Complex sentence: Late on September 20, this set-up caused the storm's forward speed to increase substantially, ultimately reaching 70 mph, the highest forward velocity ever recorded in the annals of hurricanes.
Split into simpler sentences: Late on September 20, this set-up caused the storm's forward speed to increase substantially. In fact, the forward speed of the hurricane would ultimately reach 70 mph, the highest forward velocity ever recorded in the annals of hurricanes.