Residents in 48 provinces, including Bangkok, have been warned of likely heavy rain and possible flash floods from Friday until Sept 18.
The Office of National Water Resources (ONWR) said on its Facebook page that the northern monsoon trough will move down to lie across upper Thailand between Sept 13 and Sept 18. At the same time, the southwest monsoon is strengthening and will cause more rain, with heavy falls in some areas.
The ONWR warned residents of flood-prone communities to brace for runoff and sudden flooding during the period.
Areas expected to be affected by flash floods from Friday until next Wednesday are in 13 provinces in the North (Chiang Rai, Chiang Mai, Mae Hong Son, Tak, Phayao, Nan, Phrae, Uttaradit, Phitsanulok, Kamphaeng Phet, Phichit, Nakhon Sawan and Phetchabun ), 13 provinces in the Northeast (Loei, Nong Khai, Bueng Kan, Nakhon Phanom, Khon Kaen, Maha Sarakham, Roi-Et, Yasothon, Nakhon Ratchasima, Buri Ram, Surin, Si Sa Ket and Ubon Ratchathani), 8 provinces in the Central Plain (Lop Buri, Saraburi, Bangkok and its five surrounding provinces), 6 provinces in the East (Nakhon Nayok, Prachin Buri, Chon Buri, Rayong, Chanthaburi and Trat), and 8 provinces in the South ( Chumphon, Ranong, Phangnga, Phuket, Krabi, Surat Thani, Trang and Satun)
The Disaster Prevention and Mitigation Department reported on Wednesday that floods and landslides caused by the influence of former typhoon Yagi have claimed four lives in northern Thailand. Two deaths occurred in a landslide in Chiang Mai province and the other two in Chiang Rai, near the Myanmar border.
The Meteorological Department also warned that more heavy rain is expected until next Tuesday, with the risk of flash floods.
"Between Sept 13-17, please be cautious of the potential danger from heavy to very heavy rainfall," the forecaster said.
A total of 29 people have died in heavy rain-related disasters since the start of the season, according to the national emergency operations centre.