A polar vortex dipping down from Siberia is bringing a cold front with frigid temperatures to millions Americans. See maps of the arctic blast.

How the Polar Vortex Can Bring Arctic Blasts to the U.S. Vast weather patterns can carry blasts of frigid air far from the polar regions—here’s how it works

Most of the U.S. will enjoy milder temperatures on Thursday, but the warmup will be brief as a polar vortex will send temperatures tumbling below freezing for most of the nation starting this weekend.

The freezing temperatures will move eastward, reaching locations like Philadelphia, where daytime highs will only reach the upper teens. The polar vortex is a gigantic circular upper-air weather pattern in the Arctic that envelops the North Pole.

In Indiana, around zero degrees was likely, the map showed, with temperatures of around three degrees possible in Northern Maine. Northern Georgia would see temperatures around 16, and in Colorado, the map showed that some areas would see -9.