The Meteorological Department has advised those living in 50 provinces to brace for heavy rain until Thursday. PBS World

An intensifying low pressure cell is expected to hit the southern tip of the Vietnam peninsula today (Sunday) or tomorrow, before moving into the Gulf of Thailand. This will bring heavy to very heavy rainfall to most of Thailand, including the capital.

The Meteorological Department has advised those living in 50 provinces to brace for heavy rain until Thursday.

Bangkok and its peripherals, as well as Kanchanaburi, Ratchaburi, Ayutthaya, Samut Songkhram, Samut Sakhon, Nakhon Pathom, Nakhon Nayok, Prachin Buri, Sa Kaeo, Chachoengsao, Chon Buri, Rayong, Chanthaburi, Phetchaburi, Prachuap Khiri Khan, Chumphon, Surat Thani, Nakhon Si Thammarat, Phatthalung, Songkhla, Yala, Pattani, Ranong, Phang-nga, Phuket, Krabi and Trang will see downpours from Tuesday.

North-eastern provinces, such as Nakhon Ratchasima, Buri Ram and Surin, will face heavy rain on Wednesday and Thursday.
People living at mountain bases and close to rivers and canals are warned of possible flash floods and overflows.
https://www.thaipbsworld.com/heavy-rain-forecast-for-most-of-thailand-this-week/