Thailand Officially Enters Rainy Season as Monsoon Arrives

Thailand’s Meteorological Department (TMD) has officially declared the start of the country’s rainy season, confirming that weather conditions associated with the annual monsoon transition fully aligned on May 15.

The announcement coincided with the traditional start of the low season in Thailand’s national parks, many of which begin seasonal closures around this time each year.

Unlike the Department of National Parks, which follows fixed annual dates, the TMD bases its seasonal declarations on actual meteorological data. In 2026, the official and climatic start of the rainy season matched exactly.

Meteorologists said several key weather indicators confirmed the seasonal shift beginning on May 15:

• Continuous rainfall covering at least 60% of northern Thailand

• A change in wind direction at an altitude of around 1.5 kilometres, with southwesterly winds bringing moisture from the Indian Ocean

• Easterly upper-level winds at approximately 10 kilometres altitude, carrying air masses from the South China Sea

According to the TMD forecasting division, this year’s rainy season is expected to be less severe overall compared to last year. However, forecasters still anticipate two traditional periods of particularly heavy rainfall — the first during the coming month through mid-June, and another during August and September.

The department outlined the following general timeline for Thailand’s 2026 rainy season:

• May 15 — official start of the rainy season

• May to June — gradual increase in rainfall• Late June to early July — temporary reduction in rain intensity

• August to September — peak rainfall period with elevated flood and landslide risks

• October — gradual weakening of rainfall and cooler weather in northern regions

While the forecast applies to Thailand as a whole, Phuket and Thailand’s Andaman coast typically experience stronger early monsoon conditions because they directly face weather systems moving in from the Indian Ocean.

Regional meteorologists at Phuket Met have already issued a weather advisory covering Phuket, Phang Nga, and Ranong provinces from May 14 to May 18.

Forecasters expect periods of heavy rain, rougher sea conditions, and increasing wave heights. Although conditions are not yet considered severe monsoon storms, waves in some areas could already reach between one and two metres.

Authorities advised boat operators to exercise extra caution at sea, while residents and tourists in coastal provinces were urged to closely monitor weather forecasts and changing conditions.