A new tropical storm is developing in the South China Sea, currently about 380 kilometres west of Manila in the Philippines, the Joint Typhoon Warning Centre (JTWC) in Hawaii warned on Friday.
The weather system is moving in a west-southwest direction at about 10km per hour, the JTWC warned.
It was expected to strengthen into a tropical storm and reach coastal areas of Vietnam on Monday or Tuesday.
The Meteorological Department in Thailand on Friday issued a warning for people in 23 provinces in four regions to brace for heavy rain this weekend.
The department said a monsoon trough is now prevailing in the lower part of the North, the Northeast, the Central Plains and the East and it will trigger widespread scattered rain with heavy falls in some areas.
People living near hillsides and waterways in Tak, Sukhothai, Kamphaengpeht, Phitsanulok, Phichit, Phetchabun, Chaiyaphum, Roi Et, Nakhon Ratchasima, Buriram, Surin, Si Sa Ket, Amnatcharoen, Ubon Ratchathani, Nakhon Sawan, Uthai Thani, Lopburi, Saraburi, Prachin Buri, Sakaeo, Nakhon Nayok, Chanthaburi and Trat should prepare for heavy rain and possible flash flooding on Saturday and Sunday, it warned.
In Khon Kaen, it was announced Ubonrat dam will begin discharging about seven million cubic metres of water per day to keep the level in the reservoir below capacity, and people living downstream were warned of sudden flooding.
Deputy provincial governor Songpol Champaphan said Ubonrat dam currently holds 1.09 billion cubic metres of water, or about 60% of its capacity.
"About 27 million cubic metres of water are coming into the dam each day and this volume is higher than the safety level.
"It is necessary to discharge about seven million cubic metres of water in preparation for more heavy rain to come," Mr Songpol said.
People in some areas of Muang and Nam Pong districts were likely to experience flooding, he said.
The Meteorological Centre in the Northeast reported that about 10% of the region was getting rain and 60% of Khon Kaen could expect more rain, especially in early October, he added.
In Si Sa Ket, flooding was reported to be easing off, with water levels in different parts of the province starting to recede.
Suksan Boontosaeng, chief of the provincial disaster prevention and mitigation office, said 21 of 22 districts in Si Sa Ket had been hit by flooding, but the situation was beginning to stabilise.
The water in the Mun river, in the north of the province, was almost four metres higher than its banks on Friday morning, but was gradually dropping by about two centimetres per hour. Water levels dropped by 12 centimetres on Thursday, he said.
Water levels should return to normal this weekend if there is no more heavy rain, he said.