Most regions of the country can expect to be drenched in the next 24 hours as heavy rain is forecast as Tropical Storm Conson approaches, the Thai Meteorological Department (TMD) says.
The TMD reported on Tuesday afternoon that the storm was moving across the central Philippines in a northwesterly direction towards Vietnam.
As it approaches the storm will likely bring a 70-80% chance of rain in all regions except the South in the next 24 hours. The downpours in many parts of the country are likely to continue until next Monday.
Persistent heavy rain could trigger downstream surges in rivers, including the Chao Phraya which flows through Bangkok, raising the risk of widespread flooding, the Royal Irrigation Department (RID), said.
RID director-general Prapit Chanma said he has issued instructions to all irrigation projects in areas predicted to be affected by the heavy rainfall to closely monitor water levels.
In Nakhon Ratchasima, all districts have been ordered to closely monitor water levels in reservoirs, for while major dams still have spare storage capacity some smaller ones are already full.
Provincial governor Kobchai Boon-orana said on Tuesday recent rain had increased flow into 27 large and middle-sized reservoirs. Many were now at 60% or more of their holding capacity.
Lam Takong Dam in Si Khiu district was holding 214 million cubic metres of water, or 66.13% of its capacity of 314 million cubic metres. Lam Phra Phloeng Dam in Pak Thong Chai district had 100 million cu/m, or 64.54 of its capacity of 155 million cu/m, the governor said.
These and two other dams had room for a lot more water, he said, provided there were no heavy storms or monsoon rains like those that pounded this northeastern province last year.
Mr Kobchai said even if the storms were severe, flooding and overflow from Lam Takhong and Lam Phra Phloeng dams would not be too bad.
Water levels in three out of 23 middle-sized reservoirs had already exceeded their holding capacities, he said. They are Lam Chiang Sa, Ban San Kamphaeng in Wang Nam Khieo and Lam Chiang Krai reservoir in Non Thai district.