It said 70% of Bangkok and surrounding areas can expect rain today.
Overflowing waterways are a danger in low-lying areas of flood-prone provinces, while flash flooding is also a threat.
Strong winds could cause waves to reach two to three metres in height in the upper Andaman Sea and upper Gulf of Thailand, and to exceed three metres during thunderstorms. Further south, waves are forecast to be one to two metres in height, though they could rise higher during thunderstorms.
The department is advising owners of small boats to keep them docked today.
