For what our readers describe as the best coverage on weather in the DR, see
http://dr1.com/forums/weather-beyond/
that is the DR Weather & Beyond Forum, where you are right now. At that page click on the Sticky: Hurricane Season 2007 and go to the last post. Check what Chris says on what may be in the area. She will warn everyone as soon as a major storm enters our area. For example, she posts today: "For us [meaning the Dominican Republic], there is very little activity out there. So, for the next few days, weather should be normal summer Caribbean weather, hot with a few tropical showers and all looks quiet out over the Atlantic. So, we relax a few days and enjoy." That will cover the time you will be here if your vacation is a week or so.
For a general overview on weather in general in the DR, including weather in September, see
http://dr1.com/travel/expect/weather.shtml
For lots of links with DR area relative weather information for those who really want to get into weather, see
http://www.dr1.com/weather/hurricanes.shtml