People in 14 provinces in the South and East have been told to be on alert for heavy rain and rough seas between now and next Wednesday.
The Disaster Prevention and Mitigation Department on Friday issued the forecast and warned operators of small boats in the Andaman and Gulf area to stay ashore.
Provincial branch offices were placed on full alert to deal with storm damage including floods and landslides.
Department director-general Chatchai Promlert said the southwest monsoon in the Andaman Sea, the South and the upper Gulf of Thailand would bring isolated heavy rain over the east and the southwest coast from yesterday to Wednesday next week. Waves in the Andaman Sea would be 2-3 metres high.
Fourteen provinces in the two regions were likely to be affected: Four provinces in the East -- Chon Buri, Rayong, Chanthaburi and Trat; and 10 provinces in the South -- Prachuap Khiri Khan, Phetchaburi, Surat Thani, Chumphon, Nakhon Si Thammarat, Ranong, Phangnga, Phuket, Krabi and Trang.
Residents in low-lying areas near mountains and canals should be on alert for possible danger from downpours and accumulated rain.
The Meteorological Department said tropical depression Mirinae over upper Vietnam had been downgraded to a low pressure zone but would bring more rain to the North and the Northeast of Thailand.
Bangkok and surrounding areas could expect cloud cover with fairly widespread thundershowers and isolated heavy rain, mostly in the afternoons and evenings, the weather office said.
Meanwhile, in Chiang Rai, a heavy downpour saw run-off from northern mountain ranges inundate a road and villagers' houses in tambon Nang Lae in Muang district yesterday morning.
The floodwater was 30-40cm deep along the road in Moo 13 of the tambon, submerging several houses, and causing widespread traffic congestion before it receded.