Weathermen forecast heavy rain in 15 northern provinces and five central provinces as tropical depression Rai became a low pressure area over the lower North on Wednesday.
Wanchai Sakudomchai, director-general of the Meteorological Department, said on Wednesday that rain was expected in the northern provinces of Chiang Rai, Chiang Mai, Kamphaeng Phet, Lampang, Lamphun, Mae Hong Son, Nan, Phayao, Phetchabun, Phichit, Phitsanulok, Phrae, Sukhothai, Tak and Uttaradit.
A storm warning was also out for the Central Plain provinces of Chai Nat, Kanchanaburi, Lop Buri, Nakhon Sawan and Uthai Thani.
Strong seas were forecast for the Gulf, and small boats advised to remain ashore until next Monday.
Government spokesman Sansern Kaewkamnerd said disaster and emergency services were on standby to respond to any flooding.
Water was being pumped out of waterways in Sukhothai and Ayutthaya provinces to accommodate the flood surge expected from the North, he said.