Return of the Polar Vortex

It's "The Return of the Polar Vortex" over the much of the eastern half of the U.S. this week, as another round of bitterly cold Arctic air plunges southwards out of Canada. Like many sequels, "The Return of the Polar Vortex" will not be as impressive as the original, with temperatures averaging about ten degrees warmer than during the original Polar Vortex episode earlier this January. Still, with temperatures 15 - 25° colder than average expected over much of the eastern half of the U.S. Tuesday through Thursday, this week's sequel is a respectable cold blast. The cold air is centered over the Upper Midwest, and low temperatures in portions of Minnesota, Wisconsin, and Michigan dropped below -20° early Tuesday morning. Crane Lake, Minnesota bottomed out at a bone-chilling -32°F this morning, and Pellston, Michigan hit -25°.

On Wednesday, the cold air spills southeastwards over much of the Ohio Valley, Mid-Atlantic, and New England, in the wake of Winter Storm Janus. The powerful storm is expected to bring 4 - 8" of snow to Washington D.C. on Tuesday, and 6 - 12" to Philadelphia, New York City, Providence, and Boston, Tuesday night through Wednesday. The most dangerous conditions are expected on Cape Cod, Massachusetts, where 8 - 12" of snow with sustained winds of 20 - 30 mph, gusting to 50 mph are expected. A blizzard warning is posted there.


Figure 1. Lake Erie lies 90% covered in ice on January 9, 2014, after the intense "Polar Vortex" cold air outbreak over eastern North America that week. The cold air brought the Great Lakes their highest ice coverage in 20 years for that time of year. Image credit: NASA Earth Observatory.

What is the Polar Vortex?
It's curious that the media latched on to the term "Polar Vortex" earlier this month. It's a term that has been around for a long time, but is generally not used by meteorologists in their public discussions of weather, since it is a rather technical term. NOAA has this explainer of the Polar Vortex, and TWC meteorologist Stu Ostro also has a nice description in his latest blog post, Polar Vortex, Global Warming, and Cold Weather. A short excerpt here:

• A polar vortex is not only not something new or mysterious, it's a standard feature of the atmosphere, strongest in winter. The term has been used in the scientific literature at least back as far as 1939 in a paper by Rossby. This meteorological phenomenon has likely been present for as long as there has been weather on Earth.
  
  

• A polar vortex is a large circulation in the upper atmosphere that has generally west-to-east winds circling the Earth. It's not a cold wave or a storm. As the name implies, polar vortexes (or vortices -- either is grammatically correct) are usually centered in the polar regions, one near the North Pole and another near the South Pole.

• Polar vortexes exist at a high altitude. They are most well-defined in the stratosphere, higher up than the portion of the atmosphere in which most of what is typically thought of as weather is (troposphere). Their circulations extend down to lower altitudes (the atmosphere is a continuum) but are more irregular and chaotic in the troposphere. As observations and understanding of higher altitudes have evolved, the American Meteorological Society Glossary definition has evolved, originally referring to the middle and upper troposphere (which is where I originally learned it was), and then the middle troposphere to the stratosphere, and the now the scientific literature mostly refers to the stratospheric polar vortex.
  
  
  Wunderblogger Lee Grenci sounds off on the misuse of the Polar Vortex term by the media in his latest post.
  
  Jeff Masters