The release of additional water from the dam in Lop Buri is expected raise the water level in lower Pasak River by about one metre, causing potential problems in Ayutthaya.
Boonchob Homkesorn, director of the South Pasak Water Maintenance Project, has alerted residents in Tha Rua, Nakhon Luang and Phra Nakhon Phra Nakhon Si Ayutthaya districts of the central province.
He said the Pasak Jolasid reservoir now contained 974 million cubic metres of water, above its maximum capacity of 960 million.
Ms Phuangphayom said children and the elderly would be given priority for food and water.
The dam will have to increase water discharges by almost 80% to 1,249 cubic metres per second in order to reduce the water level as soon as possible.
The dam website reported that as of 9am Saturday, the water volume had reached 1,015 million cubic metres, 29% above the normal storage level of 785 million.
The dam on Saturday morning was discharging 700.54 cubic metres of water per second, against an inflow of 1,163.14 cubic metres per second.
The water discharged from Pasak Jolasid will flow to the Phra Ram VI dam in Ayutthaya's Tha Rua district and be released into the Rapeepat Canal at 150 cubic metres per second.
The Phra Ram VI dam will also discharge 800 cubic metres per second into lower Pasak River, which will increase the water level by one metre, which may cause floods in the three districts.
During the great flood in 2011, the Phra Ram VI dam released water at 1,249 cubic metres per second into the Pasak River.
In a related development, two dams in Kanchanaburi are still able to accept almost 4 billion cubic metres of water.
The Srinakarin dam currently holds 15.2 billion cubic metres of water, or 85.7% of capacity, and could take in 2.53 billion more. The inflow into the dam on Friday was 66.1 million cubic metres and the outflow was 15.1 million.
The Vajiralongkorn dam has 7.43 billion cubic metres, or 83.87% of capacity, leaving room for another 1.43 billion cubic metres. It had an inflow of 37.1 million cubic metres and was discharging 15.05 million cubic metres on Friday.
The Meteorological Department on Saturday issued a severe weather warning for people in 8 provinces in the East and South.
The department warned that people living near hillsides and waterways in Chachoengsao, Chon Buri, Rayong, Chanthaburi, Trat, Phetchaburi, Prachuap Khiri Khan and Chumphon should brace for heavy rain and possible flash flooding on Sunday and Monday.
There will also be strong wind-generated waves in the upper part of the Gulf of Thailand. All shipping should proceed with caution and small boats should remain ashore during this period, it said.
Thirty people had died as a result of the flooding across the country as of Friday, the Disaster Prevention and Mitigation Department reported.
It said 11 deaths had taken place in Surin, nine in Si Sa Ket, five in Prachin Buri, two each in Nakhon Nayok and Yasothon, and one in Sa Kaeo.
Official figures based on residency rolls listed 14,703 houses damaged by floodwaters, along with 2,168,466 rai of farmland, 4,947 roads, 201 bridges, 218 schools, 325 temples and 56 state office buildings.
In Nakhon Ratchasima, at least 200 households faced flooding after more than 5 million cubic metres of water were released from the full Lam Chiang Klai reservoir.
In Ubon Ratchathani and Si Sa Ket, more than 15,000 people were moved to higher ground for their safety.
In Bangkok, Governor Sukhumbhand Paribatra said water levels could rise by two metres above sea level on Oct 15 and 16 due to water runoff from the North.
The government has said that Bangkok would likely be spared from major flooding this year, and said a repeat of the massive flooding of 2011, in which 813 people were killed nationwide and economic losses topped $40 billion, was unlikely.