Heavy overnight rainfall from Sunday evening caused the flash flood, said Sawai Kerdkaew, chairman of the Phakkuang Tambon Administration Organisation in Uttaradit.
Chai Nat governor Suttipong Juljarern also warned residents in the western districts of Noen Kham, Hankha, Nong Mamong and Wat Sing to prepare for possible evacuation if water overflows the banks of the Chao Phraya River.
The river in the Chai Nat area continued to rise from heavy rains over the past several days, he said. The water has risen 25cm since last week.
With the ground saturated, Mr Suttipong warned that people living near hills should be wary of possible mudslides.
The authorities will monitor the situation and immediately communicate with people in the affected areas, the governor said.
During major flooding in 2011, homes and farms in Chai Nat were hit hard.
Meanwhile, locals in Ayutthaya's Bang Ban district fear flash floods due to sharply rising water in the Chao Phraya River combined with the southwestern monsoon approaching the Gulf of Thailand.
The Thai Meteorological Department said the monsoon is likely to cause downpours in about 40% of the country.
Bang Ban resident Mali Lertdee, 57, said the rising water has turned reddish brown, which is a sign of a flash flood.
She and her local neighbours have begun to empty the ground floors of their houses.
Mrs Mali also said she had moved all of her vegetables into pots for the flood season.
Meanwhile, tourist groups were prohibited from playing in the water at Kaeng Sopha waterfall at Thung Salaeng Luang National Park in Phitsanulok's Nakhon Thai district.
Park official Sayan Timhaew said heavy rains in Phetchabun and Phitsanulok have caused the water level to swell.
She said the park was still open, but officials have been deployed at several spots to keep people out of the water.