Status:Ongoing analysis Material from: Linus, Ahmed, Rebecca
1. Impact
A severe weather event that affected the Maldives on Sunday 31st December. A low pressure area formed west of the Maldives late 30th Dec and resulted in very heavy rainfall over the country, especially the central region and capital island Male'. Hulhule station (VRMM), Central Maldives, reported a total of 179.7mm within 24 hours for 31st December 2023. This broke the previous record of 175.9mm on 23 Dec 1977. The main city island Male' was completely flooded and was dewatered using pumps. National Disaster Management Authority reported 140 households were assisted during the flood response and 42 people from 8 households are currently in temporary shelters.
2. Description of the event
The evaluation will focus on the 1-day rainfall on 31 December inside a 1x1 degree box centred on 4N, 73E.
Satellite picture below showing the convective setup.
The timeseries below shows the observed rainfall over the main city (Male).
The measured amount between 0 and 12UTC on 31st was about 178mm.
3. Predictability
3.1 Data assimilation
3.2 HRES
The plots below show 24-hour precipitation (31 December 00UTC - 1 January 00UTC) in concatenated short forecasts (first plot) and ENS control forecasts with different lead times. The box marks 0.5x0.5 degree centred on Male'. While all forecast plotted here had some rainfall, it was first the shortest forecast (from 31 December 00UTC) that captured the severe rainfall.
3.3 ENS
The plots below shows EFI and SOT for 1-day precipitation (31 December 00UTC - 1 January 00UTC) from different initial dates. An early signal was present for having a wetter than normal day but it was only the shortest ensemble that indicated the extreme around Male.
The plot below shows the forecast evolution plot for 24-hour precipitation (31 ecember 00UTC - 1 January 00UTC) a 0.5x0.5 box centred on Male. Concatenated 6-hour forecasts - green dot, ENS control –red, ENS blue box-and-whisker, Model climate – cyan box-and-whisker. Ensemble mean as black diamonds. Triangle marks the maximum in the model climate based on 1800 forecasts. While the wetter-than normal signal is visible also here, the extreme was only captured in the last forecast.
3.4 Monthly forecasts
The plot below below shows the weekly mean precipitation anomaly for 25 December - 31 December. All lead-times here showed a wet anomaly around 10-15N, and the forecast from 16 Decmber had already a very strong anomaly.
Also the seasonal forecast from 1 November predicted a wet season for the region around the Maldives.
The plot below shows MJO forecasts from 31 December and 26 December. A strong MJO event was present in the western Indian Ocean around the day of the extreme.
The plot below shows tropical wave identification in the analysis OLR on 31st December, also indicating a large area of convection associated with the MJO (purple) and with a large Kelvin wave (grey & dashed), alongside total 24-hour precipitation.
The plot below shows the tropical wave identification in the extended-range control forecast produced 1 week ahead on 24th December, indicating the convection associated with all 3 wave types in the area. The HRES forecast did not contain any MJO signal at this lead time. Extended-range control forecasts produced before 20th December did not show any MJO signal in this area.
The plots below show the wave-filtered OLR MJO signal from the tropical wave identification, for the forecasts produced on 4th January (showing analysis fields for time of the event), and for 25th December (right)
3.5 Comparison with other centres
4. Experience from general performance/other cases
5. Good and bad aspects of the forecasts for the event
- Early signal for wet weather, probably related to the MJO
- The extreme around Male appeared only in the last forecast before the event