Status: Finalised Material from: Linus, Esti, David L.
1. Impact
In the first days of October, storm Alex impacted large parts of western Europe. On the end of 1 October the cyclone rapidly developed and a sting jet feature formed and hit the coast of Brittany in France. The next day parts of south-eastern France and north-western Italy saw record rainfall and left at around 15 people dead. On the Floodlist link below you can see the impact.
http://floodlist.com/europe/france-italy-floods-storm-alex-october-2020
https://www.bbc.co.uk/news/world-europe-54402096
The storm also resulted in the wettest day on record for UK: https://www.bbc.co.uk/news/science-environment-54561601
2. Description of the event
The plots below show analyses of t850 and z500 from 30 September to 4 October, every 24h.
The plots below show analyses of MSLP and 6-hour forecasts of precipitation from 1 October 00UTC to 4 October 00UTC, every 12th hour.
Warnings on Meteoalarm for 2 October from the morning of 1 October.
The images below show satellite images from 1 October 12UTC to 2 October 00UTC, every 3rd hour to illustrate the development of the sting jet over Brittany.
3. Predictability
3.1 Data assimilation
3.2 HRES
The plots below show 24-hour maximum winds gusts (1 Oct 12UTC to 2 Oct 12UTC) from observations and HRES forecasts with 1 day apart (all 00UTC).
The plots below show 24-hour precipitation (2 Oct 06UTC to 3 Oct 06UTC) from observations and HRES forecasts with 1 day apart (all 00UTC).
3.3 ENS
The plots below show EFI for 24-hour maximum wind gusts on 1 October.
The plot below shows the forecast evolution for 24-hour maximum (1 Oct 12UTC - 2 Oct 12UTC) wind gusts in the box over Bretagne outlined in the EFI plots above. The plot includes ENS (blue), HRES (red dot), ENS control (purple dot), observation (green) and model climate (red).
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The plots below show EFI for 24-hour precipitation on 2 October.
The plot below shows EFI for the 1-day forecast for integrated water-vapor flux on 2 October. At this time there was a strong atmospheric river towards the south-western Alps.
The plot below shows the forecast evolution for 24-hour accumulated precipitation (2 Oct 06UTC - 3 Oct 06UTC) in the box over northern Italy outlined in the EFI plots above. The plot includes ENS (blue), HRES (red dot), ENS control (purple dot), observation (green) and model climate (red).
Below, Raw ENS (red) and ecPoint (blue) ensemble distribution comparison for the same location as the maximum precipitation in the 24h observations station and >100mm probability comparison (middel map raw ENS, right map ecPoint) for 36h lead time (top) and 42h lead time (bottom).
MISTRAL products (COSMO post-processed with a variable nearest neighbourhood technique blended with ecPoint-Rainfall 6h). 99th percentiles and probability >50mm in 6h from 15 UTC on 2nd October to 09 UTC 3rd October.
3.4 Monthly forecasts
The plots below show ensemble mean anomalies for precipitation 28 Sept to 4 October from extended-range forecasts with different initial times.
The plots below show the same as above but for surface pressure anomalies.
3.5 Comparison with other centres
4. Experience from general performance/other cases
5. Good and bad aspects of the forecasts for the event
6. Additional material
The plots correspond to the observations every 6 hours from 00-06z on 2 Oct (Fri) to 06-12z on 3 Oct (Sat). Last plot is the 24h precipitation observations from 12 UTC on 2nd October to 12 UTC of the 3rd October.
