THIS should give you an idea what to expect.

Pippy

We found May to be very humid in comparison to when we usually go in December. Every time I left the air conditioned room my glasses fogged up.
We have never really had a problem with rain in the DR, yes when it does rain its usually heavy but its over very quickly and still very warm and more often than not it was during the early evening or night.
Please remember that even on slightly overcast days (you might get a couple but it will still be very hot) that the sun is still really powerful and you will burn if you don't use high factor protection. I've seen so many lobster coloured people lose days of their holiday sitting in the shade because they didn't take enough care.
All that sitting about in the sunshine with free drinks and food is brilliant, just relax and enjoy.