I doubt you saw anywhere that it said rain for "long periods of time". The summer forecast will typically show a daily chance of showers. They roll in usually in the afternoon and clear up before sunset. The majority of our trips have been during the summer rain season. It about 7 trips we have only saw two days in which I would call it a long period of rain. Those were actually some really fun days as everyone finds things to do until it quits.

Rain is part of a tropical climate and will not ruin a Negril vacation.