The Royal Irrigation Department (RID) has warned residents along the lower Chao Phraya River to move their belongings to higher ground as persistent rain has increased the risk of flooding.
The river has already overflowed in parts of Ayutthaya province, damaging more than 1,000 homes.
Regional Irrigation Office 12 director Darongkorn Somton issued the warning Tuesday for residents living downstream from the Chao Phraya dam in Chai Nat province.
He said a strong monsoon is causing torrential rain in the lower North and the upper part of the Central Plains.
As a result, water flow through the Chao Phraya dam will increase from 897 to 1,300-1,500 cubic metres per second this Friday.
This will cause water levels downstream to rise by one metre, he said.
On Monday, the river rose about 50 centimetres in Ayutthaya. That caused the Phong Pheng canal and the Noi River to overflow into 41 villages in Sena district.
About 1,454 homes have been inundated. Local roads are mostly submerged and residents are using boats to commute.
Department of Disaster Prevention and Mitigation director-general Chatchai Phromlert also warned of possible heavy rains, flash floods, run-off and landslides in 14 provinces from today until Sunday.
He issued the warning to residents in Kanchanaburi, Ratchaburi, Phetchaburi, Prachuap Khiri Khan, Chon Buri, Rayong, Chanthaburi, Trat, Ranong, Phangnga, Phuket, Krabi, Trang and Satun.
He said a strong southwest monsoon is threatening those provinces, as is the possibility of a low pressure area developing into a tropical storm in the South China Sea.
He advised people with small boats to remain ashore in the Andaman Sea and the upper Gulf.
Bangkok Metropolitan Administration deputy clerk Sanya Shinimit said yesterday that water from the North posed no danger to Bangkok as the Chao Phraya was flowing through the capital at a rate of only 1,000 cubic metres per second.
He was, however, concerned by the prospect of heavy rain in the capital, saying Bangkok was a basin and its drainage capacity could handle rainfall of no more than 60 millimetres per hour.
Heavy rain is forecast throughout the capital from tomorrow until Friday.
In the southern province of Trang, several landslides have been reported on Khuan Sung mountain, on Palian district's Sukorn island, following heavy downpours.
Damage was reported in Ban Siem Mai and Ban Thung villages, both of which have been cut off.
Landslides also covered most of a five-kilometre-long road which leads to the island's highest viewpoint.