Short answer
The Thai Meteorological Department's seasonal outlook of 15 October 2026 says the cool season should run from about the third week of October to around mid-February 2027, with average minimum temperatures of 22 to 23°C, about two degrees above normal, because of El Niño. The upper North may see 10 to 12°C and Bangkok 19 to 21°C. On 28 October the department said morning temperatures had not fallen enough to declare the season's start. It also warned of heavy rain on the Gulf coast from Chumphon southwards in November and December.
A cool season that starts late and runs warm
In its seasonal outlook of 15 October 2026, the Thai Meteorological Department (TMD) said temperatures should fall around the third week of October, about a week later than usual, and that the cool season should last until around mid-February 2027, Thaiger reported. It expects average minimum temperatures of 22 to 23°C, about two degrees above normal, and blames El Niño, which it expects to strengthen towards the end of the year.
Not everywhere will be mild. The upper North and upper Northeast may see lows of 10 to 12°C, and Bangkok 19 to 21°C. The coldest period should run from late December to early February, with frost possible on mountain summits. In January and February the upper country may see thunderstorms, squalls and hail.
Not declared yet on 28 October
On 28 October, Somkuan Tonjan, director of the TMD's Aeronautical Meteorological Division, said morning temperatures had not fallen enough to declare the start of winter, Thaiger reported. He said the weather would stay variable into early November, which could make this cool season later than usual.
The mountains have already turned cold. Doi Inthanon's summit recorded a low of 9°C on 20 October and Kew Mae Pan 12°C, according to the park, as the Nation reported. Daily visitor numbers exceeded 2,700, and forecasters expected the temperature to fall a further one to two degrees.
The Gulf coast stays wet in November and December
The TMD warned of heavy rain along the Gulf coast from Chumphon southwards in November and December, with flash floods, runoff and rivers overflowing their banks. Waves in the Gulf could reach two to four metres, and storm surges of four to five metres are possible in some locations, according to the outlook.
Koh Samui is in that zone. The department said it planned to update the forecast in the last week of October, and we did not find the update on the pages we opened.
What it means for Chiang Mai and Samui days
On our Chiang Mai elephant sanctuary and Doi Inthanon tour, the top of the mountain is much cooler than the city, so bring a light jacket. The national park fee, 300 baht per adult and 150 per child, is paid on the day unless your quote says otherwise.
On the Gulf side, our Koh Tao and Nang Yuan day trip crosses open sea, which can be bumpy, roughest from late October to December. If the sea is unsafe, the operator moves the trip to another date or refunds it. Send us your date on WhatsApp and we tell you what we know.
Practical checklist
- Pack a light jacket for Doi Inthanon and for evenings in the north from November to February.
- Expect Bangkok mornings of 19 to 21°C on the coldest days, still mild for most visitors.
- Check the sea forecast for your date if you plan a Gulf island trip in November or December.
- Keep a spare day in your Samui plan in case a boat trip has to move.
- Look at the TMD's updated outlook before you finalise winter plans.
Questions and answers
When does the cool season start in Thailand in 2026?
The TMD expected temperatures to fall around the third week of October, about a week later than usual. On 28 October it said it could not yet declare the start, and conditions would stay variable into early November.
Will it be colder or warmer than normal?
Warmer. The outlook of 15 October puts average minimum temperatures at 22 to 23°C, about two degrees above normal, because of El Niño. Some northern and northeastern areas may still see 10 to 12°C.
What will the weather be like on Koh Samui in November?
The TMD warned of heavy rain on the Gulf coast from Chumphon southwards in November and December, with waves of two to four metres. Our Koh Tao tour moves or refunds a trip if the sea is unsafe.
When is Doi Inthanon coldest?
The TMD's coldest period for Thailand runs from late December to early February, with frost possible on mountain summits. The summit already recorded 9°C on 20 October.