Complex sentence: Unfavorable conditions produced by an El Niño prevented the development of any late season storm, as the final named storm, Hurricane Lili, was upgraded to a tropical storm on September 21.
Split into simpler sentences: Unfavorable conditions produced by an El Niño prevented the development of any late season storm. In fact, Hurricane Lili, the last system to reach named status, was upgraded to a tropical storm on September 21.