You can research the history and frequency of hurricanes for St. Kitts (and any other island for that matter) at www.stormcarib.com. Use the "Climatology" link then scroll down and click on "Hits & Misses". Then click on "St. Kitts (or any other island you wish) - you'll be directed to a chart showing the dates and intensity of every hurricane/tropical storm to affect the island since the early 1850's.

FYI - if there are no storms brewing during your visit the weather can be quite nice in September.

As Carol said, September is "off season" so many resorts, restaurants and tour operators are closed or operating under reduced hours or staff (or both).