Jamaica weather in September is very much like weather in August with one notable exception...
...the possibility of more rain during this month.
However, the days are still hot, still humid, and still filled with long hours of daylight.
Average High in September: 90°F / 32°C
Average Low in September: 75°F / 24°C
Average Rain in September: 6.1" / 155mm
The hurricane season is now in high gear usually with the formation of several tropical depressions later developing into hurricanes.
Although Jamaica is often spared a direct hit by a hurricane, on numerous occasions our weather is influenced by systems hundreds of miles away.
So be prepared with having to deal with more than the normal share of rain that the island usually experience.