"If our forecast is correct, it will be the first tropical storm to hit the country this year," said Prawit Jampanya, acting director of the Weather Forecast Bureau.
The warning, which raises new concerns over flooding, was announced yesterday by the Meteorology Department. It said a tropical depression, located about 600 kilometres east of Da Nang, Vietnam yesterday, is expected to turn into a storm in the middle of this week and is likely to hit central Vietnam between Thursday and Friday.
The storm is expected to move westward on Saturday and Sunday bringing heavy rains on its path through Laos, northeastern and central Thailand to Tak's Mae Sot district and Myanmar, Mr Prawit said.
Affected provinces will be mainly those located downstream from various dams, so people in these areas need to brace for flooding, he said.
"But the storm will not last long," Mr Prawit said. "It will keep on moving. That means it will only stay in Thailand a short time, for only three days."
Her Royal Highness Princess Chulabhorn said yesterday that His Majesty the King is worried about the country's flood situation.
Speaking during a function in Udon Thani province yesterday, the princess said the King and the Queen's health were good.
His Majesty, however, was concerned about the current floods in many parts of the country, she said. He always studies a map on his hospital bed and thinks about flood prevention projects.
But the princess said: "When I asked if there will be a [major] flood this year, His Majesty said flooding will be minor and won't last long," she said.
Royal Irrigation Department spokesman Boonsanong Suchatpong said the water levels in the Chao Phraya River have been regulated to stay between two and four metres below the banks in Ayutthaya and Nakhon Sawan to cope with the extra run-off.
Bangkok is also preparing for the impact of the storm, which is expected to bring an average rainfall of 90 millimetres, said Bangkok Governor MR Sukhumbhand Paribatra yesterday.
Floodwaters from the rain will be drained from the streets in one hour, except in 206 flood-prone spots where drainage times may last two to three hours, he said.
The city is expected to experience drier weather this week, allowing officials to reinforce flood prevention measures, said Royol Chitradon, director of the Hydro and Agro Informatics Institute. Mr Royol said he has inspected Khlong Chakhe, Khlong Bang Chalong, Khlong Bang Sao Thong, Khlong Phra Ong Chao Chaiyanuchit and Khlong Lat Krabang and has discovered that some of these canals need water-pushing devices to speed up the water flow.
These canals must be drained so they can handle more water from rainfall that is expected to continue until next Wednesday, he said.
In inner Bangkok, water levels in Khlong Lat Phrao and Khlong Saen Saep are being reduced to make room for more water.
Still vulnerable to floods are Bangkok's outer districts of Klong Sam Wa, Min Buri, Nong Chok and Lat Krabang as they are located outside the city's flood wall, City Hall spokesman Wasan Miwong said.
Meanwhile, the city's Department of Drainage and Sewerage has decided to raise the sluice gate at Khlong Song in Sai Mai district by 15 centimetres after locals threatened to block a road. The residents have demanded that the city raise the sluice gates at Khlong Song, Khlong Lam Mo Taek and Khlong Phraya Suren, which have been shut to protect Bangkok from floods.
The closure has caused floods in communities near Khlong Hok Wa and in tambon Khukhot in Pathum Thani.