Heavy rain and flash floods threaten upper Thailand

Heavy to very heavy rain is forecast across parts of northern, northeastern, central and eastern Thailand on Thursday (August 20, 2026), raising the risk of flash floods, forest run-off and overflowing waterways.

The Thai Meteorological Department (TMD) issued the warning in its 24-hour forecast covering the period from 6am on Thursday to 6am on Friday (August 21).

The conditions are being driven by a monsoon trough extending across the upper North and upper Northeast into a strong low-pressure cell over Hainan Island, China. A fairly strong south-westerly monsoon is also prevailing over the Andaman Sea, Thailand and the Gulf of Thailand.

Residents have been advised to watch for dangers from heavy to very heavy rain and accumulated rainfall, particularly in foothill areas near waterways and in low-lying communities.

Regional forecast


North

• Thunderstorms are forecast across 80% of the region.
• Heavy rain is expected in Chiang Mai, Chiang Rai, Lamphun, Lampang, Phayao, Nan, Phrae and Kamphaeng Phet.
• Very heavy rain is forecast in parts of Mae Hong Son, Uttaradit, Sukhothai, Tak, Phitsanulok, Phichit and Phetchabun.
• Temperatures will range from lows of 22-27°C to highs of 27-31°C.

• South-westerly winds will reach 10-20 kilometres per hour.

Northeast

• Thunderstorms are forecast across 80% of the region.
• Heavy rain is expected 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 forecast in parts of Loei, Bueng Kan, Sakon Nakhon and Nakhon Phanom.
• Temperatures will range from lows of 22-25°C to highs of 26-32°C.
• South-westerly winds will reach 10-20 kilometres per hour.

Central region

• Thunderstorms are forecast across 70% of the region.
• Heavy to very heavy rain is expected in Nakhon Sawan, Uthai Thani, Chai Nat, Lop Buri, Saraburi, Kanchanaburi, Ratchaburi, Nakhon Pathom, Samut Sakhon and Samut Songkhram.
• Temperatures will range from lows of 23-26°C to highs of 29-33°C.
• South-westerly winds will reach 10-25 kilometres per hour.

East

• Thunderstorms are forecast across 70% of the region.
• Heavy rain is expected in Nakhon Nayok, Prachin Buri, Sa Kaeo and Chanthaburi.
• Very heavy rain is forecast in parts of Trat.
• Temperatures will range from lows of 23-28°C to highs of 30-33°C.
• South-westerly winds will reach 20-40 kilometres per hour.
• Waves will reach 2-3 metres and exceed 3 metres during thunderstorms.

South, east coast

• Thunderstorms are forecast across 40% of the region.
• Heavy rain is expected in Phetchaburi and Prachuap Khiri Khan.
• Temperatures will range from lows of 23-27°C to highs of 32-36°C.
• From Surat Thani northwards, south-westerly winds will reach 20-35 kilometres per hour. Waves will be 1-2 metres high and exceed 2 metres offshore and during thunderstorms.
• From Nakhon Si Thammarat southwards, south-westerly winds will reach 15-35 kilometres per hour. Waves will be about 1 metre high, rising to 1-2 metres offshore and exceeding 2 metres during thunderstorms.

South, west coast

• Thunderstorms are forecast across 40% of the region, mainly in Ranong and Phang Nga.
• Temperatures will range from lows of 23-27°C to highs of 31-34°C.
• From Phuket northwards, south-westerly winds will reach 20-40 kilometres per hour. Waves will reach 2-3 metres and exceed 3 metres during thunderstorms.
• From Krabi southwards, south-westerly winds will reach 20-35 kilometres per hour. Waves will be about 2 metres high and exceed 2 metres during thunderstorms.

Bangkok and surrounding provinces

• Thunderstorms are forecast across 70% of the area, with isolated heavy rain.
• Temperatures will range from lows of 25-27°C to highs of 32-34°C.
• South-westerly winds will reach 10-25 kilometres per hour.

Small boats told to remain ashore

Winds and waves will remain fairly strong in the upper Andaman Sea and upper Gulf of Thailand, with waves reaching 2-3 metres and exceeding 3 metres during thunderstorms.

Waves in the lower Andaman Sea will be about 2 metres high, rising above 2 metres in storm-affected areas.

Mariners in the Andaman Sea and Gulf of Thailand should proceed with caution and avoid areas experiencing thunderstorms. Small boats in the upper Andaman Sea should remain ashore until conditions improve.