
In a second warning of fast-changing weather conditions issued at 11am on Thursday, the department said people in the south should also get prepared for heavy downpours.
The department explained that from November 18-21, another high pressure from China would extend to upper Thailand and the South China Sea.
At the beginning of the period, outbreaks of rain are likely, with temperatures plunging by 2-4 degrees Celsius and strong winds.
The warning added that a strong northeast monsoon across the Gulf would cause abundant rain in the south. Isolated heavy rains are forecast while high waves are expected to about 2 metres. People should be alert for severe rain and possible flash flooding.
The warning added that a low-pressure cell over the Philippines will move to the lower South China Sea on November 18 or 19 and that may later have some impact on Thailand.