Video: 'Polar vortex' grips the US causing traffic chaos A whirlpool of frigid, dense air known as a 'polar vortex' was expected to suppress temperatures in more than half of the continental US starting into Monday and Tuesday, with wind chill warnings stretching from Montana to Alabama. Roads in the Midwest were particularly dangerous and officials in Missouri warned it was too cold for rock salt to be very effective. Authorities also urged residents to check on elderly and disabled relatives and neighbours. It has not been this cold for almost two decades in many parts of the US. School was called off for Monday for the entire state of Minnesota, as well as cities and districts in Wisconsin, Illinois, Indiana and Iowa, among others.