People have been urged to beware of the severe weather conditions.
The moderate southwest monsoon covers the upper Andaman Sea, Thailand and the Gulf of Thailand, while the low-pressure cell over Vietnam and the Bay of Tonkin is weakening, the department said.
The weather forecast for different regions of the country:
North: Very cloudy with scattered thundershowers and isolated heavy rains in Phayao, Phrae, Nan, Uttaradit, Phitsanulok and Phetchabun provinces. Minimum temperature 23-24 degrees Celsius, maximum 28-30°C; southwesterly winds 10-25kph.
Northeast: Very cloudy with scattered thundershowers and isolated heavy rains in Udon Thani, Nong Khai, Buengkan and Nakhon Phanom provinces; minimum temperature 22-25°C, maximum 27-31°C; southwesterly winds 10-25kph.
Central: Very cloudy with scattered thundershowers mostly in Ratchaburi, Kanchanaburi, Uthai Thani, Chainat, Nakhon Sawan, Lop Buri, Sara Buri, Pra Nakhon Si Ayutthaya and Samut Songkham provinces; minimum temperature 22-24°C.m, maximum 30-32°C, southwesterly winds 10-25kph.
East: Very cloudy with scattered thundershowers mostly in Chachoengsao, Nakhon Nayok, Prachin Buri, Sakaeo, Chonburi, Rayong, Chanthaburi and Trat provinces; minimum temperature 23-27°C, maximum 30-33°C, southwesterly winds 15-35kph; waves 1-2 metres high and above two metres in thundershowers.
South (east coast): Cloudy with scattered thundershowers mostly in Phetchaburi, Surat Thani, Nakhon Si Thammarat, Phatthalung, Songkhla, Pattani, Yala and Narathiwat provinces; minimum temperature 24-25°C, maximum 32-34°C; southwesterly winds 15-30kph; waves about a metre high and above two metres offshore.
South (west coast): Cloudy with isolated thundershowers mostly in Phuket, Krabi, Trang and Satun provinces; minimum temperature 23-26°C, maximum 32-35°C.
Phuket northward: Southwesterly winds 15-35kph; waves 1-2 metres and above two metres in thundershowers.
Krabi southward: Southwesterly winds 10-30kph; waves about a metre high and 1-2 metres in thundershowers.
Bangkok and its vicinity: Very cloudy with scattered thundershowers; minimum temperature 25-27°C, maximum 31-33°C, southwesterly winds 10-25kph.