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Material from: Linus


 


1. Overview

Between 24 and 26 September 2026, Bangkok experienced one of its most severe rainfall events in decades. A slow-moving low-pressure system along the monsoon trough produced nearly continuous heavy rain, with many districts receiving close to 300 mm in about 48 hours and some locations exceeding 300 mm. The deluge overwhelmed the city's drainage and canal network, causing widespread flooding, major transport disruption, and inundation of homes and businesses. Authorities declared all 50 districts disaster areas, opened hundreds of emergency shelters, and operated pumping stations at full capacity. Around the same time, the Dawei district in Myanmar was also hit by flooding. 

2. Analyses and Observations

2.1 Event Definition

The evaluation will focus on the 3-day precipitation (24-26 September) for a 0.5x0.5 degree box centred on Bangkok (13.7N, 100.6E).

2.2 Analyses

The plots below show analyses of MSLP and 6 hour rainfall from 22 September 00UTC to 27 September 00UTC, every 12th hour.

The plots below show z500 and T850 from 18 to 27 September 00UTC, every 24 hour.

2.3 Observations


2.4 Climatological perspective

3. Forecasts

3.1 Data assimilation and observation usage

 

3.2 Single Forecasts for the event based on 00UTC forecasts (defined in Section 2.1)

Observations and analysis for the event

Control forecast (IFS 9-km resolution)

DestinE (IFS 4.4km resolution)

AIFS-single (AIFSv1.0 ~0.25 resolution)

AIFS-ENS control (~0.25 degree resolution)

3.3 Ensemble distribution

EFI (based on IFS 9-km ensemble)

The plots below show the EFI for 3-day precipitation for 24-26 September, from different initial dates.

Forecast Evolution plot

The plot below shows the evolution of the forecasts for 3-day precipitation (24-26 September) for Bangkok.

The  plot below shows the forecast evolution for 3-day precipitation in the Dawei district in Myanmar (0..5x0.5 box centred on 14.0N, 98.2E).

3.4 Sub-seasonal forecasts

 

3.5 Comparison with other centres


4. Hazard Forecasts


5. Dedicated Experiments


6. Event Summary

6.1 Good and bad aspects of the forecasts for the event

  • Not much signal in EFI from the IFS ensemble beyond two days before the onset of the event.
  • More consistent prediction from AIFS-single, especially over Myanmar

6.2 Experience from general performance/other cases


7. Additional material

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