Thailand Weather Warning: Heavy Rain and Rough Andaman Seas Through September 20

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Update — September 17, 2026: Thailand has a new Thai Meteorological Department (TMD) warning in effect. Heavy to very heavy rain is forecast for September 17–20, spreading from the North and Northeast on September 17 into Central Thailand, the East and the South from September 18. The TMD has also issued a nationwide five-day flash flood and landslide alert, and is warning of rough seas of 2–3 metres in the upper Andaman Sea, with small boats told to remain ashore until September 20.

Thailand is facing several more days of heavy rain and rough seas, with the Thai Meteorological Department warning of widespread downpours, flash floods and dangerous marine conditions through September 20, 2026. The weather pattern is expected to spread from the North and Northeast into Central Thailand, the East and the South over the following days.

For travelers, the main concerns are temporary flooding, difficult road conditions, landslide risk in hilly areas, and rough seas affecting boat trips and ferries on the Andaman coast. This is an update to the TMD warning we first covered on September 9 — the earlier warning period (through September 13) has passed, and this new warning covers the days ahead.

Where Will the Heaviest Rain Fall?

The weather pattern is changing across the country rather than affecting every region equally at the same time.

Period	Areas to Watch
September 17	North and Northeast Thailand
September 18–20	Central Thailand, Bangkok, the East and the South

The TMD is also warning that accumulated rainfall during September 17–20 could trigger flash floods and forest runoff, with particular caution advised on foothill slopes near waterways and in low-lying areas nationwide.

Bangkok Could See More Heavy Rain

Bangkok and surrounding provinces are among the areas that should pay particular attention from September 18 onward, as the rain band moves south and east.

Heavy accumulated rainfall can produce temporary flooding, especially on low-lying roads and in areas where drainage cannot handle a sudden downpour. Travelers staying in Bangkok should allow extra time for road journeys and avoid planning tightly connected activities during periods of severe rain.

If you are arriving or departing during this period, remember that Bangkok has two airports. Our Bangkok Airport Guide explains the differences between Suvarnabhumi and Don Mueang and the main ways to travel between them.

Phuket, the Andaman Coast and Sea Conditions

This update carries a specific warning for anyone with boat trips, ferries or island-hopping plans on Thailand’s west coast. The TMD is forecasting waves of 2–3 metres in the upper Andaman Sea, rising to more than 3 metres during thunderstorms, through September 20. Small boats in the Andaman Sea have been told to stay ashore until September 20. The lower Andaman Sea is forecast at around 2 metres, higher during storms.

If you have a boat trip, snorkeling or diving excursion booked out of Phuket, Krabi, Phi Phi or nearby islands during this window, confirm with your operator on the day rather than relying on an earlier booking confirmation — operators routinely cancel or reschedule trips when TMD issues this kind of advisory.

Phuket is currently in its rainy season, and September is one of the wetter months on the Andaman coast. Our Thailand Islands Guide explains the different monsoon patterns on the Andaman and Gulf coasts and why the choice of island matters depending on the month.

If you’re already staying in Phuket, our Phuket area guide can also help when deciding where to stay if poor weather changes your plans for the day.

Flash Floods and Landslides Are the Main Risk on Land

The TMD is warning that heavy accumulated rainfall during September 17–20 may cause flash floods, rapid runoff and landslides, particularly around foothills, waterways and low-lying areas. This is a nationwide five-day alert, not a single-region warning.

This does not mean that Thailand is expected to experience nationwide flooding. The risk is local and can develop quickly after intense rainfall.

Travelers should be especially careful with road trips to waterfalls, national parks, mountain areas and other locations where access roads can become difficult after heavy rain.

Could the Weather Affect Flights?

Heavy rain does not automatically mean widespread flight cancellations. Thailand’s airports normally continue operating during the rainy season, although thunderstorms and poor visibility can sometimes cause delays or changes to individual flights.

The bigger issue for travelers may be getting to or from the airport by road if a severe downpour causes temporary flooding or traffic congestion.

If you have a flight during the warning period, check your airline’s latest status before leaving for the airport and allow more time than usual for the journey.

What Should Tourists Do?

• Keep an eye on the weather forecast if you are travelling between September 17 and 20.
• Allow extra time for road journeys in Bangkok and other areas affected by heavy rain.
• Postpone or reconfirm boat and ferry trips on the Andaman coast — small boats have been told to stay ashore through September 20, with waves of 2–3 metres forecast (higher in storms).
• Avoid flood-prone roads and do not attempt to drive through rapidly rising water.
• Stay away from foothill and waterway routes where flash floods and landslides are more likely during this five-day alert.
• Check flights directly with your airline if thunderstorms are affecting your departure or arrival time.
• Keep outdoor plans flexible, particularly for mountain, waterfall and marine activities.

Should You Cancel a Thailand Trip?

Not necessarily. Thailand is in its southwest monsoon season, and heavy rain during September is normal. The current warning does not mean that the entire country will be affected by continuous rain or that tourism has stopped.

The important difference this time is the combination of nationwide flash flood risk and a specific marine warning for the Andaman coast. Travelers should be prepared to change individual plans — especially boat trips — rather than automatically cancel an entire trip.

Bottom Line

Thailand is entering several more days of potentially disruptive weather, with heavy to very heavy rain and rough Andaman seas forecast through September 20. The North and Northeast should watch conditions from September 17, with Bangkok, Central Thailand, the East and the South increasingly affected from September 18.

For tourists already in Thailand, the best approach is to keep plans flexible, allow additional travel time, and reconfirm boat trips and marine excursions before heading out. The situation is expected to evolve, so updated forecasts from the Thai Meteorological Department should take priority over earlier predictions.

FAQ

How long will the heavy rain last in Thailand?

The current TMD warning covers September 17–20, with heavy to very heavy rain possible in different parts of Thailand during this period, alongside a nationwide five-day flash flood and landslide alert.

Will Bangkok be affected by the heavy rain?

Yes. Bangkok and surrounding areas are expected to see increased rainfall from September 18 onward, with localized heavy to very heavy rain possible.

Is it safe to take a boat trip in Phuket or the Andaman islands right now?

The TMD has told small boats in the Andaman Sea to stay ashore through September 20, with waves forecast at 2–3 metres (higher during thunderstorms). Confirm any boat, snorkeling or diving trip with your operator on the day rather than relying on an earlier booking.

Will flights in Thailand be cancelled because of the rain?

There is no indication of a nationwide flight disruption. However, severe thunderstorms can cause individual delays or schedule changes, so passengers should check directly with their airline.