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Thai Weather Warning: Flash Floods and Landslides Forecast for 5 September
The Thai Meteorological Department (TMD) issued a forecast on the morning of 5 September 2026 warning of isolated heavy rain, flash floods, and rough seas across large parts of Thailand. The cause is a monsoon trough sitting over the North and upper Northeast, stretching into a low pressure cell over the Gulf of Tonkin, combined with a moderate southwest monsoon moving across the Andaman Sea, mainland Thailand, and the Gulf of Thailand. For Koh Lanta and the surrounding Krabi coast, the practical impact is choppier seas rather than heavy rainfall, so anyone with boat trips or water activities planned should keep an eye on conditions.
The general picture
TMD is warning of isolated heavy rain leading to flash floods, water overflowing riverbanks, and landslides, particularly near waterways close to foothills and in low-lying areas. Seas in the upper Andaman are expected to run around 2 metres, rising higher during thunderstorms, and all vessels are being told to proceed with caution and steer clear of storm activity.
Region by region
<table> <tr><th>Region</th><th>Conditions</th><th>Temps (min–max)</th><th>Wind / Sea</th></tr> <tr><td>North</td><td>Scattered thundershowers, isolated heavy rain (isolated very heavy rain in Nan, Uttaradit, Phitsanulok, Phetchabun)</td><td>22–27°C to 27–35°C</td><td>SW 10–20 km/h</td></tr> <tr><td>Northeast</td><td>Fairly widespread rain, isolated heavy falls (isolated very heavy rain in Loei, Nakhon Phanom, Mukdahan, Yasothon, Amnat Charoen, Si Sa Ket, Ubon Ratchathani)</td><td>22–26°C to 31–35°C</td><td>SW 10–20 km/h</td></tr> <tr><td>Central</td><td>Scattered thundershowers, isolated heavy rain</td><td>23–27°C to 34–36°C</td><td>SW 10–20 km/h</td></tr> <tr><td>East</td><td>Scattered thundershowers, isolated heavy rain</td><td>23–27°C to 31–35°C</td><td>SW 15–35 km/h; waves 1–2m, higher in storms</td></tr> <tr><td>South East Coast</td><td>Scattered thundershowers</td><td>23–26°C to 33–36°C</td><td>SW 15–35 km/h; waves ~1m, 1–2m offshore, higher in storms</td></tr> <tr><td>South West Coast</td><td>Scattered thundershowers</td><td>23–27°C to 31–34°C</td><td>Phuket north: SW 20–35 km/h, waves ~2m; Krabi south: SW 15–35 km/h, waves 1–2m, higher in storms</td></tr> <tr><td>Bangkok and Vicinity</td><td>Scattered thundershowers, isolated heavy rain</td><td>26–28°C to 33–36°C</td><td>SW 10–25 km/h</td></tr> </table>
What this means for Koh Lanta
Krabi province, including Koh Lanta, falls under the "South West Coast" region in the "Krabi southward" wind and wave zone. The forecast here calls for scattered thundershowers rather than the heavier rain warnings given to the North, Northeast, Central, East, and Bangkok areas. Southwest winds of 15 to 35 km/h are expected, with waves of 1 to 2 metres, rising above 2 metres during thunderstorms.
This means the rain risk on Koh Lanta itself is comparatively light for 5 September, but the sea conditions are worth planning around. Small boat transfers, ferry trips, snorkeling, and diving tours may be affected or delayed, so it's worth checking with your tour operator before heading out on the water. Boat operators have been told to proceed with caution and avoid areas with thunderstorm activity.
Elsewhere in Thailand
Travelers in the North and Northeast face the highest flood and landslide risk, with isolated very heavy rain warnings in several provinces, so low-lying areas, riverside accommodation, and foothill trekking routes are best avoided on 5 September. Bangkok can expect scattered thunderstorms and heavy rain alongside warm temperatures up to 36°C, so sudden downpours are likely. Southern Gulf coast spots like Koh Samui, Songkhla, and Narathiwat have a lighter forecast, with scattered thundershowers only, though wave heights offshore are still elevated.
Information sourced from The Thaiger.