The Northeast will see the mercury plummet by 2-4 degree Celsius this week, while the North, Central and East regions will experience falls of 1-3C, the department added.
It added that isolated heavy rains would hit the lower Central region on Tuesday. Upper Thailand will also see strong winds.
Meanwhile the South can expect isolated heavy to very heavy rain.
In its sixth variable weather warning issued at 5am on Tuesday, the department explained that temperature drops and heavy rain were the result of a high-pressure system over upper Thailand and a monsoon trough in the South, respectively.
Central: Ratchaburi and Samut Songkhram.
South: Phetchaburi, Prachuap Khiri Khan, Chumphon, Surat Thani, Ranong, Phang Nga and Phuket
The department said the Gulf of Thailand will see waves about 2 metres high and all ships should proceed with caution until Friday.