Heavy rain puts all Thai provinces on five-day flood alert

The Department of Disaster Prevention and Mitigation (DDPM) has warned all provinces nationwide, including Bangkok, to prepare for very heavy rain during September 16-20, 2026, with risks of flash floods, forest run-off, landslides, waterlogging and rough seas.

Personnel and machinery are also being prepared for round-the-clock deployment in at-risk areas.

Nationwide flood watch and Bangkok drainage risks

All provinces must remain alert for flash floods, forest run-off, landslides and waterlogging. In Bangkok, particular attention is needed in urban areas prone to recurring flooding or where drainage cannot keep pace with rainfall.

Strong winds and high waves are a concern in six southern provinces: Ranong, Phang Nga, Phuket, Krabi, Trang and Satun.

Rough seas in six southern provinces

Coastal and island areas in the six southern provinces must monitor conditions closely. Waves in the Andaman Sea are expected to reach around 2-3 metres, potentially exceeding 3 metres in areas affected by thunderstorms.

Officials will coordinate warnings for tourists and coastal residents, while urging mariners and boat operators to exercise greater caution. Authorities should consider prohibiting boats from putting to sea if conditions become severe.

Reservoirs above 80% capacity under watch

Small and medium-sized reservoirs holding more than 80% of their capacity require close monitoring in Chiang Mai, Chiang Rai, Phayao, Nan, Phetchabun, Tak, Uthai Thani, Chaiyaphum, Sakon Nakhon, Chanthaburi, Trat, Kanchanaburi, Krabi and Ranong.

Waterways requiring close monitoring

Areas along several major waterways face risks from rapidly rising water levels, overflowing banks and flooding in low-lying areas. The waterways requiring monitoring are:

• Yuam River in Mae Hong Son.
• Sai and Kok rivers in Chiang Rai.
• Nan River in Nan.
• Moei River in Tak.
• Khwae Noi River in Phitsanulok.
• Pa Sak River in Phetchabun.
• Songkhram River in Sakon Nakhon.
• Kolok River and Tanyong Mat Canal in Narathiwat.

Heavy rain forecast for September 17-20

The Thai Meteorological Department forecasts heavy to very heavy rain in some areas of Thailand during September 17-20, 2026. The North and Northeast are expected to be affected first on September 17, before the rain spreads to the Central region, East and South on September 18.

The main factors are a cold air mass extending southwards from China, pushing the monsoon trough down across Thailand, and a strengthening south-westerly monsoon.