Janice,
The weather in Hawaii in September is not much different that the weather in Hawaii most any other time of the year, generally sunny in the mid-80's. The only possible difference at that time of the year is that sometimes the trade winds stop and the humidity may get a little uncomfortable. Also, the weather may vary from place to place on the islands, with one side of the mountain (leeward) being in sun while the other side of the mountain (windward) has some showers.