TMD lists 55 Thai provinces in September 21 heavy rain alert
Thailand faces flash-flood and landslide risks, with very heavy rain forecast in Nakhon Nayok and Prachin Buri on September 21–22
- The Thai Meteorological Department (TMD) has issued a heavy rain alert for 55 provinces, including Bangkok, for the 24-hour period starting at 6 am on September 21.
- The warning highlights the risk of flash floods, landslides, and forest run-off in foothill and low-lying areas due to heavy and accumulated rainfall.
- Very heavy downpours are specifically forecast for parts of Nakhon Nayok and Prachin Buri, while thunderstorms are expected across 80% of Greater Bangkok, the Central region, and the East.
The Thai Meteorological Department (TMD) warned of heavy rain across Thailand on Monday (September 21, 2026), naming 55 provinces in its regional forecasts and also warning Bangkok and surrounding areas. Very heavy downpours are expected in parts of Nakhon Nayok and Prachin Buri.
The forecast reported by Thai Post covers the 24 hours from 6am on September 21 to 6am on September 22, 2026, Thailand time.
Foothill communities and low-lying areas face flood risks
The TMD warned that heavy to very heavy rain, combined with accumulated rainfall, could trigger flash floods and landslides, as well as forest run-off. People on foothill slopes near waterways and in low-lying areas were advised to remain particularly cautious.
The TMD also advised travellers to avoid routes prone to repeated flooding, especially through low-lying areas where water could accumulate over a short period.
Monsoon trough remains across lower Central, upper South and East
A monsoon trough continues to cross lower central, upper southern and eastern Thailand, extending towards a strong low-pressure area over the central South China Sea, according to the TMD.
The TMD said the high-pressure system, or cool air mass, from China covering upper northern and northeastern Thailand would begin to weaken. A moderate south-westerly monsoon continues to prevail over the Andaman Sea, southern Thailand and the Gulf of Thailand.
Upper Andaman Sea and upper Gulf waves reach about 2 metres
The TMD forecasts moderate winds and waves in the upper Andaman Sea and upper Gulf of Thailand, with waves around 2 metres high and exceeding 2 metres during thunderstorms.
The TMD advised mariners in the Andaman Sea and Gulf of Thailand to proceed cautiously and avoid thunderstorms. Small boats in the upper Andaman Sea should remain ashore.
Storms forecast across 80% of Greater Bangkok, Central and East
The TMD forecasts thunderstorms across 80% of Bangkok and surrounding areas, the Central region and the East during September 21–22. The complete regional outlook is as follows:
North: The TMD’s September 21–22 forecast puts thunderstorm coverage at 70%, with isolated heavy rain in Chiang Mai, Chiang Rai, Lampang, Sukhothai, Tak, Uttaradit, Kamphaeng Phet, Phichit, Phitsanulok and Phetchabun. Minimum temperatures will range from 21–26°C and maximum temperatures from 32–35°C. Easterly winds will reach 10–20 km/h.
Northeast: Thunderstorms will cover 70% of the region during September 21–22, according to the TMD, with isolated heavy rain in Loei, Chaiyaphum, Khon Kaen, Maha Sarakham, Roi Et, Mukdahan, Yasothon, Amnat Charoen, Nakhon Ratchasima, Buri Ram, Surin, Si Sa Ket and Ubon Ratchathani. Minimum temperatures will be 22–24°C and maximum temperatures 31–34°C, with easterly winds of 10–20 km/h.
Central: The TMD forecasts thunderstorms across 80% of the region during September 21–22, with isolated heavy rain in Nakhon Sawan, Uthai Thani, Chai Nat, Lopburi, Saraburi, Suphan Buri, Phra Nakhon Si Ayutthaya, Kanchanaburi, Ratchaburi, Nakhon Pathom, Samut Songkhram and Samut Sakhon. Minimum temperatures will range from 23–25°C and maximum temperatures from 32–34°C. South-easterly winds will reach 10–25 km/h.
East: The TMD’s September 21–22 forecast indicates thunderstorms across 80% of the region, with isolated heavy rain in Sa Kaeo, Chachoengsao, Chonburi, Rayong, Chanthaburi and Trat, and very heavy rain in parts of Nakhon Nayok and Prachin Buri. Minimum temperatures will be 23–25°C and maximum temperatures 30–34°C, with variable winds of 20–35 km/h. Waves will reach around 2 metres and exceed 2 metres during thunderstorms.
South, east coast: Thunderstorms will cover 70% of the region during September 21–22, according to the TMD, with isolated heavy rain in Phetchaburi, Prachuap Khiri Khan, Chumphon, Phatthalung, Songkhla, Pattani, Yala and Narathiwat. Minimum temperatures will be 22–25°C and maximum temperatures 31–34°C. From Surat Thani northwards, variable winds will reach 20–35 km/h, with waves of 1–2 metres, around 2 metres offshore and more than 2 metres during thunderstorms. From Nakhon Si Thammarat southwards, south-westerly winds will reach 15–35 km/h, with waves around 1 metre, rising to 1–2 metres offshore and exceeding 2 metres during thunderstorms.
South, west coast: The TMD forecasts thunderstorms across 60% of the region during September 21–22, with isolated heavy rain in Ranong, Phang Nga, Phuket and Krabi. Minimum temperatures will range from 23–25°C and maximum temperatures from 30–33°C. From Phang Nga northwards, south-westerly winds will reach 20–35 km/h, with waves around 2 metres and above 2 metres during thunderstorms. From Phuket southwards, south-westerly winds will reach 15–35 km/h, with waves of 1–2 metres and more than 2 metres during thunderstorms.
Bangkok and surrounding areas: The TMD’s September 21–22 forecast puts thunderstorm coverage at 80%, with isolated heavy rain. Minimum temperatures will be 24–26°C and maximum temperatures 31–33°C. Variable winds will reach 10–25 km/h.
