Heavy rain and high waves are forecast throughout the country from Thursday to Monday, brought by the annual monsoon and a developing tropical storm due to hit Vietnam on Friday.

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.

There could be runoff and flash floods on low-lying ground and near waterways.

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, the department said. It would land in central Vietnam on either Friday or Saturday, and then gradually weaken.

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.