Bundle Up: Polar Vortex Returns for an Encore Frigid weather just won't let up By Matt Cantor, Newser User Posted Feb 25, 2014 10:00 AM CST 15 comments Comments Visitors take pictures of an ice filled Chicago River Wednesday, Feb. 19, 2014, in downtown Chicago. (AP Photo/Kiichiro Sato) (Newser) – The wait for spring isn't getting any easier. The northern US is in for more Arctic chills, LiveScience reports. While it won't be the season's coldest weather in absolute terms, it could be the coldest compared to what's normal for this time of year, an expert tells the site. He notes, however, that the "polar vortex" everyone's talking about has actually always been around; it's just that normally, the jet stream restricts it further to the north. Things get cold when the jet stream moves further south than usual as weather patterns alter. The northern US can expect cold all week, while areas further south will see a midweek drop in temperatures, Weather.com reports. The Upper Midwest should brace for temperatures significantly below zero tonight, Accuweather.com adds. The Midwest and Northeast may also see some snow in the coming days. "One reason for the cold blast carrying more weight than you might expect is the fact that the Great Lakes are largely frozen over," notes a meteorologist. "The air will not moderate to the extent as if most of the lakes were not frozen."