Rainy season to begin officially on Friday

Thailand officially entered its rainy season on May 15, 2026, though forecasters expect overall rainfall to remain below typical levels. The Meteorological Department announced the start of the season on May 14, 2026, noting that total rainfall is projected to be approximately 10% lower than the seasonal average.

The official designation of the season’s start is based on three specific meteorological conditions. According to Sukanyanee Yawinchan, the director-general of the Meteorological Department, these conditions include the presence of continuing rain over 60% or more of upper Thailand, a shift in lower-level winds to the southwesterly and a transition of upper-altitude winds to the easterly.

Regional Timelines and Rainfall Patterns

The duration of the wet season varies by region. The rainy season in upper Thailand is expected to persist until approximately mid-October 2026. In the eastern parts of the South, rain is forecast to continue until December 2026.

The Meteorological Department expects rainfall frequency to increase steadily from May 15, 2026, through mid-June, with the most notable increases occurring in the East and the western parts of the South.

Following this initial increase, the department forecasts a shift in intensity. From the end of June 2026 into July 2026, both the frequency and intensity of rainfall are expected to drop, resulting in a temporary dry spell.

Rainfall is expected to resume in August and September 2026. Forecasters warned that this latter period requires close monitoring due to the potential for torrential rain, forest runoff, rivers overflowing, and flash floods.

Climate Patterns and El Nino Risks

The Meteorological Department is closely monitoring the El Nino climate pattern. While the pattern is currently considered to be in a neutral condition, Ms. Sukanyanee stated there is a possibility of a stronger El Nino developing during the second half of 2026.

This outlook aligns with warnings from scientists regarding an increasing risk of a Super El Nino in the latter half of the year. Such a development is expected to hit Southeast Asia particularly hard, leading to significantly drier conditions across the region.

Tropical Storms and Marine Monitoring

In addition to the general seasonal rainfall, the Meteorological Department expects one or two tropical storms to approach Thailand during August and September 2026. These storms are projected to primarily affect the North and Northeast regions of the country.

To manage these risks, the department has opened a monitoring centre specifically for strong winds and heavy rain in the Andaman Sea.