It said a monsoon trough lies across the lower North, the upper Central and the Northeast regions, while the southwest monsoon lies above the Andaman Sea and the Gulf of Thailand.
The department warned of possible flash floods and overflowing waterways in low-lying areas. Waves will be one to two metres in height in the upper Andaman Sea and the upper Gulf of Thailand, rising above two metres during thundershowers. In the lower Andaman Sea and the lower Gulf of Thailand waves will average one metre in height, but could be twice as high during thundershowers.
The department urged all ships to avoid thundershowers.
North
Fairly widespread thundershowers and isolated heavy rains in Mae Hong Son, Chiang Mai, Lamphun and Tak. Minimum temperature 23-24° celcius. Maximum temperature 30-34°C. Winds five to 5 kilometre per hour.
Northeast
Fairly widespread thundershowers and isolated heavy rains in Bueng Kan, Nakhon Phanom, Sakon Nakhon Mukdahan, Yasothon, Amnat Charoen, Nakhon Ratchasima, Burirum, Surin, Sisaket and Ubon Ratchathani. Minimum temperature 23-24°C. Maximum temperature 33-34 °C. Winds ranging from 10-25 km/hr.
Central
Scattered thundershowers and isolated heavy rains in Kanchanaburi, Ratchaburi, Phra Nakhon Sri Ayutthaya , Nakhon Pathom, Samut Songkhram and Samut Sakhon. Minimum temperature 24-25°C. Maximum temperature 33-34°C. Southwesterly winds ranging from 10-20 km/hr.
East
Scattered thundershowers in Nakhon Nayok, Prachinburi, Chanthaburi and Trat. Minimum temperature 24-26°C. Maximum temperature 32-34°C. Southwesterly winds 15-35 km/hr.
South (East Coast)
Isolated thundershowers in Phetchaburi, Prachuap Khiri Khan, Pattani, Yala and Narathiwat. Minimum temperature 24-26°C. Maximum temperature 33-36°C. Southwesterly winds 15 to 30 km/hr.
South (West Coast)
Isolated thundershowers in Krabi, Trang and Satun. Minimum temperature 24-25°C. Maximum temperature 32-35°C. Southwesterly winds 15-35 km/hr.
Bangkok
Scattered thundershowers and isolated heavy rains. Minimum temperature 26-27°C. Maximum temperature 33-35°C. Southwesterly winds 10-20 km/hr.