Complex sentence: On the September 18, Roslyn had a brief period of strengthening, On the September 21, deep convection began to rapidly increase, followed by a visible eye on satellite imagery the next day, which indicated that Roslyn had reached hurricane intensity.
Split into simpler sentences: On the September 18, Roslyn had a brief period of strengthening. By September 21, deep convection began to rapidly increase, followed by a visible eye on satellite imagery the next day, which indicated that Roslyn had reached hurricane intensity.