Thailand’s weather is two wind reversals, not a single “rainy season” you can circle on a UK calendar. The Andaman and the Gulf have opposite wet windows. The north has a third mood. Book the wrong sea and you will watch a red flag from a lobby.
The planning fork: Which coast. The climate-zone version: Climate zones. Gateway month voice: When to visit.
Southwest monsoon (May–October)
Asian land heats, low pressure draws wet air off the Indian Ocean, that air hits the Andaman first.
- When: late May–October in the north; June–November in parts of the south
- Direction: southwest → northeast
- Andaman: the full force — Phuket, Krabi, Phang Nga, islands. Rough seas. Similan and Surin close about 16 May–14 October 2026.
- North: afternoon storms, peak often August–September
- Bangkok / Isan: inland leftovers; Bangkok peak often September–October, street floods on a bad day
- Gulf: partly sheltered. Samui, Phangan, Tao often the better beach bet May–September
Thai monsoon rain is usually not British all-day grey. Mornings clear and hot; cumulonimbus by mid-afternoon; a violent hour; then evening. Multi-day soaks happen, especially at peak, and El Niño/La Niña shove the totals around.
Ranong on the upper Andaman can take 4,000 mm+ a year. Phuket ~2,300 mm. That is why “Thailand in September” is not one forecast.
Northeast monsoon (November–February)
Land cools, dry air from China/Siberia flows southwest. Cool and dry for most of the map. Wet for the Gulf.
- North / Isan: cool season. Chiang Mai nights 10–15 °C. Doi Inthanon (2,565 m) can frost. Doi Inthanon.
- Bangkok: the liveable months, 25–32 °C, less sticky.
- Andaman: dries, islands reopen, November–April is the classic window.
- Gulf: northeast moisture. Samui’s wettest month is often November, with flood days. Eases late December.
The offset is the whole trick: Andaman wet May–October while Gulf is often fine; Gulf wet November–December while Andaman is at its best.
| Month | Andaman | Gulf | North & central |
|---|---|---|---|
| January | Dry / prime | Drying | Cool |
| February | Dry / prime | Dry | Cool |
| March | Dry, heating | Dry | Hot starts |
| April | Transition; Songkran 13–15 Apr 2026 | Dry / hot | Hottest |
| May | Monsoon on; Similans shut ~16 May | Mostly dry | Rains begin |
| June–August | Wet / swell | Often the Gulf high window | Rainy; north peak Aug |
| September | Peak Andaman rain | Still often the better sea | Central peak |
| October | Transition; Similans ~15 Oct | Gulf wet arriving | Easing |
| November | Dry starting | Often wettest | Cool starting; Loy Krathong 24 Nov 2026 |
| December | Dry / prime | Wet easing | Cool |
These bars are suitability for a beach/boat week, not millimetres. A “dry” month can still dump at 16:00.
ENSO and storms
- El Niño — drier, hotter, drought risk in Isan and the plains, bleaching risk up. Coral reefs.
- La Niña — wetter, flood risk up. 2011 mega-flood sat on a La Niña. Rivers and waterways.
Pacific storms that have already hit Vietnam/Cambodia can still dump on the Gulf, Isan, and the east, especially September–November. Full typhoons rarely keep strength over Thai mountains; remnants still flood.
Practical monsoon advice
- Do not skip Thailand because a website said “rainy season.” Rain is often a dramatic hour.
- Pack a packable rain jacket, not only a disposable poncho. Umbrella for city, jacket for boats.
- Andaman May–October is the wrong window for Similan snorkel. North and Gulf still work. Inland: Khao Sok.
- Gulf October–December, especially November, is the caution.
- Shoulder months (May, November) are often value: fewer crowds, prices down, weather “usually fine” in the way that still cancels a longtail.
Park fees do not pause for rain: Khao Yai 400 THB, Inthanon 300, Khao Sok HQ ~200–300, Cheow Lan ~300, Similan ~500. Park permits. A dry bag on any coast.
Festivals sit on the calendar anyway: Songkran 13–15 April 2026 (hot), Loy Krathong 24 November 2026 (cool/dry north, Andaman opening, Gulf maybe wet). Songkran, Loy Krathong. BE 2569 = 2026.
Gateway packing and first week: Packing, First week. Beaches by job: Best beaches.
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Which coast, Climate zones, Andaman coast, When to visit, Park permits.