Gaemi is forecast to hit Thailand from Sunday until Tuesday, particularly the lower Northeast, the East and parts of the central region, including Bangkok.
Bangkok residents have been warned of possible flash floods from heavy rain that could exceed 100mm, while City Hall said it could manage well if rainfall does not exceed 60mm.
In the Northeast, several provinces have suffered drought and farmers and irrigation authorities are waiting for heavy rains to relieve the situation.
"Heavy rain brought with Gaemi would benefit local agriculture," said Kumjorndech Songyai, chief of the water management and irrigation improvement section of the Regional Irrigation Office 7.
Without rainfall, it is predicted that Ubon Ratchathani and surrounding provinces will face a serious drought by the end of the year, he said.
Heavy rain from Gaemi could fill up water sources in the provinces.
Mr Kumjorndech said Buri Ram, Si Sa Ket and Surin provinces have also been short of water.
According to a survey conducted by the irrigation office, Kalasin's Lam Pao Reservoir, one of the major water sources in the Northeast, is down to only 24% of its water capacity.
The Sirindhorn Dam in Ubon Ratchathani is operating at 72% of its capacity.
Mr Kumjorndech said the water level in the Mun River is 3.14m lower than its bank, while the level of the Chi River is 3.82m below its bank.
"If there is no rainwater to fill up water sources, Ubon Ratchathani and nearby provinces will be in trouble with a serious drought," Mr Kumjorndech said.
Ubon Ratchathani rice farmer Supon Buasri, 51, said he needed water for his 10 rai rice field, which has almost died, though he was afraid of possible flooding caused by Gaemi after seeing news reports.
Since July, he said, he has lost 30% of the grain he should get from his rice field because of the water shortage.
Yasothon farmer Buakan Vajasata, 64, said the region has been dry for three months. As a result of the water shortage, her 56 rai rice field has lost 50% of the yield she would have expected to take in the harvest season, which runs until the end of this month. About 3.5 tonnes of paddy have been lost to the drought.
"We're not afraid of Gaemi. We need water," she said.
Pairin Limjaroen, chief of Buri Ram disaster prevention and mitigation office, said water has been scarce in some parts of the province since last summer.
"If Gaemi causes heavy rainfall for a couple of days, we would have more water to fill up reservoirs and use for irrigation and to revive farmers' crops," he said.
Mr Pairin said 13 reservoirs in Buri Ram have water levels averaging about 63% of capacity.
However, he said the province has also prepared for possible flooding in some areas due to the storm.
Provincial authorities have cleared obstructions from waterways after learning a lesson from last year when Buri Ram municipality was inundated under 1.4m-deep floods.
"The outcome of the storm is unpredictable," said Mr Pairin.
In Nakhon Ratchasima, ML Anumas Thongthaem, chief of Regional Irrigation Office 8, said it would be good to have storms in lower Northeast provinces so that heavy rain would fill up reservoirs that are mostly down to only half of their capacity.
The large Lam Ta Kong Reservoir is at only 45% of its capacity.