What is a blizzard? Official criteria and where they hit the most

A blizzard needs more than heavy snow: the National Weather Service requires 35 mph winds, quarter-mile visibility and three hours of whiteout conditions. Blizzards strike most often across the Northern Plains of North Dakota, South Dakota and Minnesota, with Nebraska, Iowa, northwest Kansas, eastern Colorado and Wyoming also at risk. Find out how a true blizzard differs from an ordinary snowstorm, and why "ground blizzards" can shut down roads even when no snow is falling.