The Northeast monsoon is expected to cross the Gulf of Thailand along with a depression from the South China Sea and will hit eight provinces in the South with heavy rain.
The provinces to be affected are Surat Thani, Nakhon Si Thammarat, Phatthalung, Trang, Songkhla, Pattani, Yala and Narathiwat.
ONWR has called on all relevant agencies to strictly monitor the situation and urge residents to move to higher ground.
Water management systems will be adjusted likewise, ONWR said, adding that levels in small sources will be maintained at 95 per cent to avoid an effect on reservoirs.