TMD warns of very heavy rain, flash floods and rough seas across Thailand

The Thai Meteorological Department has warned of heavy to very heavy rain across parts of Thailand from May 20-21, with flash floods and forest run-off possible in foothill areas, near waterways and in low-lying communities.

In Weather Advisory No 16 (76/2026), issued at 5pm on Wednesday, the department said a rather strong southwest monsoon was prevailing over Thailand and the upper Andaman Sea, while a low-pressure cell was covering the lower North and upper Northeast.

The weather system is expected to bring continuous rain and isolated heavy showers nationwide, with very heavy rain possible in the North, upper Northeast and the South’s west coast.

Flood warning for vulnerable areas

The department urged people in risk areas to beware of heavy rainfall and accumulated rain, which could trigger flash floods and overflows, especially along foothills, waterways and lowlands.

The warning covers the period from May 20-21, with the next advisory scheduled for 5am on May 21.

Upper Andaman small boats told to stay ashore

The strong monsoon is also generating rough seas.

Waves in the upper Andaman Sea are forecast to reach two to three metres, and more than three metres in thundershowers. In the lower Andaman Sea and upper Gulf of Thailand, waves are expected to be about two metres, rising above two metres during thundershowers.

All vessels have been advised to proceed with caution and avoid thundershowers, while small boats in the upper Andaman Sea should remain ashore.

North, Northeast face heaviest rain risk

In its 24-hour forecast from 6pm Wednesday to 6pm Thursday, the department forecast scattered thundershowers and isolated heavy to very heavy rain in the North, particularly in Nan, Sukhothai, Uttaradit, Tak, Kamphaeng Phet, Phichit, Phitsanulok and Phetchabun.

Temperatures in the North are expected to range from 23-26°C at the lowest to 31-38°C at the highest.

The Northeast is forecast to see fairly widespread thundershowers and isolated heavy to very heavy rain in Sakon Nakhon, Nakhon Phanom, Khon Kaen, Kalasin, Maha Sarakham, Nakhon Ratchasima, Loei, Nong Khai, Bueng Kan, Nong Bua Lamphu, Udon Thani and Chaiyaphum.

Temperatures in the region are expected to range from 23-25°C to 30-34°C.

Heavy rain forecast in Central and Eastern provinces

The Central region is expected to see scattered thundershowers and isolated heavy rain in Nakhon Sawan, Chai Nat, Lop Buri, Saraburi and Sing Buri, with temperatures ranging from 24-27°C to 31-35°C.

In the East, scattered thundershowers and isolated heavy rain are forecast in Nakhon Nayok, Prachin Buri, Chanthaburi and Trat. Temperatures are expected to range from 24-29°C to 32-34°C.

Waves in eastern coastal areas are forecast at about two metres, rising above two metres during thundershowers.

South warned of heavy rain and high waves

On the southern east coast, scattered thundershowers and isolated heavy rain are forecast in Chumphon, Surat Thani, Nakhon Si Thammarat, Phatthalung and Songkhla. Temperatures are expected to range from 24-26°C to 30-35°C.

From Surat Thani upwards, waves are expected to be about two metres, and above two metres in thundershowers and offshore. From Nakhon Si Thammarat downwards, waves are forecast at one to two metres, about two metres offshore, and above two metres in thundershowers.

The southern west coast faces a higher risk, with fairly widespread thundershowers, scattered heavy rain and isolated very heavy rain forecast in Ranong, Phang Nga, Phuket, Krabi, Trang and Satun.

From Phuket upwards, waves are expected to reach two to three metres and above three metres in thundershowers. From Krabi downwards, waves are forecast at about two metres and above two metres in thundershowers.

Bangkok to see scattered storms

Bangkok and surrounding areas are forecast to see scattered thundershowers from Wednesday evening to Thursday evening.

Temperatures in the capital are expected to range from 26-28°C to 33-36°C, with southwesterly winds of 10-20 kilometres per hour.