Complex sentence: The tornado was one of a series of tornadoes (called a family of tornadoes) that was part of a supercell thunderstorm that travelled through most of North Dakota and into parts of Minnesota.
Split into simpler sentences: The tornado was part of what its called a family of tornadoes which is a series of tornadoes that is part of a supercell thunderstorm. That particular storm travelled through most of North Dakota and into parts of Minnesota.