This cold snap is by far the coldest I've experienced in Bangkok in ~ 15 years. Feel for the locals, and folks in the country, who are unused to the low temps (lots of people getting sick and dying), and don't have hot water heaters.
The good news is that things should normalize (high: 35, low: 25) as the week progresses. By the time you get here you won't need a coat. If you're here already it's honestly not that bad (low of 15).
http://www.tmd.go.th/en/province.php?id=37