Pra Pirun was yesterday centred in the South China Sea about 550 kilometres southeast of Hainan island. Maximum wind velocity recorded near its centre was about 110kph.

It was moving northwest at about 15kph and would affect Thailand's weather.

The storm has already caused heavy rain in northern provinces including Chiang Mai, Chiang Rai, Lamphun, Lampang and Mae Hong Son.

Kriengsak Khet-in, an official of the Northeast region meteorological centre, said people in lowland areas of Nong Khai, Nakhon Phanom, Ubon Ratchathani, Mukdahan and adjacent provinces should beware of flash-floods in the next few days.

"The tropical storm will increase in power and reach land on Sunday. It will bring heavy rain to 80 per cent of the Northeast," Kriengsak said.

Meanwhile, the unprecedented flooding in Chiang Mai and several northern provinces on Monday has left businesses suffering huge losses, the tourism industry in tatters, schools closed and train services suspended.

Rain still fell on the northern capital yesterday, but the flood water was receding as local officials used 20 pumping machines to drain the water.

Heavy rain in Chiang Rai province on Tuesday night caused water to overflow from two reservoirs and submerge 300 houses and about 1,000 rai of farmland in Pan district.

"The flood situation will worsen if there is more rain. Residents in areas near the reservoirs have been evacuated," said a Pan district official.

Samart Choknapitak, the director-general of the Royal Irrigation Department, said the severe floods in northern provinces were caused by the huge quantity of rainwater, not by the construction sites at Chiang Mai's Night Safari or the Royal Flora Ratchaphruek expo allegedly blocking watercourses.

The department had already built a one-million-cubic-metre reservoir in the Night Safari and a two-million-cubic-metre reservoir at the construction site of the Royal Flora expo to prevent flooding, Samart said.

As a long-term measure to prevent flooding in Chiang Mai, the department planned to build another two reservoirs, which could contain about 100 million cubic metres of water, at a cost of Bt14 billion, he said.