
to the forums! If you're dead set on heading to Thailand in May, then pretty much wherever you go will be HOT HOT HOT - approaching and even exceeding 30 degrees Centigrade. That aside, if you stay in the Gulf of Thailand (Koh Samui etc) then it should be fairly dry.
That said, the weather's a lottery and you could end up having a week of rain in peak dry season, or a dry week in the wet season - but IMHO your chances are better to avoid rain in May if you avoid the Andaman side (Phuket, Phi Phi etc) and stay Gulf Side instead (Samui).