If you go to the national weather service website you can get the statistical info in terms of "odds".
It's hurricane season, so obviously there's a chance.
CarnivalSensation February 2015
Carnival Dream November 2015
Carnival Fascination April 2014
Carnival Elation March 2011
Carnival Imagination Sept 2007
Carnival Sensation Dec 2006
RCI Sovereign of the Seas Sept 2006
Carnival Miracle Sept 2005
Carnival Glory Sept 2004
Carnival Fantasy Jan 2004
Late August through early October is the peak of hurricane season. But no one can predict whether you will have any kind of storm. They can't even predict where the storm will go when they know one is out there. Living on the Gulf Coast, I live with the chance of a hurricane striking us every year. We also have beautiful weather in September. The ship will go around a storm though, so don't get scared off by the possibility.
the chances are less the further south and west you go. The eastern Caribbean seems to get hit the most. Its hurricane season so the possiblity always exists.
Your safety is the Captain's main concern, he will steer around the storm, you might miss a port of call or call on another port. If he cannot steer around the storm safely he will find a safe haven and put into port. You might get some rain and some rough seas but that's usually about it.
we did western caribbean last sept 7-14 and had no major problems - there were a couple of huricanes out there but we didn't really feel anything. just be prepared for the fact that a port might get changed just in case. i think the ships do whatever they can to avoid problems, so it might mean missing or changing a port. if you're ok with that, i wouldn't worry at all.