A monsoon trough prevails across the lower Northeast, the Central and the East of Thailand while a rather strong southwest monsoon prevails over the Andaman Sea, Thailand and the Gulf of Thailand.
People should beware of the severe conditions and flash floods, the department said.
Waves in the upper Andaman Sea are expected to rise 2-3 metres high and above three metres in thundershowers. All ships should proceed with caution and small boats should keep ashore, the department said.
The weather forecast for the next 24 hours:
North: Thundershowers in 60 per cent of the areas with isolated heavy rains; temperature lows of 23-25 degrees Celsius, highs of 30-35°C.
Northeast: Thundershowers in 70 per cent of the areas with isolated heavy rains; temperature lows of 23-25°C, highs of 33-35°C.
Central: Thundershowers in 70 per cent of the areas with isolated heavy rains; temperature lows of 24-25°C, highs of 32-34°C.
East: Thundershowers in 70 per cent of the areas with isolated heavy rains; temperature lows of 23-26°C, highs of 32-35°C; waves 1-2 metres high and over two metres during thundershowers.
South (east coast): Thundershowers in 70 per cent of the areas with isolated heavy rains; temperature lows of 23-24°C, highs of 31-35°C; waves a metre high and 1-2 metres during thundershowers.
South (west coast): Thundershowers in 60 per cent of the areas with isolated heavy rains; temperature lows of 21-25°C, highs of 31-35°C; waves 2-3 metres high and over three metres during thundershowers.
Bangkok and surrounding areas: Thundershowers in 60 per cent of the areas; temperature lows of 24-26°C, highs of 32-34°C.