The useful thing about Indonesian season forecasting is how precisely it is defined. There is a threshold, a unit of time, and a zone map, which means a traveller can check whether a given place is in the wet season rather than guessing from a month name.
How the season is actually defined
The unit is a dasarian, a ten-day period, with each month split into days 1 to 10, 11 to 20, and 21 to month end, giving 36 a year.
The wet season begins, in the agency's own words, at the first dasarian with rainfall of 50 mm or more that is followed by two more dasarian of 50 mm or more. Alternatively, if the first reaches 50 mm but one of the next two falls below it, the first still counts as onset provided the three together reach 150 mm or more. The dry season definition mirrors it with the inequalities reversed.
Seasons are also graded against normal: above normal at more than 115 percent, normal at 85 to 115 percent, below normal under 85 percent, all against the 1991 to 2020 baseline. A separate guide takes up the Lebaran travel shutdown.
Why there is no national calendar
Indonesia is divided into 699 season zones, defined as areas sharing relatively homogeneous season characteristics. The agency identifies three rainfall pattern types: monsoonal, with a U shape and rain at both ends of the year; local, with one mid-year peak; and equatorial, with two peaks.
The sub-types make the point sharper still. Some zones are classified as having a wet season all year, others as having two dry and two wet seasons, and one local sub-type as having a dry season all year.
The agency states directly that the wet season in Indonesia does not begin at the same time, and gives examples: parts of Sumatra and Kalimantan had entered the 2025/2026 wet season before September 2025 while most areas started between September and November; several areas in Maluku and Papua were reclassified as wet year-round; and the dry season begins in Nusa Tenggara first.
The current forecast
On 22 September 2026 the agency reported an ENSO index of +2.63 for the July to September quarter, a Nino3.4 sea surface temperature anomaly of +2.83 at the end of August 2026 and an Indian Ocean Dipole of +0.376, describing this as a very strong El Nino.
Its forecast for the 2026/2027 wet season is onset later than normal in 529 zones, 61.08 percent of the land area, earlier in 11 zones and the same in 26. Onset falls mainly in November 2026 for 185 zones and December 2026 for 266, with the December group explicitly including most of Java. The peak is put at December 2026 for 155 zones, January 2027 for 162 and February 2027 for 190, with Java appearing under both January and February.
Duration is forecast shorter than normal in 482 zones, 51.90 percent, and the character normal in 510 zones with 147 wetter than normal. The agency's own advice in that release is to optimise drainage and evacuation readiness against flood and landslide at the start and peak of the wet season.
What flooding in Greater Jakarta looks like in the records
The disaster agency's report on the early March 2025 Jabodetabek floods gives the clearest recent picture. In Bekasi regency, 18 villages across 10 districts with 13,704 households, 51,320 people, affected and water not yet receded. In Bekasi city, 25 urban villages across 12 districts with 18,738 households, 61,233 people, affected, 47 households displaced, and power still out in flooded areas under a declared local emergency. In Jakarta itself, 770 households affected and 313 displaced.
The most recent event on record is 28 to 29 January 2026: eight districts of Bekasi city affected with 80 people displaced and water still at 150 cm on the 29th, and in West Jakarta the Ciliwung and Pesanggrahan rivers overtopping, affecting 446 houses across six urban villages with water 30 to 150 cm.
For historical scale, a February 2020 report recorded 23 Jakarta districts flooded with water 10 to 170 cm in the east and 891 households affected. A companion guide sets out what stops a ferry sailing.
The rainfall numbers behind it
The agency's daily intensity scale runs light at 0.5 to under 20 mm, moderate at 20 to under 50, heavy at 50 to under 100, very heavy at 100 to 150, and extreme above 150 mm per day.
For the Jakarta flooding of 18 to 19 February 2021 it recorded the highest daily rainfall at Pasar Minggu at 226 mm, with Sunter Hulu 197 mm, Halim 176 mm and Lebak Bulus 154 mm, having warned beforehand of 100 to 150 mm. Its deputy noted that this was still lower than the January 2020 rainfall that also flooded the region, without giving a figure for it.
On mechanism, the agency lists three contributors to Jakarta flooding: rain falling around the wider metropolitan area that drains into Jakarta, rain falling on Jakarta itself, and high tide, with environmental carrying capacity also a factor.
When in the day it rains
This is worth getting right because the two patterns are different. For the onset and transition period the agency describes hot middays with uneven rain in the late afternoon or evening, driven by local convective activity and strong daytime heating.
For Jakarta's peak season it has said the opposite: rain generally occurs from night into the small hours and continues to morning, and described those as the critical hours to watch. Both statements are official and no single page reconciles them, so the safe reading is that afternoon storms characterise the shoulder months and overnight rain the deep wet season.
What to check, and where
The disaster agency publishes a kecamatan-level flood potential forecast at three levels. For the first ten days of January 2026 it put no Jakarta district at high risk but all five Jakarta cities at medium, along with twelve districts of Bekasi city and a long list in Bekasi regency.
For live conditions, Jakarta's provincial disaster agency publishes an inundation monitor showing areas flooded above 30 centimetres, and hourly river levels at the main floodgates including Katulampa, Depok and Manggarai. The weather agency publishes nowcast warnings defined as covering the next zero to six hours, with start and end times per province, plus a three-day extreme weather outlook graded waspada for moderate to heavy rain and siaga for heavy to very heavy. Jakarta's emergency number is 112. Our guide to Batavia's canal system covers that separately.
What this guide does not cover
No forecast is offered here; the agency's own current bulletins are the only source for a given date. Typical calendar onset months for the Asian and Australian monsoons are not given, because the agency declares monsoon activity per dasarian from monitoring rather than by fixed dates. The rainfall thresholds behind the third warning tier, awas, could not be verified and are omitted. The January 2020 Jakarta daily rainfall total is unverified. No commuter rail flood suspension is described: the operator's own pages were blocked and only article titles were visible. The national disaster database was unreachable, so no aggregate flood statistics appear. Jakarta's dedicated flood portal was not reachable on the verification date.