Temperatures are forecast to drop by 5C to 8C until Friday in the upper part of the country, while the lower parts are being warned to brace for torrential rain, strong winds and rough seas.
In its fifth warning issued yesterday, the Meteorological Department (MD) said upper Thailand as well as Bangkok and surrounding provinces will experience a cold spell.
Frost is forecast for certain highland areas in the North and the Northeast.
Temperatures are forecast to drop by 3-5C today, and further drop by another 2-3C from tomorrow until Thursday, said the department, adding the cold weather may last until Friday.
Colder weather in the northeastern province of Kalasin, for instance, has already attracted tourists to several attractions in the province including Laem Non Wiset, a newly discovered scenic spot by one corner of Lam Pao dam, that covers three districts of the province.
Laem Non Wiset, in Sahatsakhan district, is now seeing a rise in the number of visitors who come to relax and dine during a raft ride on the dam and enjoy the other recreational activities which the tourist attraction has to offer.
Kittikorn Thep-yu-amnuay, head of Prachuap Khiri Khan's disaster prevention and mitigation, said the Southern Meteorological Centre has warned Prachuap Khiri Khan and the rest of the South to prepare for heavy rain, strong winds and high sea waves today.
As a high pressure system approaches upper Thailand, it normally results in a monsoon period in the lower part of the country.
In Prachuap Khiri Khan, yellow flags have been raised to warn tourists about the dangers of swimming at certain areas, said Mr Kittikorn, adding local fishing trawlers were also being warned to refrain from going out to sea.
Meanwhile, four provinces in the South are still inundated by flood water, affecting more than 90,000 households, Disaster Prevention and Mitigation Department chief Chayapol Thitisak said yesterday.
They are Songkhla, Phatthalung, Nakhon Si Thammarat and Surat Thani.
In Songkhla, 37 villages in six tambons of Sathing Phra, Krasae Sin and Ranot districts are still flooded.
In Phatthalung, flooding continues to ravage 422 villages in 40 tambons of Muang, Khuan Khanun, Pak Phayun, Khao Chaison and Bang Kaeo districts.
In Surat Thani, 155 villages in 30 tambons of Khian Sa, Ban Nasan, Ban Nadoen and Phun Phin districts are still flooded.