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

An Arctic blast that began in the northern plains on Saturday spread bitter cold from the Rockies through the East Coast and as far south as Florida by Tuesday morning. A blizzard warning was in effect for southern Louisiana, the furthest south such a warning has ever been posted.

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

Still, several Colorado school districts will start late on Tuesday as the state waits out the last of subzero temperatures. Denver, Jefferson County, Aurora, Littleton, St. Vrain Valley and Weld, among others, are all on a two-hour delay.

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.

A series of cold fronts the last few weeks has kept Florida downright chilly, as another winter storm starts to take aim this weekend