Stacker used data from NOAA's National Centers for Environmental Information to find the counties in North Dakota that have experienced the most tornadoes since 2000. While data is available going back to 1950, tornado tracking by the National Weather Service became more robust and accurate in the 1990s. Since counties can vary in size, they are ranked by the number of tornadoes per 100 square miles. Counties that experienced less than five tornadoes are not included.