The downpours are caused by the active monsoon prevailing across Myanmar, the upper North and the upper part of Laos converging with a low-pressure cell over upper Vietnam and Tonkin Bay.
The department advised people in affected areas to be beware of rain accumulation that could cause flash flooding and overflows, especially along foothills, near waterways and low-lying areas. Road users should also take more caution during thundershowers.
Meanwhile, the moderate southwest monsoon prevailing across the Andaman Sea and the Gulf will cause waves in the upper Andaman Sea and the upper Gulf to rise to 1-2 metres and over 2 metres high in thundershowers. All ships should proceed with caution and keep away from storms, it advised.