You never can tell. Just pack some ponchos.
It is PEAK hurricane season which doesn't necessarily mean there will be hurricanes but there is lots of tropical weather during the month of Sept. so it can mean lingering rainy systems.
Sometimes they create off/on rain, other times it can rain for several days. Otherwise it's still usually very HOT and humid. You won't know until you're there.
We've been there in rainy Septembers and we've lucked out sometimes with little rain and surprisingly comfortable temps (which ironically was due to a huge hurricane out in the Atlantic traveling parallel to the coast but still far out at sea). It seemed to create a nice breeze and sucked out some of the humidity! SO you never know!
The nice thing is that this is a very slow time of the year.