Weathermen forecast heavy rain and high waves throughout the country from Thursday to Monday, brought by the annual monsoon and a developing tropical storm due to hit Vietnam on Friday.
Kornrawee Sitthichivapak, director-general of the Meteorological Department, said on Wednesday that heavy downpours and strong winds were forecast for the North, Northeast, Central Plain including Greater Bangkok, the East and South from Sept 19 to 23.
Consequently, there could be runoff and flash floods on sloping and low-lying terrain, and near waterways, she said.
The rain was being brought by a depression located over the upper South China Sea, about 550 kilometres east of Da Nang at 10am on Wednesday.
The depression had wind speeds of 55 kilometres per hour at its centre, was moving westwards and was likely to develop into a tropical storm, Ms Kornrawee said. It would land in central Vietnam on either Friday or Saturday, and then gradually weaken.
Ms Kornrawee also said that from Thursday to next Sunday the monsoon over the Andaman Sea, the South and the Gulf of Thailand would bring heavy rain to the South.
Waves could be two to four metres high in the Andaman Sea and the upper part of the Gulf, and about two metres high in the lower part of the gulf.
Boats should remain ashore in the Andaman Sea and the Gulf of Thailand during this period.
The Royal Irrigation Department said on Wednesday it would gradually increase the discharge rate from the Chao Phraya barrage in Chai Nat province, from 1,099 to 1,500 cubic metres per second, ahead of the arrival of the storm now heading for Vietnam.
The levels of waterways in 11 provinces in the Chao Phraya River basin were expected to rise by 60 to 100 centimetres as a result. They are: Ang Thong, Ayutthaya, Bangkok, Chai Nat, Lop Buri, Nonthaburi, Pathum Thani, Samut Prakan, Sing Buri, Suphan Buri and Uthai Thani.
Thanasith Iamananchai, deputy director-general of the Meteorological Department, on Wednesday afternoon confirmed the developing storm could have a serious impact. It would affect the upper and central Northeast, the lower North and the upper part of the Central Plains, he told Thai PBS news.