Heavy rain will blanket most of the country until Monday, according to the Thai Meteorological Department.
The strong southwest monsoon prevails over the Andaman Sea, Thailand and the Gulf of Thailand while the monsoon trough lies across Myanmar, Laos and Vietnam, according to the No. 8 statement issued on Saturday.
More rain and isolated heavy to very heavy rain is forecast in the North, Northeast, Central region, East and upper South.
For Saturday, the affected areas are:
North: Mae Hong Son, Chiang Mai, Chiang Rai, Phayao, Nan, Phrae, Lamphun, Lampang, Tak, Kamphaeng Phet, Sukhothai and Phichit.
Northeast: Nong Khai, Bung Kan, Sakon Nakhon, Nakhon Phanom, Mukdahan, Yasothon, Amnat Charoen and Ubon Ratchathani.
Central: Nakhon Sawan, Uthai Thani, Chai Nat, Suphan Buri, Kanchanaburi and Ratchaburi.
East: Chonburi, Rayong, Chanthaburi and Trat.
South: Phetchaburi, Prachuap Khiri Khan, Chumphon, Surat Thani, Ranong, Phang Nga, Phuket and Krabi.
For Sunday and Monday, the affected areas are:
North: Mae Hong Son, Chiang Mai, Chiang Rai, Lamphun, Lampang, Phayao, Nan, Phrae, Uttaradit, Sukhothai, Tak, Kamphaeng Phet, Phitsanulok, Phichit and Phetchabun.
Northeast: Loei, Nong Bua Lam Phu, Udon Thani, Nong Khai, Bung Kan, Sakon Nakhon, Nakhon Phanom, Mukdahan, Chaiyaphum, Nakhon Ratchasima, Amnat Charoen and Ubon Ratchathani.
Central: Nakhon Sawan, Uthai Thani, Chai Nat, Lop Buri, Saraburi, Suphan Buri, Kanchanaburi and Ratchaburi.
East: Nakhon Nayok, Chachoengsao, Prachin Buri, Sa Kaeo, Chon Buri, Rayong, Chanthaburi and Trat.
Strong wind and waves are likely 2-3 metres in the Andaman Sea and the upper Gulf from Surat Thani northward.
All ships should proceed with cautions and small boats should keep ashore. People in the coastal areas of the East and the southwest coast should watch out for storm surge, the statement said.
For Bangkok, scattered to fairly widespread thundershowers and isolated heavy rain are forecast until Monday, to be followed by scattered thundershowers until Friday, with temperatures in a range between 24C and 34C.