The annual wet season is officially here, with heavy rain reported over at least 60% of the upper regions of the country.
The Meteorological Department on Tuesday announced the end of the summer season on May 20 and commencement of the rainy season.
In upper Thailand, heavy rain has been reported over at least 60% of the region for three consecutive days, the department said.
Trade winds blowing over Thailand at around 1,500 metres in height had changed direction. Southwest winds would continuously carry moisture over the land from the Andaman Sea.
At about 10,000 metres height, the winds had changed direction to blow from the east.
With these conditions, the rainy season had started in Thailand, the department said.
The rainy season in upper Thailand was expected to end in mid-October, while heavy rain in the South, particularly on the east coast, would continue to pound the region until mid-January.
The department's immediate forecast is for heavy rain in areas in the North, Northeast, Central Plains and East and on the south coast, from May 21-24.
Residents are warned to prepare for heavy to very heavy rain, runoff and possible flash flooding during this period.