John Nelson has used 56 years of data gathered by the National Oceanic and Atmospheric Administration on tornadoes across the United States to create this spectacular image.
In the comments, some have mentioned they are surprised the paths of the tornadoes appear in straight lines. Nelson explains, based on the data, the vectors could be created using touchdown points and liftoff points. Another commenter -- OKWeatherGirl -- shares this information:
In response to the comment about the tornado paths all being linear the tornado lengths are correct. The data file that was used connects a straight line between the lat/long of the starting point to the lat/long of the ending point. Those are historically the only points that were stored for every known tornado. It was not until recent years that the usefulness and practicality of storing an approximate path was discovered. Remote sensing is also used to more accurately locate exact tornado paths in addition to the damage survey information and all of this data is easier to store and manage now using GIS(geographic information systems)than has been in the past.
This map was created using a raw or unprojected shapfile which definitely increases the error in path location, but the error with this data set is already pretty high.
Yet another commenter points out when you zoom in closer the lines are less linear than they appear suggesting they could be accurate.
Still, as GeekOWire says, it may not be the most scientific of maps, but "it’s still a startling look these enormous storms."