A blocking system has split a polar vortex in two pieces, with one piece settling over North America. That's why it's so terribly cold, said Anthony Lupo, professor of atmospheric science in the University of Missouri College of Agriculture, Food and Natural Resources. Bitter cold imposed its will across mid-Missouri on Monday, with temperatures hovering just below zero and total snow accumulation forecast to be between 3 to 6 inches. More snow may be approaching Columbia on Wednesday. Also called a blocking anticyclone, the blocking pattern in the Pacific is a ridge in the jet stream that prevents other weather systems from moving in.