. The polar vortex suddenly weakened, and a huge high-pressure system formed over Greenland. The high-pressure system blocked the escape of all that cold air in the jet stream, and allowed part of the polar vortex to break off and move southward. Places as far south as Tampa, Florida experienced the wrath of this wandering polar vortex. Most of Canada and parts of the Midwestern United States had temperatures colder than Alaska at the height of this cold snap!

It’s important to remember that not all cold weather is the result of the polar vortex. While the polar vortex is always hanging out up north, it normally minds its own business. It takes pretty unusual conditions for it to weaken or for it to migrate far south, and other things can cause cold arctic air to travel our way, too.

The Polar Vortex in 2015

From the discussion above, it should be clear that there is always a polar vortex. The real question is to what extent will the polar vortex meander? The answer to this question is not an easy one. The farther in advance a weather prediction is made, the less accurate it will be.  Forecasts of a few days in advance are usually accurate.  Such a long range forecast is based more on the “art” of forecasting than on the science. That is not to say that this long range forecasting ignores science – it’s just that the current state of weather science does not support accurate long range forecasts.

The answer to whether or not the polar vortex meandering will bring us extremes in col weather is – maybe. Not an entirely satisfying answer, but the best answer that can be given at this time. There is some difference of opinion among the major forecasters on this, however.

AccuWeather predicts the return of the polar vortex, at least for the Northeast portion of the country, with the South also experiencing cold temperatures.

The Weather Channel is also calling for a return of the polar vortex.

The National Weather Service released their long-range forecast which calls for a warmer-than-average west and cooler- and wetter-than-average south.  The return of the polar vortex was seen as less likely.

The differences in these forecasts result from emphasis on different weather elements at play.  These include such factors as the state of snowfall levels in Siberia, the state of the El Nino pattern, and the phase of the Arctic Oscillation, among others.

One thing we do know and that is that Wisconsin winters bring their share of cold weather and it won’t be long until tho uncertain long range winter forecasts give way to actual winter weather and more accurate short term forecasts.

Acknowledgement is made to the National Weather Service and National Snow & Ice Data Center for material used in this article.

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