The Polar Vortex was in the news the week of January 6. It seemed every paper and TV broadcast headlined it. Sometimes the reports accurately described what is meant by such a creature. More often, it was described as some sort of monster that had emerged from beneath the arctic ice this winter to wreak havoc. And, it might be an effect of the so called global warming phenomenon. Well, it appears the polar vortex is coming back this week.
The facts are less spectacular. The rotating pool of cold air above the polar region in winter is bounded on the southern edge by the polar jet stream. This jet is a fast moving stream of air in the upper atmosphere going west-to-east. It forms a loose loop around the polar region. Several times in the winter, the jet gets less west-to-east and more wavy. At those times, the jet often comes off the Pacific entering British Columbia before diving southeast across the middle of the U.S. A strong low pressure forms over Canada and draws exceptionally cold air south. This large lobe of cold air pours into the mid-west and eastern states. The cold air can flow as far south as the Gulf and Florida. That scenario occurred the week of January 6. It appears it will happen again this week.
The NASA Jet Propulsion Laboratory AIRS Mission scientists published this excellent video which describes exactly the nature and behavior of the Polar Vortex and why it sometimes impacts the United States.