The conditions are being created by the moderate high-pressure system from China that has extended to upper Thailand, the weak monsoon trough prevailing across the middle South, and the southwest monsoon over the Andaman Sea, the lower South and the lower Gulf of Thailand, the department added.
People should beware of the severe conditions that may cause flash floods and overflows, especially along foothills near waterways and lowlands. All transport should exercise caution, the department warned.
Waves in the Andaman Sea and the Gulf could rise to 1-2 metres high and over two metres high in thundershowers. All ships should proceed with caution and avoid the thundershowers, the department said.
The areas to be affected by thundershowers are:
North: Mae Hong Son, Chiang Mai, Chiang Rai, Phayao, Nan, Lampang, Phrae, Sukhothai, Tak and Kamphaeng Phet.
Northeast: Nakhon Ratchasima, Buri Ram, Surin, Si Sa Ket and Ubon Ratchathani.
Central: Saraburi, Suphanburi, Kanchanaburi, Ratchaburi, Samut Songkhram, Samut Sakhon, Nakhon Pathom, Ayutthaya and the Bangkok metropolitan area.
East: Nakhon Nayok, Chachoengsao, Chonburi, Rayong, Chanthaburi and Trat.
South: Phetchaburi, Prachuap Khiri Khan, Surat Thani, Nakhon Si Thammarat, Phatthalung, Songkhla, Yala, Narathiwat, Ranong, Phang Nga, Krabi, Trang and Satun.