Complex sentence: Odile continued to weaken as its convection sped northeastwards into Mexico and the southwestern United States; at 21:00 UTC on September 17, the NHC determined that the tropical storm had degenerated into a remnant area of low-pressure inland over northwestern mainland Mexico.
Split into simpler sentences: Odile continued to weaken as its convection sped northeastwards into Mexico and the southwestern United States. At 21:00 UTC on September 17, the NHC determined that the tropical storm had degenerated into a remnant area of low-pressure inland over northwestern mainland Mexico.