The release was ordered by Prime Minister Yingluck Shinawatra after the weather office forecast heavy rainfall in the northeastern, eastern and central regions.
The National Water and Flood Management Policy Office yesterday predicted the Northeast, especially Ubon Ratchathani, would be hit by heavy downpours brought by Gaemi on Friday.
The storm would then travel past lower northeastern and eastern provinces before hammering the Central region on Sunday and Monday with heavy rain expected in Bangkok and Pathum Thani.
Gaemi, which has recently developed from a depression system, was about 750km east of Da Nang, Vietnam, yesterday and packing winds of up to 65kph, the Meteorology Department said.
Heavy rainfall in the Central region is expected to increase the water level in the Pasak Jolasid dam where the water volume reached 79% of its capacity on Monday, according the Hydro and Agro Informatics Institute.
The reservoir, located in Lop Buri and Saraburi, blocks runoff in the Pasak River, which merges with the Chao Phraya River in Ayutthaya.
For the Bhumibol dam in Tak, which Gaemi is also expected to pass before entering Myanmar, the reservoir reached 60% of its capacity on Monday while the Sirikit dam in Uttaradit was at 67% of its capacity.
The two dams help delay runoff from the North that passes into the Chao Phraya which flows through the Central Plains and Bangkok.City officials have been told to keep a watch on the water level in Khlong Thawi Watthana in western Bangkok because canal-side communities could flood if it receives more rainwater.
Bangkok Governor MR Sukhumbhand Paribatra, meanwhile, said yesterday the city is considering raising the sluice gate at the Pracha Ruamchai section of Khlong Saen Saep-Min Buri from Sunday to improve drainage in eastern Bangkok when Gaemi hits.
Min Buri district, where the watergate is located, has 13 flood-prone areas which are outside the city's flood walls.
The Royal Irrigation Department is also bracing for heavy rainfall by preparing 665 pumps for areas at risk of flooding, he said.