For the Grenadines in May:
Average Daily Temp - 79 F.
Average Low Temp. - 77 F.
Average High - 88 Average Daily Relative Humidity - 75% .
Average Monthly Rainfall - 3.1"
Number of Days with some rain - 12

If you want more detailed statistics go here:
https://www.wunderground.com/weather/vc/bequiao www.wunderground.com
Click on the Almanac section then search history for May. You can find data for actual weather in the Grenadines for every day in May during the last 10 years.