Weathermen forecast heavy rain nationwide throughout Thursday, and then gradual improvement until next Wednesday.
Chomparee Chompurat, director-general of the Meteorological Department, said heavy rainfall was still being reported in provinces in the North, the Northeast, the Central Plains, the East and the South on Thursday.
It was due to a strong southwesterly monsoon over the Andaman Sea and the Gulf of Thailand and the monsoon trough, which stretched from the North to a low-pressure system in the Northeast.
Residents were warned to keep alert for hillside runoff in areas of persistent rain, she said.
Small boats should remain ashore in the Gulf and the Andaman Sea on Thursday, due to waves that were 2-3 metres high.
From Friday to Sunday the monsoon would be weaker and the trough would move northwards to Myanmar, upper Laos and upper Vietnam, she said. Heavy rain would continue only in some areas of the East and the western coast of the South. Waves would drop to 1-2 metres high.
From next Monday to Wednesday the monsoon over the Andaman Sea and the Gulf of Thailand would further lose strength and rain would become scarce. Waves would be 1-2 metres high.