There's a kind of cold that's legitimately dangerous. It can causes frostbite in minutes to unexposed skin. In parts of the Lower 48 states, such extreme cold is not uncommon, for a few days to at most a week.
But over the entire month of February and even into March, such exceptional, life-threatening cold never departed parts of Montana. Temperatures averaging 20 to 30 degrees below normal gripped huge areas in the state, as well as parts of the Dakotas.
These places are normally cold but this chill was unlike anything seen in the contiguous United States in decades, for both its intensity and its duration.