The Royal Irrigation Department has increased the water discharge rate from the Chao Phraya Dam in Chai Nat province in preparation for taking more runoff from upstream, warning downstream communities to brace for possible flooding.
The release rate was raised to 2,100 cubic metres per second (m³/s) on Tuesday at 1am, from 2,000 m³/s earlier. It will be maintained at that level until further notice, the department said on Wednesday.
Communities in downstream provinces -- particularly Ang Thong, Ayutthaya and Sing Buri -- have been urged to monitor updates, prepare for possible inundation and be ready to evacuate if necessary.
The department said a water-measuring station in Muang district of Nakhon Sawan -- upstream of the dam -- had begun to register a decline in water levels by 6am on Wednesday, while levels further downstream were rising following the increased discharge.
Authorities are diverting some of the incoming runoff from the North into canals and water catchment fields before it reaches the dam. The measures are aimed at slowing the northern deluge from flowing rapidly downstream into the Central Plains and Bangkok.
The Office of National Water Resources (ONWR) reported that, as of Tuesday, flood-affected zones had been declared in 45 districts across 12 provinces, mainly in the lower North and Central regions.
In Bangkok, thunderstorms have been seen or forecast for 60-80% of the city, with heavy rain expected in some locations until Saturday, the Meteorological Department said on Wednesday.
It also forecasts torrential rain over the East and western parts of southern provinces within the next 24 hours, warning of flash floods in vulnerable areas. Heavy rainfall is expected across 70% of Bangkok and surrounding provinces during the same period.
The southern provinces of Ranong and Phangnga, as well as Chanthaburi and Trat in the East, were also told to prepare for rising rainfall over the coming days, which could trigger flooding in low-lying areas.
The weather bureau attributed the conditions to a strengthening southwest monsoon prevailing over the Andaman Sea and the Gulf of Thailand, coupled with a low-pressure system forming over the South China Sea. The monsoon is also expected to whip up waves as high as two metres in the Andaman and one to two metres in the Gulf.
Several areas in the lower North and the Northeast are already struggling with floods caused by overflowing rivers. Ubon Ratchathani, Yasothon and Phitsanulok have been among the hardest hit.
In Phitsanulok, the well-known Bang Rakam floodplain has so far absorbed 364 million cubic metres of excess water, or about 91% of its holding capacity, officials said.