TL;DR:The rainy season in Thailand is mainly May–October (southwest monsoon), though the Gulf coast’s wettest months often shift toward October–December.
Thailand’s main rainy season is driven by the southwest monsoon and runs broadly from May through October. Rainfall usually begins in May, strengthens through June–August and peaks around August–September before tapering off in October. Regional variation matters: the Andaman coast (Phuket, Krabi) gets heavy rain May–October; central Thailand including Bangkok and the north/northeast also have their wet season May–October; the Gulf coast (Koh Samui, Hua Hin) often has a different pattern with its wettest months around October–December. Short, intense afternoon storms are common during the season, with the highest flood risk during peak months. For operational forecasts, onset/cessation dates vary year-to-year—check the Thai Meteorological Department for current-season bulletins and the official tourism site for regional summaries: https://www.tmd.go.th/en/climate/summarymonthly and https://thailand.go.th/issue-focus-detail/009_142
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