The department said a high-pressure system would linger over upper Thailand until October 14 (Friday), leading to thundershowers with isolated gusty winds and heavy rain for the next two days.
Temperatures in the Northeast will fall by 3-5 degrees Celsius and by 1-3C in the North, Central and East regions, according to the forecast.
"People should beware of variable weather conditions and stay safe," the department said.
The prevailing easterly wind across the Gulf and the South will strengthen bringing heavy to very heavy rain in the South until Tuesday, it added. The department forecasts waves of 2 metres high in the Gulf, and higher in thundershowers.
All ships should proceed with caution until October 14 (Friday), it added.