Unstable weather as Thailand nears rainy season in mid-May

The current weather — marked by heat, thunderstorms, and sudden rain — is a sign of this seasonal shift.

Authorities are warning the public to prepare for severe weather, including heavy rain, strong winds, lightning, flash floods, and changing wind patterns.

The Pre-Monsoon Phase

This transition period, known as the pre-southwest monsoon phase, occurs when southerly and southeasterly winds start covering the country, bringing humidity from the Andaman Sea. This results in increased rainfall in the upper northern and northeastern regions.

During this time, weather conditions can be highly unstable, with shifting wind directions leading to wind convergence zones. The air remains hot and humid, with a high chance of thunderstorms, especially in areas directly influenced by the prevailing southerly winds. The Andaman coast and the southern west coast are now under the influence of southwesterly winds.

When Will the Rainy Season Begin?

The TMD forecasts that the 2025 rainy season will begin around mid-May, though the official declaration depends on specific meteorological conditions being met.

What to Watch For During the Seasonal Shift

The TMD urges the public to stay alert and prepare for the following risks during the transition:

Lightning strikes: Avoid open areas, large trees, and buildings without lightning rods during thunderstorms.

Strong gusts: Steer clear of billboards, weak structures, and large trees that could fall during high winds.

Structural safety: Secure or reinforce trees and structures around homes to prevent storm damage.

Flash floods and runoff: Stay informed in flood-prone areas and be prepared to evacuate if necessary.

Persistent rainfall in the upper regions: Continued rainfall in northern areas signals the onset of the monsoon.

Criteria for Official Rainy Season Declaration

The rainy season is declared based on three main criteria:

Consistent and widespread rainfall across most parts of the country.

Lower-level winds (surface to 3.5 km altitude) must shift to steady southwesterly winds, carrying moist air from the Andaman Sea for at least 1–2 weeks.

Upper-level winds (above 5.5 km altitude) must switch to northeasterly winds.

Public Advisory

As the 2025 rainy season approaches, the TMD advises everyone to closely follow weather forecasts, take precautions against severe storms, flooding, and temperature swings, and maintain good health during this changing weather.