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For much of Thailand September sees a general improvement in the weather. The rain starts to back off in more provinces while the temperatures continue to fall (ever so slightly). The islands in the southern Gulf of Thailand are your best bet in September while Bangkok and Central Thailand see a last blast of heavy rain before the rains really back off.
Where are the best places in Thailand in September? Consider:
September is similar to August in much of the country, though the rain does tend to back off in Northern destinations like Chiang Mai, Lamphun and Phitsanulok, as well as in Northeastern points like Khon Kaen and That Phanom. The lower Gulf coast provinces of Chumphon, Surat Thani, Nakhon Si Thammarat, Phatthalung and Songkhla remain considerably drier than the rest of the country. Beyond this region, waterfalls like those in Kanchanaburi, Umphang and Khao Yai tend to be extremely powerful and potentially dangerous. Floods are common in the Central plains, including parts of Ayutthaya and Lopburi provinces.
Overall, September is not a great time to hit the islands. Ko Phi Phi, Ko Lanta and other Andaman Sea islands are still soggy and quiet as can be, while flooding and mudslides have occurred on Ko Chang and other places in the eastern Gulf region at this time of year. The lower Gulf islands of Ko Tao, Ko Pha Ngan and Ko Samui are your best bets, though even here some rain should be expected particularly towards the end of the month.
The weather in Bangkok in September, along with Central Thailand, sees a sharp increase in the rain — in Bangkok is increases by over 50%, but after September the rains beat a speedy retreat. The waterfalls in Kanchanaburi can be especially impressive this year — almost as memorable as the flooded streets in Bangkok! Temperatures remain very warm.
Finally! September sees an almost halving of the rain on Ko Chang. It is still going to be raining most days, but nothing like how it was coming down in August. From here onwards the rain continues to back off, forming a steady decline to the glorious November, December, January period. If you can put off your trip here for a month, do so.
September sees the rainfall levels begin their steady decline as the wet season prepares to wind down. The daily minimum and maximum temperatures don't change much from the previous month though — you'll need to wait another month before the first signs of a serious cooling will begin. If it has been a very wet wet season, then flooding in Chiang Mai and other urban centres may occur, along with rural flooding if the major rivers break their banks.
While most provinces in northeast Thailand will see a further reduction in rainfall in September, some provinces, notably Buriram and Khon Kaen, see an uptick in rainfall before drastic drops across the region in October. By northeast standards, the temperatures are becoming more moderate.
September on the Gulf islands can be lovely. It is the last month of especially good weather before the wet months of October and November. Yet again, the islands of Ko Samui, Ko Pha Ngan and Ko Tao remain the best choice weather-wise in September. This time of year the Gulf Coast mainland is actually a little wetter than the islands.
September is the wettest month of the year on the Andaman Coast. The seas will still be rough and the rainfall heavy. Surfers may be able to catch some waves on Phuket in September — good for surfers, but that is about all. On a positive note, the weather should improve somewhat in October, so if you can, organise to spend September on the Gulf Coast and October on the Andaman Coast.