In its advisory issued at 11 am Thursday, the department said the drop in the temperature would be caused by a strong high pressure from China that extends to cover the Northeast, the Central and the East of Thailand as well as the North.
During the next two days, frost could happen on mountaintops in the North, the department added.
It also warned that the northeast monsoon across the Gulf and the South of Thailand will intensify and it will generate waves that will be 2-4 meters high in the Gulf of Thailand.
It said small boats should keep ashore during 17-20 December.