In a special warning issued at 5am Saturday, the department said there would be thundershowers, gusty winds and heavy rain in upper Thailand Saturday and Sunday.
The department attributed the weather conditions to a monsoon trough sitting over the lower North, the upper Central and the lower Northeast and a low pressure system over the South China Sea while a moderate high-pressure system from China hung over upper Vietnam and Laos.
The department said a sharp fall of temperature from Monday to Thursday would be caused by a moderate high-pressure system from China.
It said the fall in temperature would start in the Northeast and spread to the rest of the country.
