The government has set up a "war room" to closely monitor the impact of Typhoon Noru, which is expected to hit the northeastern provinces as a tropical storm on Thursday.
Noru is forecast to bring heavy rain to Thailand, similar to Dianmu which struck in September last year.
Government spokesman Anucha Burapachaisri said the war room, chaired by Digital Economy and Society Minister Chaiwut Thanakamanusorn, will gather information about the storm, assess its severity and take actions to mitigate its impact.
According to the Meteorological Department, the typhoon will make landfall in central Vietnam on Wednesday before hitting Thailand early on Thursday morning.
"Those residing in low-lying areas should watch out for flash flooding and run-off from higher ground. Local agencies have been ordered to mobilise their resources to ensure they can provide timely assistance to those affected," the spokesman said.
Chomparee Chompurat, director-general of the Meteorological Department, said Noru would have become a tropical storm by the time it reaches Thailand.
Together with the strengthening southwesterly monsoon in the Andaman Sea and the Gulf of Thailand, the storm will bring heavy rain to most parts of the country, including Bangkok and its surrounding provinces between Sept 28-Oct 1.
According to Ms Chomparee, the department has detected a new tropical storm forming in the Pacific Ocean, but as the system is moving north towards Taiwan and Japan, it is unlikely to affect Thailand.
Another storm system is expected to form in the middle of October, but it is still too early to forecast its path, he said.
Mr Chaiwut, in his capacity as the war room's chairman, said the government is concerned about the approaching storm because several provinces, including some in the northeast, are still inundated.
The minister warned of possible floods and landslides as the storm is expected to bring around 100 millimetres of rain in several areas. He urged local authorities to provide constant updates on the flood situation so local residents can prepare.
He said companies should consider allowing their employees to take a couple of days off to help alleviate traffic congestion caused by flooding across the city.
Bangkok governor Chadchart Sittipunt said while City Hall is prepared for the coming storm, there are still concerns about possible floods caused by water discharges from Pasak Jolasid Dam in Lop Buri and the rising water level along the Chao Phraya River.
He said City Hall's Drainage and Sewerage Department is to boost the city's flood defences, especially in the city's east and along the Chao Phraya River.
The governor said City Hall is diverting water away from the city's canals to ensure they are capable of receiving the additional rainfall. It is also reinforcing several areas along the Chao Phraya River and other major waterways in Bangkok with around 2.5 million sandbags.
Mr Chadchart said Bangkok residents are also being urged to protect themselves against floods by installing water pumps and placing sandbags around their property. "City Hall is ready to provide assistance and equipment," he said.
Thaweesak Thanadechopol, deputy director-general of the Royal Irrigation Department, said the country's large and medium-sized reservoirs are currently at 72% capacity.
Together, the nation's four major dams -- Bhumibol in Tak, Sirikit in Uttaradit, Kwae Noi Bamrung Dan in Phitsanulok and Pasak Jolasid in Lop Buri -- hold about 16 billion cubic metres of water. They can store an additional 8.904 billion cubic metres of water.
Mr Thaweesak said the department is closely monitoring reservoirs which have reached 80% capacity, to ensure authorities can release the water in a timely manner.
Meanwhile, the Department of Disaster Prevention and Mitigation reported that 37 districts in 18 provinces have been hit by flooding between Sept 22-26. They are Chiang Mai, Lampang, Lamphun, Sukhothai, Tak, Phetchabun, Kamphaeng Phet, Loei, Khon Kaen, Chaiyaphum, Amnat Charoen, Si Sa Ket, Nakhon Ratchasima, Mukdahan, Yasothon, Rayong, Prachin Buri and Samut Prakan.