It issued a warning of flash floods and runoffs in the South, especially in foothills, near waterways and in low-lying areas.
Waves in the Andaman Sea are expected to rise to 2 metres, and higher in thundershowers, while waves in the Gulf will be around 1 metre, rising in stormy weather.
"All ships should proceed with caution and keep off thundershowers," the department added.
The 24-hour forecast for different regions:
North: Morning cool with fog and isolated light rain; temperature lows of 21-24C and highs of 31-33C.
Northeast: Morning cool with light fog and isolated light rain; temperature lows of 20-22C and highs of 30-33C.
Central: Morning light fog and isolated thundershowers mostly in Suphanburi, Kanchanaburi, Ratchaburi, Nakhon Pathom, Samut Sakhon and Samut Songkhram; temperature lows of 23-25C and highs of 31-34C.
East: Morning light fog and isolated thundershowers mostly in Chonburi, Rayong, Chanthaburi and Trat; temperature lows of 23-26C, highs of 30-34C; waves about 1 metre high and 1-2 metres offshore.
South (east coast): Fairly widespread thundershowers and isolated heavy rain in Chumphon, Surat Thani, Nakhon Si Thammarat, Phatthalung and Songkhla; temperature lows of 23-24C, highs of 30-33C; waves about 1 metre high, rising in thundershowers.
South (west coast): Scattered thundershowers in Ranong, Phang Nga, Krabi, Trang and Satun; temperature lows of 23-25C, highs of 31-33C; waves 1-2 metres high, rising in thundershowers.
Bangkok and surrounding areas: Morning light fog and isolated thundershowers; temperature lows of 24-25C and highs of 32-34C.