Thailand forum

If this December is normal, the south-west would be the better bet.
I'm a weather nut - this is my spiel for December.
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The southern Gulf islands (Samui, Phangan, Tao) tend to be wetter in December. They have a different wet season - normally Oct into Jan.
Prolonged wet periods are not too common in Thailand but I have to say in all the years I've been following the wet season (I'm a weather nerd, got a weather page in the link below) the most complaints of this have come from the Samui area islands in Oct, Nov and early Dec. But things are variable - I've only visited this area once during THAT period, in Nov 97, and I got good weather. It seems these real wet ones come up about in about 20-25% of the years - but not necessarily every 4th-5ths year.
Late December and early Jan should see a taper of rainfall - the fact that it is high season in Samui etc points towards rain not normally excessive enough to scare tourists away like say Phuket in August (I like Phuket in August, but there is a distinct lack of summering Euros and Americans).

In other regions early November often sees the wet season tapering off - by mid November often dry season has kicked in and so December normally should be dead-set dry season.
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As a point of interest, the Thai Bureau of Meteorology reckons both areas will have SLIGHT BELOW normal rainfall this december. Check the table bottom of page here -
http://www.tmd.go.th/programs%5Cuploads%5Cforecast%5C2009-10-05_3months_EN_084110.pdf