Monsoon rains raise flash-flood risk across upper Thailand
A monsoon trough and a rather strong southwest monsoon will bring more rain, while waves in the upper Andaman Sea will exceed 3 metres during thunderstorms.
- Thailand's Meteorological Department has issued a warning for heavy to very heavy rain in upper Thailand, effective from August 29 to September 2, 2026.
- The intense rainfall poses a significant risk of flash floods and run-off, particularly on foothill slopes, near waterways, and in low-lying areas.
- The severe weather is caused by a monsoon trough moving south combined with a strong southwest monsoon over the Andaman Sea and Gulf of Thailand.
- In addition to flooding, the weather system will create strong waves of 2-3 meters in the Andaman Sea and Gulf of Thailand, with small boats in the upper Andaman Sea advised to stay ashore.
The Thai Meteorological Department (TMD) issued an advisory at 5pm on Friday (August 28, 2026) on heavy to very heavy rain in upper Thailand, effective from Saturday (August 29, 2026) to Wednesday (September 2, 2026).
During the period, rain is expected to increase across Thailand, with isolated heavy to very heavy falls, particularly in the western parts of the North and Central regions, the upper and eastern parts of the Northeast, and the East.
The monsoon trough is forecast to move south across the North and upper Northeast, extending into an active low-pressure cell over the northern coast of Vietnam and the Gulf of Tonkin.
The southwest monsoon prevailing over the Andaman Sea, Thailand and the Gulf of Thailand will also be rather strong.
People in these areas were advised to beware of heavy to very heavy rain and accumulated rainfall, which could cause flash floods and run-off, particularly on foothill slopes near waterways and in low-lying areas.
They should avoid routes repeatedly affected by flooding, especially in low-lying areas where short-term flooding could occur.
Wind and waves in the upper Andaman Sea will be rather strong, with waves 2–3 metres high and exceeding 3 metres in areas with thunderstorms.
Waves in the lower Andaman Sea and upper Gulf of Thailand will be about 2 metres high, exceeding 2 metres in areas with thunderstorms.
Mariners in the Andaman Sea and Gulf of Thailand were advised to navigate with caution and avoid sailing in areas with thunderstorms.
Small boats in the upper Andaman Sea should remain ashore throughout the period.
To ensure safety and reduce potential impacts, the department urged people to plan their daily activities and travel carefully during the period.
People should follow the department’s announcements; information is available 24 hours a day at tmd.go.th or by calling 0-2399-4012-13 or 1182.
