Why hasn't Thailand's cool season started yet, and how cold will Bangkok and Chiang Mai get this winter?

It is November, Bangkok is still getting evening thunderstorms, and afternoon highs in the capital were forecast at 34 to 36 degrees on 1 November. So where is the cool season? According to the Thai Meteorological Department (TMD), Thailand has not yet officially entered winter, and this year's cool spell is forecast to be both later and milder than usual.

Here is why the season has not been declared, what the TMD's own winter outlook says about temperatures in Bangkok, Chiang Mai and the south, and how to plan trips and daily life around it.

Why has winter not been declared yet?

The TMD does not declare the cool season just because mornings feel fresher. It looks at three things across upper Thailand: rainfall has to fall away, low-level winds have to settle into a steady north-easterly or easterly pattern, and minimum temperatures have to drop below 23 degrees over a wide area.

Speaking to TNN on 1 November, the department's deputy director-general and spokesman said only the rainfall test had been met, with rain covering less than 20 per cent of upper Thailand. The temperature test needs more than 60 per cent of the area below 23 degrees at night, and only about 40 per cent qualified, mostly in the upper north. The winds were still too changeable. He said the strong cold air mass from China had mostly veered east towards Japan, and that the season may start around mid-November.

How late is that?

A mid-November start would not be unusual. The 2023 season began on 14 November, and the TMD spokesman noted that in some El Nino years the department has not been able to declare winter even by 15 November.

What the official winter outlook says

The TMD's seasonal outlook, issued on 9 October 2026, expected upper Thailand to enter the cool season about a week later than average and to stay cool until about mid-February 2027. It also forecast a milder winter than last year:

• Average minimum temperatures across upper Thailand of about 22 to 23 degrees, roughly 2 degrees above the normal 19.9 degrees and warmer than last winter's 19.6 degrees.
• Lowest temperatures of about 10 to 12 degrees, mostly in the upper north and upper north-east.
• Lowest temperatures of 19 to 21 degrees in Bangkok and 18 to 20 degrees in the surrounding provinces.
• The coldest spell from late December 2026 to early February 2027, with cold to very cold mornings and frequent frost on mountain peaks.
• El Nino expected to strengthen from late 2026 into early 2027, one reason the department expects a warmer, less chilly season.

City by city: how cold will it get?

Those are the coldest single readings expected, not typical nights. For Bangkok and its neighbouring provinces the TMD forecast average minimums of 25 to 27 degrees in November, 24 to 26 in December and 23 to 25 in January, warming again through February. In other words, Bangkok's cool season is more about drier air and pleasant mornings than anything you would call cold.

What about the south?

The Gulf of Thailand side of the south runs on a different calendar. The outlook expected heavy, widespread rain from Chumphon southwards in November and December, with flash floods and run-off possible in places and waves of 2 to 4 metres at times in the Gulf. It also warned that low-pressure systems in November and December could become tropical storms and move towards the Gulf and the south. The Andaman side, including Phuket, was forecast to see rain over 30 to 50 per cent of the area until mid-November, with waves of 1 to 2 metres.

TNN reported on 1 November that the TMD expected another round of rain on the Gulf side of the south from 7 to 13 November, so anyone planning Koh Samui or Koh Phangan in the next fortnight should watch the daily forecasts.

What it means for your plans

• Mountain trips in the north: if you want misty mornings and the best chance of a chill at viewpoints, the TMD's coldest window is late December to early February. A trip in early November may feel more like late rainy season.
• Driving in the north and north-east: the TMD warns of dense fog in many areas in December and January, especially in the north and north-east, so allow extra time and drive with care in the early morning.
• Bangkok: expect evening storms to linger for a while yet, then cooler, drier mornings once the season starts. A light layer is enough for most nights.
• Gulf islands: plan around the rain. November and December are the wettest months there in this outlook, while the Andaman coast dries out sooner.
• Later in the season: the TMD also flagged possible thunderstorms with gusty winds and hail in upper Thailand in January and February.

Is a warmer winter good news?

For anyone who moved to Thailand to escape the cold, a mild season hardly sounds like a problem. But a short, warm winter means fewer of the crisp mornings that make the north so popular from December, and the TMD itself links this year's outlook to El Nino. Our advice is simple: keep plans flexible, follow the TMD's daily forecasts rather than the calendar, and save the mountain weekend for the coldest weeks.