The Interior Ministry has warned seven provinces in the lower Central Plains to brace for overflow from the Chao Phraya River.
The expected surge is due to stronger water run-off from the North that has led to more water being released from the Chao Phraya dam.
This will increase water levels in the river, which runs through these provinces, by between 1 and 1.5 metres, Chatchai Promlert, chief of the ministry's Disaster Prevention and Mitigation Department, said.
People in riverside areas in Uthai Thani, Chai Nat, Sing Buri, Ang Thong, Suphan Buri, Lop Buri and Ayutthaya have been advised to closely monitor the situation.
More rainfall, which is expected by the Meteorology Department to hit most parts of the country until tomorrow, will increase water levels in the river, causing irrigation officials to release water from the Chao Phraya dam to ease possible flooding upstream.
Earlier, farmers in Nakhon Sawan, to the North of Chai Nat where the dam is situated, asked officials to release more water after thousands of their paddy fields were flooded.
The discharge rate increased from 70 to 649 cubic metres a second and, with more downpours in the upper parts of Thailand, the rate will further go up to 700 cu/m a second, Suchat Charoensi, chief of irrigation office 12 in Chai Nat, said yesterday.
The result is water levels of a section of Chao Phraya River from Chai Nat's Sapphaya district to parts of Ayutthaya's Bang Ban and Sena districts will increase, he said. The areas include tambon Ban Krathum and tambon Hua Wiang in Sena district, which sit on the lowest-lying area in the Central Plains.
Residents in five villages from the two tambons are complaining about increasing water levels in the Noi River which receives more water from the Chao Phraya River via Phong Pheng canal. Riverside houses along the Noi River are experiencing strong overflows which is expected to leave river banks under 30 centimetres of water.
"Flooding came rapidly this year," said Somphon Kitket, a 74-year-old resident in tambon Hua Wiang.
Usually the seasonal flooding is expected in late August or early September but its early arrival has left villagers with little time to prepare, she said.
Officials are carefully watching water levels to ensure appropriate dam management.
The Chao Phraya dam has played a vital role in controlling the river's flow of water which runs downstream to Ayutthaya, Pathum Thani, Nonthaburi and Bangkok.
Yesterday, downpours continued to hit many parts of the capital throughout the day, causing flooding and worsening traffic gridlock.
Phaya Thai was the hardest-hit area with the 24-hour rainfall amount measured at 70.5 millimetres, according to City Hall as of 3.45pm yesterday.
Rain also prompted city officials to closely monitor major canals as higher water levels will block floodwater drainage. Overflows were reported yesterday at parts of Prem Prachakon canal in the Don Muang area, Lat Phrao canal near Wat Lat Phrao, and the Bang Waek and Bang Kaeo canals near Ratchadaphisek Road.