Discussed in the following Daily reports:
Material from: Linus
Discussed in the following Daily reports: |
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. 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.
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).
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.
Observations and analysis for the event
Many stations in south-eastern Thailand received more than 200 mm (purple) over this 3-day period. Also note the grey dot (>350mm) in the Dawei district in Myanmar.
Control forecast (IFS 9-km resolution)
IFS control had a reasonable signal for Thailand from 21 September and onwards, but missed the extreme around Bangkok in the shortest forecast from 24 September.
DestinE (IFS 4.4km resolution)
AIFS-single (AIFSv1.0 ~0.25 resolution)
Compared to to IFS control, AIFS-single had a more consistent signal both for the Bangkok area and especially for Dawei.
AIFS-ENS control (~0.25 degree resolution)
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. In forecasts from 19-21 September the signal was east of Bangkok and clearly missed Dawei.
Forecast Evolution plot
The plot below shows the evolution of the forecasts for 3-day precipitation (24-26 September) for Bangkok.
Theis was a consistently stronger signal in AIFS-single compared to the other forecasts (both IFS and AIFS-ENS mean). For Dawei the early signal in AIFS-single is even for striking.
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).
The plots below show verification of weekly mean precipitation for the week 21-27 September from the IFS sub seasonal forecasts. Some signal in forecasts from 9 September for a wetter than normal week for the Bay of Bengal, that might relate to this event.
