Gulf of Thailand weather forecast: what the outlook means for your trip
This Gulf of Thailand weather and tide forecast covers Koh Samui, Koh Phangan, Koh Tao and the mainland gateway of Surat Thani for the period 16 to 21 September, 2025. It is a planning aid for travellers with ferry, bus, minivan or private transfer tickets on this stretch of coast.
The headline: a wet, unsettled week. Every day in the window shows heavy rain across part of the region, daytime highs of 32–33 °C and overnight lows around 26 °C, with winds building from about 13 km/h at the start of the period to roughly 17 km/h by the weekend.
Day-by-day weather and tide forecast
Tuesday, September 16 — Temperature: 32–26 °C. Weather: Heavy rain, 60% of area. Sailing Conditions: Winds ~13 km/h, moderate seas. Tide: Low tide morning, high evening.
Wednesday, September 17 — Temperature: 33–26 °C. Weather: Heavy rain, 40% of area. Sailing Conditions: Winds ~13 km/h, choppy seas possible. Tide: Low tide morning, high evening.
Thursday, September 18 — Temperature: 33–26 °C. Weather: Heavy rain, 40% of area. Sailing Conditions: Winds ~15 km/h, moderate to rough seas. Tide: Low tide morning, high late evening.
Friday, September 19 — Temperature: 32–26 °C. Weather: Heavy rain, 60% of area. Sailing Conditions: Winds ~15 km/h, rough seas at times. Tide: Low tide early morning, high at night.
Saturday, September 20 — Temperature: 33–26 °C. Weather: Heavy rain, 40% of area. Sailing Conditions: Winds ~17 km/h, unsettled seas. Tide: Low tide morning, high late night.
Sunday, September 21 — Temperature: 33–26 °C. Weather: Heavy rain, 40% of area. Sailing Conditions: Winds ~17 km/h, moderate seas. Tide: Low tide morning, rising afternoon.
How conditions shift across the week
Wind speeds trend upward through the period, and sea states follow: moderate on Tuesday, choppy midweek, moderate to rough on Thursday, rough at times on Friday, and unsettled over Saturday before easing back to moderate on Sunday. Thursday to Saturday is the roughest stretch on paper.
Who this affects
Sea conditions matter most to passengers crossing between Koh Samui, Koh Phangan and Koh Tao, and to anyone travelling to or from the Surat Thani mainland piers. Smaller, faster boats feel choppy and rough seas more than larger vehicle ferries, so the same forecast can produce very different rides.
Road legs are affected differently. Heavy rain across 40–60% of the area can slow bus and minivan journeys and combined bus-and-ferry itineraries, particularly on the mainland approach to the piers.
Practical tips for travelling this week
- Low tide falls in the morning on most days, with high water in the evening or late at night — worth knowing if your pier or beach transfer is tide-sensitive.
- Leave buffer time between connecting services rather than booking the tightest possible link.
- Pack a light rain layer and keep documents and electronics in a dry bag on deck.
- If you are prone to seasickness, favour departures on the calmer days and sit low and central on board.
- Confirm departures with your operator on the day, since sailings can be delayed or cancelled at short notice in rough conditions.
Book your crossing
Compare operators, departure times and travel times for Koh Samui to Koh Phangan ferries, Koh Samui to Koh Tao ferries and Surat Thani to Koh Samui transfers, or browse everything serving Koh Samui, Koh Phangan and Koh Tao. Live prices and timetables are shown at the time of booking.
Disclaimer: Weather, sea conditions, and tide information are subject to change. Always check local updates and forecasts before planning activities.
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