Heavy to very heavy rain forecast across much of the country over the next five days is expected to increase flood risks in Bangkok and the central plains, with water levels in parts of Ayutthaya and Ang Thong potentially rising by up to 1.25 metres.
The Thai Meteorological Department (TMD) has forecast heavy rain from Wednesday until Sept 27, with Bangkok and surrounding provinces expected to be particularly affected. Rainfall during the critical three-day period from Sept 24-26 could reach 125-250 millimetres.
The heavy rain is expected to affect parts of the North, Northeast, Central Plains, East and South, with Chanthaburi and Trat among the eastern provinces forecast to receive particularly high rainfall.
The Department of Disaster Prevention and Mitigation has warned communities along the Chao Phraya River to prepare for rising water levels of 0.61-1.50 metres over the next one to nine days, depending on rainfall and upstream runoff.
The warning comes as water is already surging down from the North. The Royal Irrigation Department (RID) said water flow at the C.2 monitoring station in Nakhon Sawan reached 1,601 cubic metres per second early yesterday, while another 140 cubic metres per second was recorded on the Sakae Krang River in Uthai Thani.
The combined flow is moving downstream towards the Chao Phraya barrage in Chai Nat, prompting irrigation officials to gradually increase downstream discharge from 1,300 to 1,500 cubic metres per second yesterday. The higher discharge is expected to remain in place as officials manage incoming water and reservoir levels.
RID director-general Wittaya Kaewmee said water management was focused on three measures: keeping upstream water levels below target thresholds, maximising diversion into eastern and western irrigation networks, and increasing dam releases incrementally in line with incoming flows.
Particular attention is being paid to low-lying areas outside flood embankments in Ayutthaya and Ang Thong, where water levels could rise by 0.75-1.25 metres. Communities protected by flood barriers are not currently expected to be directly affected by the increased discharge.
Officials said the flood risk would depend not only on dam releases but also on additional rainfall across the upper river basins between Sept 23 and 27. Authorities have urged farmers in low-lying floodplain areas to accelerate harvesting and prepare land for possible inundation.