TMD warns of heavy rain and 2–3m Andaman waves on Sept 18

The Thai Meteorological Department (TMD) warned on Friday (September 18, 2026) of heavy rain across Thailand, with very heavy downpours forecast in parts of the North, Northeast, East and South. Waves in the upper Andaman Sea are expected to reach 2–3 metres, rising above 3 metres during thunderstorms.

TMD’s 24-hour forecast covers the period from 6am on September 18 to 6am on September 19, Thailand time. Thunderstorms are expected across 80% of the Northeast, while Bangkok and surrounding areas face 70% coverage and isolated heavy rain.

Heavy rain brings flood and landslide risks across Thailand

Heavy to very heavy rain and accumulated rainfall could cause flash floods and forest run-off, as well as landslides, TMD warned. Particular risks apply to foothill slopes near waterways and low-lying areas.

TMD advised travellers to avoid routes with recurring flooding, especially low-lying roads where water could accumulate temporarily.

A monsoon trough crossing the lower North, upper Central region and Northeast is driving the rainfall, together with a relatively strong southwest monsoon over the Andaman Sea, Thailand and the Gulf of Thailand, according to TMD.

Small boats in the upper Andaman Sea advised to stay ashore until September 20

Small boats in the upper Andaman Sea should remain ashore until Sunday (September 20), TMD advised, as relatively strong winds and waves affect the Andaman Sea and Gulf of Thailand.

TMD forecasts waves of 2–3 metres in the upper Andaman Sea, exceeding 3 metres in thunderstorms. The lower Andaman Sea and upper Gulf of Thailand are expected to have waves of about 2 metres, rising above 2 metres in thunderstorms.

Mariners in both the Andaman Sea and Gulf of Thailand should navigate cautiously and avoid areas with thunderstorms, TMD warned.

Northeast has the widest forecast thunderstorm coverage at 80%

The Northeast has the highest thunderstorm coverage in TMD’s September 18–19 forecast, at 80% of the region. The regional outlook retains separate warnings for heavy and very heavy rainfall.

North: TMD’s September 18–19 forecast puts thunderstorm coverage at 70%, with isolated heavy rain in Chiang Mai, Chiang Rai, Lampang, Phayao, Nan, Phrae, Tak, Sukhothai, Kamphaeng Phet and Phichit. Very heavy rain is expected in some areas of Uttaradit, Phitsanulok and Phetchabun. Minimum temperatures will range from 22–27°C and maximum temperatures from 30–36°C, with variable winds of 10–20 km/h.

Northeast: Thunderstorms are forecast across 80% of the region for September 18–19, according to TMD, with isolated heavy rain in Nong Khai, Bueng Kan, Udon Thani, Sakon Nakhon, Nakhon Phanom, Kalasin, Mukdahan, Maha Sarakham, Roi Et, Yasothon, Amnat Charoen, Buri Ram, Surin, Si Sa Ket and Ubon Ratchathani. Very heavy rain is expected in some areas of Loei, Nong Bua Lam Phu, Chaiyaphum, Khon Kaen and Nakhon Ratchasima. Minimum temperatures will be 22–25°C and maximum temperatures 29–34°C, with variable winds of 10–20 km/h.

Central region: TMD forecasts thunderstorms across 70% of the region for September 18–19, with isolated heavy rain in Nakhon Sawan, Uthai Thani, Chai Nat, Lop Buri, Saraburi, Sing Buri, Ang Thong, Phra Nakhon Si Ayutthaya, Suphan Buri, Kanchanaburi, Ratchaburi, Nakhon Pathom, Samut Songkhram and Samut Sakhon. Minimum temperatures will range from 23–25°C and maximum temperatures from 33–35°C. Southwesterly winds will reach 10–25 km/h.

East: Thunderstorms will cover 70% of the region in TMD’s September 18–19 forecast, with isolated heavy rain in Sa Kaeo, Chachoengsao, Chon Buri and Rayong, and very heavy rain in some areas of Nakhon Nayok, Prachin Buri, Chanthaburi and Trat. Minimum temperatures will be 23–27°C and maximum temperatures 30–34°C, with southwesterly winds of 20–35 km/h. Waves will reach about 2 metres, exceeding 2 metres in thunderstorms.

South, east coast: TMD forecasts thunderstorms across 60% of the region for September 18–19, with isolated heavy rain in Phetchaburi, Prachuap Khiri Khan, Nakhon Si Thammarat and Songkhla. Very heavy rain is expected in some areas of Chumphon and Surat Thani. Minimum temperatures will range from 22–25°C and maximum temperatures from 31–35°C.

From Surat Thani northwards, TMD’s September 18–19 outlook forecasts southwesterly winds of 20–35 km/h and waves of 1–2 metres, reaching about 2 metres offshore and exceeding 2 metres in thunderstorms.

From Nakhon Si Thammarat southwards, TMD forecasts southwesterly winds of 15–35 km/h for September 18–19. Waves will be about 1 metre high, reaching 1–2 metres offshore and exceeding 2 metres in thunderstorms.

South, west coast: Thunderstorms are expected across 70% of the region for September 18–19, according to TMD, with isolated heavy rain in Krabi, Trang and Satun. Some areas of Ranong, Phang Nga and Phuket face very heavy rain. Minimum temperatures will be 23–26°C and maximum temperatures 28–34°C.

From Phuket northwards, TMD’s September 18–19 forecast gives southwesterly winds of 20–40 km/h and waves of 2–3 metres, exceeding 3 metres in thunderstorms.

From Krabi southwards, TMD forecasts southwesterly winds of 20–35 km/h for September 18–19. Waves will reach about 2 metres, rising above 2 metres in thunderstorms.

Bangkok and surrounding areas: TMD forecasts thunderstorms across 70% of the area for September 18–19, with isolated heavy rain. Minimum temperatures will range from 24–26°C and maximum temperatures from 32–35°C. Southwesterly winds will reach 10–25 km/h.