Complex sentence: Tropical Storm Arlene was an unusually large and early-forming tropical storm, being the first of twenty-eight different storms during the 2005 Atlantic hurricane season, which would become the most active season on record.
Split into simpler sentences: Tropical Storm Arlene was an unusually large and early-forming tropical storm forming during the 2005 Atlantic hurricane season. It was the first storm of the season that would become the most active season on record.