Complex sentence: It is estimated that at 1800 UTC on that day, the system became a subtropical storm; the classification as a subtropical cyclone was because a ship reported gale force winds as far as from the center.
Split into simpler sentences: It is estimated that at 1800 UTC on that day, the system became a subtropical storm. Initially it was classified as a subtropical cyclone because a ship reported gale force winds as far as from the center.