Status: Finalised Material from: Linus


 

1. Impact

In August 2020 Western Europe underwent a strong heatwave. For example, Paris measured above 35C and London Heathrow above 33C during 6 consecutive days (7-12 August). The heatwave later propagated to the north-west and affected Scandinavia. In the end of the heatwave, heavy thunderstorms and flash-floods affected north-western Europe and UK (e.g https://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-53778891 )

2. Description of the event

The plots below show analyses of T850 and z500 from 5 August to 12 August (all 00UTC).


The plot below shows a Howmoller diagram of v200 (left) and z500 (right) averaged between 35N-60N cover 1 July to 17 August. A Rossby wave packet started on around 27 August over Western Pacifici (~ 120E) and propagated eastward and reached the longitude of Europe (0E) around 6 August, in the time for the onset of the heatwave.


3. Predictability

  

3.1 Data assimilation

The plot below shows the observations statistics for 2-metre temperature observations from the land data assimilation, from the box 49N-51N, 2E-6E. The first guess forecasts seem to have missed the magnitude of the cooling during night.

 

3.2 HRES


3.3 ENS

The plots below show EFI for 5-day mean temperature valid for 7-11 August.

The plot below shows the forecast evolution for 5-day (7-11 August) temperature averaged inside the box in the EFI plots above (49N-51N,2E-6E), for ENS (blue box and whisker), HRES (red dot), model climate (red box and whisker) and the observations (green dot). The ensemble stated to pick up the signal for the anomaly around 28-30 July, and the ensemble median (black dot) was consistently above the 99th percentile of the model climate from 3 August and onwards.


3.4 Monthly forecasts

The plots below show 7-day anomalies of 2-metre temperature (shade) and winds (arrows) for the period 6-12 August (Thursday to Thursday), in 1,2 and 3 week forecasts. The gain of the signal for the heatwave happened between 23 and 30 July, as expected from the ensemble evolution plot above.


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