Heavy rain and thunderstorms are expected in most parts of the country including Bangkok until Sunday, while the cold season is forecast to arrive mid-month, according to the Meteorological Department (MD).
At least three provinces in the South -- Phuket, Surat Thani and Trang -- have been badly affected by downpours since Monday night.
The heavy rain with thunder and lightning was brought by a monsoon trough now stretching across the country from the North to the South, said Phuwiang Prakhammin, acting director-general of the department.
The rainy weather is forecast to have an impact on the North, Central Plains, East and upper South until Sunday, said Mr Phuwiang.
Residents of communities near mountains in the North, especially Mae Hong Son and Phitsanulok, where the soil has absorbed a great volume of water from the last rainy period, are being warned of possible landslides and forest runoff during this fresh period of heavy rain, he said.
Those mountain slopes in the East and West are also at high risk of landslides and forest run-off, he added.
Super Typhoon Kong-Rey that is moving from the Pacific Ocean towards Taiwan and China, but it won't have any significant impact on Thailand, he said.
In Surat Thani, flash floods and runoff ravaged three districts overnight, affecting 25 households and claiming the life of a 55-year-old man, whose body was found early Tuesday morning.
Heavy rain started pounding several villages in Khian Sa, Phanom, Khiri Ratthanikhom and Phrasaeng districts at around 9pm on Monday. The downpours lasted several hours, triggering floods and forest runoff in the four districts.
Three villages bore the brunt -- Ban Khao Samyod in Khian Sa, Ban Tham Phueng in Phanom and Ban Tham Khrob Nam in Phrasaeng -- with 83 people from 25 households affected, said Chamnong Sawatwong, chief of the Surat Thani disaster prevention and mitigation office.
In Phrasaeng district, one man, identified later as Arunrat Phetcharat, 55, was swept away by the strong current when he left his flooded house to seek safety on higher ground.
Searchers found his body at around 4.45am lodged in a clump of bamboo along a canal, about 200 metres from his house.
Praphat Khomkla, kamnan of tambon Bang Sawan in Phrasaeng district, said he hadn't seen a flood reach such critical levels for the past three decades.
After only three hours of heavy rain, the water rose rapidly forcing 11 Buddhist monks and novices at Tham Nam Rot monastery to first flee to the roof of the monastery and then escape to a nearby mountain one hour later, said the kamnan.
In Phuket, forest run-off from Kamala mountain in Kathu district triggered a flash flood in about six villages, prompting urgent evacuations. The situation was chaotic as there was no flood warning, leaving most residents unprepared.
A landslide was also reported on a section of the Patong-Kamala road, making it impassible. Motorists travelling between Kamala and Patong beaches were forced to make detours.