Zacharias Wuragil, Jakarta – Indonesia's Meteorology, Climatology, and Geophysics Agency (BMKG) has officially declared the start of the rainy season, with several regions already transitioning from dry weather this September. Typically, the rainy season begins later in the year, but this time it has come earlier than expected.

Where the rains have started

The first signs of the rainy season appeared in August across parts of Sumatra and Kalimantan. By September, the seasonal shift had spread southward and eastward. BMKG noted that this year's dry season has been shorter and wetter compared to previous years.

"The majority of regions will enter the rainy season between September and November," said BMKG Head Dwikorita Karnawati on Friday, September 12, 2025.

When will the peak occur?

• Sumatra & Kalimantan: November – December 2025
• Java, Sulawesi, Maluku & Papua: January – February 2026

Dwikorita warned that because the peaks are staggered across regions, Indonesia may face prolonged risks of hydrometeorological disasters, such as floods, landslides, and strong winds, stretching over several months.

Bali floods a warning sign

BMKG's announcement followed a major flood in Bali on September 10, when rainfall reached 385.5 millimeters in a single day, more than double the threshold for extreme rain, which is 150 mm per day.

What's driving the early rains?

BMKG cited several climate patterns and weather systems fueling the heavy rainfall:

• Equatorial Rossby wave, Kelvin wave, and Madden-Julian Oscillation (MJO), which are forecast to remain active for at least another week.
• Tropical cyclone seed 93S forming in the Indian Ocean, west of Sumatra.
• A cyclonic pattern detected near North Kalimantan.

Which areas should be on alert?

Between September 15-18, BMKG urged residents in the following regions to prepare for heavy rain, thunderstorms, and strong winds:

• Central Java
• East Java
• West Kalimantan
• West Sulawesi
• South Sulawesi
• Central Papua
• Papua Highlands
• South Papua

What this means

Indonesia's early rainy season underscores the growing unpredictability of climate patterns in the archipelago. While the rains provide relief after the dry season, they also heighten the risk of natural disasters across vulnerable regions.

Source: https://en.tempo.co/read/2048870/why-is-indonesias-rainy-season-starting-early-in-202