Complex sentence: The 2012 Christmas tornado outbreak was the largest tornado outbreak ever to occur in the United States on December 25, and a severe weather event that affected the United States Gulf Coast and East Coast over a two-day span.
Split into simpler sentences: The 2012 Christmas tornado outbreak was the largest tornado outbreak ever to occur in the United States on December 25. It was a severe weather event that affected the United States Gulf Coast and East Coast over a two-day span.