BANGKOK — 20 August 2026, the Thai Meteorological Department (TMD) has warned of heavy to very heavy rain in parts of the North, Northeast, Central Plains, and East over the next 24 hours, with risks of flash flooding, forest runoff, and overflowing waterways.

The weather bureau said a monsoon trough lies across the upper North and upper Northeast, extending towards a strong low-pressure area near China’s Hainan Island. A moderately strong southwest monsoon is also prevailing over the Andaman Sea, Thailand, and the Gulf of Thailand.

Residents in affected areas, particularly foothills, areas near waterways, and low-lying locations, should remain alert for heavy and accumulated rainfall, which could trigger flash floods, runoff, and river overflows.

Winds and waves are also expected to be relatively strong in the upper Andaman Sea and upper Gulf of Thailand, where waves are forecast at 2–3 metres and may exceed 3 metres during thunderstorms. The lower Andaman Sea is expected to see waves of about 2 metres, rising above 2 metres in thunderstorms.

Mariners in the Andaman Sea and Gulf of Thailand are advised to navigate with caution and avoid thunderstorms. Small boats in the upper Andaman Sea should remain ashore during this period.

The forecast is valid from 6am on 20 August to 6am on 21 August.

North

Thunderstorms are forecast across 80% of the region, with heavy rain in Chiang Mai, Chiang Rai, Lamphun, Lampang, Phayao, Nan, Phrae, and Kamphaeng Phet.

Very heavy rain is possible in Mae Hong Son, Uttaradit, Sukhothai, Tak, Phitsanulok, Phichit, and Phetchabun. Temperatures are forecast at 22–27°C, with highs of 27–31°C. Southwest winds are expected at 10–20 km/h.

Northeast

Thunderstorms are forecast across 80% of the region, with heavy rain in Nong Khai, Nong Bua Lamphu, Udon Thani, Chaiyaphum, Khon Kaen, Kalasin, Mukdahan, Maha Sarakham, Roi Et, Yasothon, Amnat Charoen, Nakhon Ratchasima, and Ubon Ratchathani.

Very heavy rain is possible in Loei, Bueng Kan, Sakon Nakhon, and Nakhon Phanom. Temperatures are forecast at 22–25°C, with highs of 26–32°C. Southwest winds are expected at 10–20 km/h.

Central Plains

Thunderstorms are forecast across 70% of the region, with heavy to very heavy rain in Nakhon Sawan, Uthai Thani, Chai Nat, Lop Buri, Saraburi, Kanchanaburi, Ratchaburi, Nakhon Pathom, Samut Sakhon, and Samut Songkhram.

Temperatures are forecast at 23–26°C, with highs of 29–33°C. Southwest winds are expected at 10–25 km/h.

East

Thunderstorms are forecast across 70% of the region, with heavy rain in Nakhon Nayok, Prachin Buri, Sa Kaeo, and Chanthaburi. Very heavy rain is possible in Trat.

Temperatures are forecast at 23–28°C, with highs of 30–33°C. Southwest winds are expected at 20–40 km/h. Waves are forecast at 2–3 metres and may exceed 3 metres in thunderstorms.

South, east coast

Thunderstorms are forecast across 40% of the region, with heavy rain possible in Phetchaburi and Prachuap Khiri Khan. Temperatures are forecast at 23–27°C, with highs of 32–36°C.

From Surat Thani northwards, southwest winds are expected at 20–35 km/h, with waves of 1–2 metres, rising above 2 metres offshore and during thunderstorms. From Nakhon Si Thammarat southwards, southwest winds are expected at 15–35 km/h, with waves of about 1 metre, rising to 1–2 metres offshore and above 2 metres in thunderstorms.

South, west coast

Thunderstorms are forecast across 40% of the region, mainly in Ranong and Phang Nga. Temperatures are forecast at 23–27°C, with highs of 31–34°C.

From Phuket northwards, southwest winds are expected at 20–40 km/h, with waves of 2–3 metres and more than 3 metres during thunderstorms. From Krabi southwards, southwest winds are expected at 20–35 km/h, with waves of about 2 metres, rising above 2 metres in thunderstorms.

Bangkok and surrounding regions

Thunderstorms are forecast across 70% of the area, with isolated heavy rain. Temperatures are forecast at 25–27°C, with highs of 32–34°C. Southwest winds are expected at 10–25 km/h.