Complex sentence: In addition, the dry line, which marks a divided line between the dry and humid air mass, was positioned over the southern High Plains, and that allowed for the initiation of scattered supercells on May 4.
Split into simpler sentences: In addition, the dry line, which marks a divided line between the dry and humid air mass, was positioned over the southern High Plains. This allowed for the initiation of scattered supercells on May 4.