The department expects rising seawater, isolated heavy rains triggered by the Southwest monsoon, and a deluge from the North to increase the water levels of the Chao Phraya, Mae Klong and Tha Chin rivers.
The office is monitoring the situation in flood-prone areas in Samut Songkhram, Samut Sakhon, Nakhon Pathom, Nonthaburi, Bangkok and Samut Prakan provinces.
The office said that the Royal Irrigation Department had gradually increased water drainage to cope with surging water levels of the Chao Phraya and Pasak rivers.
The water drainage at Chao Phraya Dam in Chainat province and Pasak Jolasid Dam in Lopburi province were 1,699 cubic metres and 200-260 cubic metres per second, respectively, the office explained.
Flooding caused 19 provinces to be inundated as of Thursday: Chiang Rai, Chiang Mai, Mae Hong Son, Lamphun, Sukhothai, Nan, Lampang, Phrae, Tak, Phitsanulok, Phetchabun, Loei, Udon Thani, Nong Khai, Chaiyaphum, Khon Kaen, Ubon Ratchathani, Ayutthaya and Prachinburi.