I have been to SD 6 times so far.... in the months of February, April, July, August, October, and December.
Everytime that I have been there, the tempature has always been warm to hot and rain just about 6 out of 7 days. But the rain has always been for just about an hour or so and then cleared up.
Perhaps I was just lucky..... But I was under the impression that a small island in the Carribean will always get a passing storm with rain often, but not stick around long enough to make you want to cancel your trip.
Good luck and travel safe
