The change in season is brought by south-westerly winds blowing moisture-laden air from the Andaman Sea, said department chief Dr Nattapon Nattasomboon. These will combine with high-altitude winds blowing from the northeast, whipping up rainstorms across most of the country, he added.
The Meteorological Department forecasts a heavier-than-usual first half of the rainy season, with high risk of flash floods in many areas.
Overall, 5-10 per cent more rain than average is forecast for this year's rainy season.
Meanwhile, there is a high probability that the Northeast and North will be hit by two or three tropical typhoons during August and September, the department warned. The rainy season in upper Thailand will end around mid-October. For the South, especially the east coast, the rains will linger on until mid-January.