BANGKOK: The Thai Meteorological Department (TMD) has issued a nationwide warning for heavy rain on Monday, September 21st, 2026, with isolated very heavy downpours expected in the East and thunderstorms covering 80 percent of Bangkok and surrounding areas.
In its 24-hour forecast, the department said Thailand would see heavy rain in some areas across all regions. Very heavy rain is possible in parts of the East, including Nakhon Nayok, Prachin Buri, Chachoengsao, Chon Buri, Rayong, Chanthaburi and Trat. The Central region, including Bangkok and its vicinity, is forecast to have thunderstorms over 80 percent of the area with isolated heavy rain.
Bangkok and the surrounding metropolitan area can expect a low of 24–26°C and a high of 31–33°C, with variable winds of 10–25 km/h. Similar thunderstorm coverage of around 80 percent is forecast for much of the Central region. The North and Northeast are expected to see thunderstorms over about 60–70 percent of those areas, with isolated heavy rain in several provinces.
The weather pattern is driven by a high-pressure system from China covering the upper North and upper Northeast, which is beginning to weaken, combined with a monsoon trough lying across the lower Central region, upper South and East, and a moderate southwest monsoon over the Andaman Sea, Thailand and the Gulf.
Authorities urged residents, especially those in foothill areas near waterways and in low-lying zones, to watch for flash floods, forest runoff and landslides caused by heavy and accumulated rainfall. People were advised to avoid routes that regularly flood, as short-term inundation is possible in low-lying spots. Small boats in the Andaman Sea were told to use caution, with waves of 1–2 meters and higher than 2 meters in thunderstorms.