Heavy rain is forecast for northern, northeastern, central and southern provinces this week, with Sukhothai in the North at risk of major flooding from the rain-swollen Yom River.
The Meteorological Department issued a warning on Monday morning.
It said heavy rain was expected in North, upper Northeast and Central regions, including Bangkok, and most parts of the southern region. It warned of possibly flash flooding from overflowing watercourses and runoff at least until Wednesday.
The Hydro Informatics Institute said heavy rain could continue until Friday, especially in Chiang Rai, Phayao, Nan and Phrae provinces.
The Disaster Prevention and Mitigation Department had earlier reported that flooding caused by severe downpours last week had finally started falling in those four provinces. The latest forecast puts them back in the danger zone.
Almost 12,000 households in the northern provinces were already affected by floods, it said.
Flooding in the northern region is due to water overflowing the banks of the Ping, Wang, Yom and Nan rivers.
The next target for downstream flooding is Sukhothai, which always absorbs water brought by the Yom River from provinces to the north. The Yom is the only major river in the region without a dam to hold back water.
Sophan Srisawangworakul, chief of the royal irrigation project in Sukhothai, told The Reporters TV that Sawankhalok, Si Samrong and Muang districts were likely to be flooded.
The situation was not yet critical. The canals in the province could manage the water in the Yom. "The situation in Sukhothai is serious only because it is receiving more water," he said.
Mr Sophan said, however, more heavy rain could overwhelm flood prevention measures in the province and cause new problems.
