People in 14 provinces in the South and the East have been told to brace for heavy rain and rough seas from today through to Aug 3.
The Disaster Prevention and Mitigation Department on Friday issued the forecast on Friday, along with a warning that smallboats in the Andaman and Gulf area should stay ashore.
Provincial branch offices were placed on full alert to react to storm damage including floods and landslides.
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 between Friday and Wednesday. 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, Chantharuri 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 of low-lying areas near mountains and canals should be 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.
The southwest monsoon across the Andaman Sea, the South and the upper Gulf of Thailand would bring isolated heavy rain in the East and the western South.
Bangkok and vicinity could expect cloud cover with fairly widespread thundershowers and isolated heavy rain, mostly in the afternoon and evening today, the weather office said.