Temperatures as much as 30 to 50 degrees below normal are entering the Northern Plains as we close out the workweek. Through the weekend, brutal conditions you might expect in a frigid January overtake the central portion of the country, from the Mexican to the Canadian borders. Heading into the first full week of March, Arctic air takes up residence in the East as well. When it’s all done, most of the contiguous United States will endure a punishing blow of frigid air from this Arctic blast. Records for cold are likely to be most numerous in the north-central United...