Department deputy director Somchai Baimuang said varying volumes of heavy rain are forecast in the North, Northeast, the central region, the East and the west coast of the South.
A weather system trough will lie above the Central and the East on Friday and Saturday, and while the southwest monsoon strengthens over the Andaman Sea and the Gulf of Thailand.
Mr Somchai warned the public of severe weather today through Monday, with rain amounting to as much as 90-100 millimetres in flood-hit provinces.
The weather agency reported all information to the Water and Flood Management Commission (WFMC) to lay out flood preventive measures.
The public should closely monitor the situation but should not panic, he said. The amount of rainfall this year is [so far] 20 per cent less than last year.
Thailand will possibly face two storms from next month onwards, he said. In that case, the department will be able to issue a warning at least ten days in advance.
Meanwhile, typhoon Sanba, centred east of the Philippines with maximum sustained wind of about 155 km/hr, is moving toward Okinawa, Japan on the weekend and will have no effect on Thailand.