Bangkok area faces 70% storm coverage on September 11–12

The Thai Meteorological Department (TMD) forecasts thunderstorms across 70% of Bangkok and surrounding areas, with isolated heavy rain, from 6am on September 11 to 6am on September 12, 2026, Thai time. Increasing rain is forecast in southern Thailand, including isolated very heavy falls in Phetchaburi and Prachuap Khiri Khan.

In its forecast issued on Friday (September 11), the TMD also predicts isolated heavy rain in central and eastern Thailand. Rainfall is expected to decrease in the North and Northeast.

Flood risks and a southward monsoon shift

The TMD warned that heavy to very heavy rain and accumulated rainfall could cause flash floods, forest runoff and mudslides. People near foothills, waterways and low-lying ground should be particularly alert.

Travellers in rain-affected areas should avoid routes with recurring flooding problems, the TMD advised. Low-lying routes may experience short-term inundation.

A high-pressure system, or cool air mass, has already spread over the upper North and upper Northeast, according to the TMD. This has pushed the monsoon trough southwards across the lower Central region, upper South, upper Gulf of Thailand and East, extending to a strong low-pressure area over the central South China Sea.

A moderate south-westerly monsoon is also covering the Andaman Sea, southern, central and eastern Thailand, and the Gulf of Thailand, the TMD reported.

Potential cyclone development and sea conditions

The TMD forecasts that the strong low-pressure area over the central South China Sea is likely to strengthen into a tropical cyclone during September 11–13. The system is expected to approach the coast of northern Vietnam during September 13–14, and the department urged the public to continue following weather forecasts.

Sea conditions in the Andaman Sea and Gulf of Thailand remain moderate, according to the TMD. Waves in the upper Andaman Sea are around 2 metres high and exceed 2 metres in thunderstorms; boat operators in both seas should navigate cautiously and avoid thunderstorms.

Regional forecast for September 11–12

The TMD’s September 11–12 regional outlook names 39 provinces, alongside Bangkok and surrounding areas. Thunderstorm coverage and rainfall intensity vary by region.

North

Northern Thailand’s September 11–12 forecast from the TMD gives thunderstorm coverage of 60%, mainly in Tak, Phichit, Kamphaeng Phet and Phetchabun. Minimum temperatures are 22–26°C and maximum temperatures 31–37°C, with easterly winds of 10–20 km/h.

Northeast

For the Northeast, the TMD forecasts thunderstorms across 40% of the region during September 11–12, mainly in Chaiyaphum, Nakhon Ratchasima, Buri Ram, Surin, Si Sa Ket and Ubon Ratchathani. Temperatures range from lows of 21–23°C to highs of 30–34°C, with north-easterly winds of 10–20 km/h.

Central

Central Thailand can expect 70% thunderstorm coverage during September 11–12, with isolated heavy rain in Phra Nakhon Si Ayutthaya, Suphan Buri, Kanchanaburi, Ratchaburi, Nakhon Pathom, Samut Sakhon and Samut Songkhram, according to the TMD. Lows are 23–24°C and highs 32–34°C, with northerly winds of 10–20 km/h.

East

The TMD’s September 11–12 outlook for eastern Thailand forecasts thunderstorms across 70% of the region and isolated heavy rain in Nakhon Nayok, Prachin Buri, Chachoengsao, Chon Buri, Rayong, Chanthaburi and Trat. Minimum temperatures are 22–25°C and maximum temperatures 30–34°C. Variable winds of 15–35 km/h will accompany waves of 1–2 metres, rising above 3 metres in thunderstorms.

South, east coast

Along the South’s east coast, the TMD forecasts 70% thunderstorm coverage during September 11–12, with isolated heavy rain in Chumphon, Surat Thani, Nakhon Si Thammarat, Phatthalung, Songkhla, Pattani, Yala and Narathiwat. Isolated very heavy rain is forecast in Phetchaburi and Prachuap Khiri Khan. Lows are 23–25°C and highs 30–36°C.

The TMD’s September 11–12 marine forecast gives westerly winds of 15–35 km/h from Surat Thani northwards, with waves of 1–2 metres and above 2 metres in thunderstorms. From Nakhon Si Thammarat southwards, north-westerly winds will reach 15–35 km/h, with waves around 1 metre, 1–2 metres offshore and above 2 metres in thunderstorms.

South, west coast

For the South’s west coast, the TMD expects thunderstorms across 70% of the region during September 11–12, with isolated heavy rain in Ranong, Phang Nga, Phuket, Krabi and Satun. Minimum temperatures are 23–25°C and maximum temperatures 28–32°C.

From Phuket northwards, the TMD’s September 11–12 marine forecast gives westerly winds of 20–35 km/h and waves around 2 metres, exceeding 2 metres in thunderstorms. From Krabi southwards, north-westerly winds will reach 15–35 km/h, with waves of 1–2 metres and above 2 metres in thunderstorms.

Bangkok and vicinity

Bangkok and surrounding areas have a 70% thunderstorm coverage forecast for September 11–12, with isolated heavy rain, according to the TMD. Lows are 24–25°C and highs 33–35°C, with north-westerly winds of 10–20 km/h.