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 about weather patterns, it’s a good starting point. It is also available as an app, but only on iOS and Windows Phone only (why on earth a Windows phone?), unfortunately not available for Android as yet.
Following is a list of towns / provinces in Thailand with a history of weather info, click on a name to find useful info.