A somewhat ominous tweet from The Weather Channel on Wednesday evening offered fair warning about a returning winter phenomenon. Showing frigid temperatures dipping into the teens, single digits, and even minuses in some parts of the country, the tweet read: "No, these are not forecast low temperatures. These are high temperatures for the second half of next week." NBC News explains that while some parts of the Rockies and Plains have already been slammed with an arctic blast—and with forecasts showing possible freezing temps in parts of all 50 states Saturday morning, per TWC—a polar vortex may make things even crisper late next week. "It's going to be a shock," a TWC senior meteorologist says. "The fall was closest to the warmest on record, so this is really back to reality."
TWC explains the polar vortex as a stratospheric "closed circulation" system that hovers over the polar regions. When parts of the vortex get weak or stretch out, however, that biting cold can stretch down and out. What this could mean: temperatures in the 20s in the South, in the teens throughout the Northeast, and below zero in certain parts of the Midwest, including Chicago and Minneapolis. How cold things will get and in what regions depends on how far out the polar vortex reaches. Although a WeatherBELL Analytics meteorologist tweeted Tuesday that this "upper-level atmosphere configuration" is "very similar in scale & magnitude" as the "life-threatening" 2014 polar vortex, the Washington Post speculates this one won't be as bad due to the fact that it's happening earlier in the season and there's not as much snow on the ground yet to help keep air masses frosty-cold. (Read more polar vortex stories.)