There is always the possibility, typhoons are rather common here in Okinawa however for the past two years there have been no "direct" hits of typhoons here on the main island of Okinawa. A few hit the Miyako and Ishigaki Island areas though.
Last year we were brushed by a couple but nothing serious. Typhoons can occur anytime of the year, however typhoon season generally runs from the end of June until October.
There is no way to predict now if or when a typhoon will hit the island, particularly in September. If you are planning to come here for a trip or something it would probably be a good idea to check the weather usually a week or so ahead of time before departure. Typhoons dont just pop up out of nowhere and can be tracked rather easily.
There are numerous sites available for tracking typhoons that is just one.
I would suggest keeping it in mind, about typhoons, but dont let it be a deterent for planning a trip or activities here in Okinawa. If it happens it happens but usually damage from typhoons here in Okinawa is much less than in mainland for any number of reasons. The biggest being that the buildings here are pretty much all constructed from steel reinforced concrete.
It's easy to ride out a typhoon, they really are not a "big" deal. Particularly if one uses common sense and stay indoors.