Southwest monsoon starts in India and brings the stream of moist to Thailand from the Indian Ocean. Thailand is also located in the Inter Tropical Convergence Zone (ITCZ), a belt of low pressure which is located near the equator. It makes Thailand exposed to thunderstorms, typhoons or extreme tropical cyclones.
Typhoon season lasts from May to October with the most extreme rainfall occurring in August and September. So don't forget to check the weather forecast for tropical storm alerts when travelling in Thailand during this time.
On the Andaman Sea, where the resort of Phuket is located, monsoon can start as early as April although the heaviest downpours occur in June–August, and last till October, when the temperatures begin to cool. Due to its location, the Andaman Coast of Thailand is hit by the monsoon the hardest. Torrential rains here also mean rough sea conditions. Ferries and boats are no longer safe. Some water transportation doesn’t operate from May to October.
The Gulf region, where includes popular islands of Ko Samui and Ko Pha Ngan, and the East Coast with the resorts of Pattaya and Ko Chang, also feel the impact of southwest monsoon but they are drier than the Andaman Coast.