One year ago this week, temperatures across most of the lower 48 states were plunging to the lowest levels seen in about three decades, spawning the rise of the weather term of 2014: "polar vortex."  Exactly a year later, it's happening again.  However, there are some differences between the extreme cold that is already snaking its way south from Siberia, across the North Pole, down across Canada, and into the Upper Midwest, and the 2014 Arctic freeze. From there, the frigid temperatures of 25 to 35 degrees Fahrenheit below average for this time of year will quickly engulf the entire U.S. east of the Rocky Mountains by midweek, like an invading army encountering weak opposition.  The cold is in fact associated with a wobble in the polar vortex, or what one meteorologist has termed a "wayward" piece of the polar vortex. It is occurring in the wake of a phenomenon that is well-known to destabilize the polar vortex, which is a sudden stratospheric warming event.  These two images compare the stratospheric polar vortex in January 2014 and 2015, with the blue colors corresponding to unusually cold conditions. It's clear from these images that a piece of the vortex has broken off very close to the U.S. this year.  The main polar vortex is a circulation of air enveloping a near-permanent area of low pressure that exists in the upper atmosphere, above typical cruising altitudes for commercial jetliners, over the Arctic. When these winds weaken, as has been happening recently, pieces of the vortex can break off, and drunkenly wander south into the U.S., Europe and parts of Asia.  This is what happened in early January of last year, and to some extent, it's occurring again now, as the animation of conditions at about 100,000 feet above the Northern Hemisphere shows.  However, this cold outbreak is going to be more brief in many areas (though not all) compared to the long-duration misery that millions of Americans endured last year.  The cold air is already spilling into Minneapolis and Chicago, where up to six inches of snow is forecast through early Tuesday morning. Navy Pier in Chicago had a low temperature of minus-5 degrees Fahrenheit on Monday.  With an exceptionally intense high-pressure area forecast to build into the Central Plains by Wednesday, the Great Lakes will be a frigid, windy region where it may be unsafe to be outside for extended periods due to the danger of frostbite.  High temperatures on Tuesday and Wednesday may not climb above 0 degrees Fahrenheit on Tuesday and Wednesday, with wind chills close to 40 below. The last time Chicago saw a subzero high temperature was Jan. 6, 2014.  The high-pressure area, which may rank among the strongest such weather features on record in the U.S., will ensure that the cold outbreak is accompanied by relentlessly blustery winds from Minnesota to Ohio.  Wind chill advisories and warnings are in effect for much of the Upper Midwest on Monday morning, with wind chills in the 40s below zero in International Falls, Minnesota, a community on the Canadian border nicknamed the "Nation's Icebox."  The Northeast is already experiencing a classic case of weather whiplash, after temperatures soared into the 50s and 60s Fahrenheit in New York and southern New England on Sunday. New York City may not get above the freezing mark from Monday afternoon until Sunday.  Two cold temperature records are in jeopardy in Chicago, including the daily record high temperature on Wednesday, which is 3 degrees Fahrenheit set last year, and the daily record low on Thursday morning, which is minus-10 degrees Fahrenheit. Other records may fall in the Ohio Valley and Southeast, but in general this will not be an unprecedented cold weather event for most areas, similar to last year's event which did not set widespread all-time benchmarks.  Sunrise on a very cold morning in #Chicago. RT Shot with a @DJIGlobal Phantom and a @GoPro Hero 4 ——> pic.twitter.com/ODd1wWZYu2  — Мax Tarlton (@MaxTarlton) January 5, 2015  Gusty winds will accompany the cold temperatures as a strong area of high pressure builds in behind the clipper system. These gusty winds will make it feel even colder. Wind chill values are expected to be below zero for much of the Midwest and Northeast overnight Wednesday into Thursday morning and in the single digits for parts of the Southeast.  The cold in New England and the Southeast will peak on Thursday and Friday, with sub-freezing temperatures likely all the way to the Gulf Coast. Parts of Tennessee and possibly North Carolina may see single digit low temperatures on Thursday.  The long-term forecast shows the frigid weather pattern, with a deep trough of low pressure in the Midwest and East, transitioning into a flatter jet stream that will transport milder Pacific air into much of the U.S. by early next week.  Have something to add to this story? Share it in the comments.
On the one year anniversary of the 2014 "polar vortex" cold snap, exceptionally frigid air is once again enveloping the lower 48 states.