Weatherbase is a good & quick reference website to find out weather averages for each month of the year. I use this website on a regular basis when planning my own or other peoples trips. The site has everything you need to know and it’s a good starting point to plan your trip so. It’s also available as an app, but only on iOS and Windows Phone unfortunately (why on earth a Windows phone?).
Following is a list of towns / provinces in Thailand with a history of weather info, click on a name to find useful info.