The department forecast continuous heavy to very heavy rainfall from the monsoon trough across the lower South and a depression over the lower Andaman Sea.
"People should beware of severe conditions that may cause flash floods and overflows, especially along the waterways near foothills and lowlands," the department said.
The forecast for upper Thailand is a cool morning with light fog in many parts, though the temperature will rise by 1-2 degrees Celsius.
An easterly wind in the lower Northeast and East will bring isolated rain," the department said.
It advised people to take health precautions in the variable weather conditions.
24-hour forecast:
North: Morning cool and light fog, temperature lows of 16-19C and highs of 31-34C. Temperature on hilltops likely to drop to 8-14C.
Northeast: Morning cool with light fog and isolated light rain mostly in lower parts. Temperature lows of 16-22C and highs of 31-32C. Temperature on hilltops likely to drop to 10-14C.
Central: Cool morning with light fog. A rise in temperature of 1-2C. Temperature lows of 20-22C and highs of 31-33C.
East: Cool morning with light fog. Temperature rise of 1-2C. Isolated rain mostly in Chonburi, Rayong, Chanthaburi and Trat. Temperature lows of 22-24C, highs of 31-33C; waves about 1 metre high, and higher offshore.
South (east coast): Fairly widespread thundershowers and isolated heavy to very heavy rain in Prachuap Khiri Khan, Chumphon, Surat Thani, Nakhon Si Thammarat, Phatthalung, Songkhla, Pattani, Yala and Narathiwat. Temperature lows of 21-24C, highs of 28-31C; waves about 1 metre high, rising higher to two metres in thundershowers from Prachuap Khiri Khan to Chumphon.
South (west coast): Fairly widespread thundershowers and isolated heavy to very heavy rain in Ranong, Phang Nga, Phuket, Krabi, Trang and Satun. Temperature lows of 23-24C, highs of 29-32C; waves 1 metre high, higher in thundershowers.
Bangkok and surrounding areas: Cool morning with light fog. A rise in temperature of 1-2C. Isolated light rain. Temperature lows of 22-23C, highs of 31-34C.
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