Given shifting weather patterns I'm not sure you can know anything for certain these days. My first visit to Thailand was in September 2006 (got to experience the coup first hand). I had been somewhat concerned about weather but it turned out to be fine. Just some afternoon showers on a few days. I spent my time in Chiang Mai/Chiang Rai in addition to Bangkok. So I wouldn't necessarily write it off either if that's where you really want to go. The other benefit is that it's still low season.