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 Status: Finalised Material from: Linus, Esti, Ivan


 


1. Impact

On 10 august the summer storm Johanne hit south-western Scandinavia with strong winds and rainfall. However, earlier forecasts predicted the storm to hit the Netherlands.

2. Description of the event


The series of plots below show analysis of MSLP and 6-hour precipitation (top) and Z500 and T850 (bottom) from 9 Aug 00z to 10 Aug 12z.


The plot below shows the maximum wind gusts in the Netherlands on 9 August.


3. Predictability

  

3.1 Data assimilation

 The plot below show the increment for the analysis 9 August 12UTC, which decreased the magnitude of the cyclone.


3.2 HRES

The plot below show observed 24-hour wind gusts (first plot) valid for 10 August and HRES forecasts for 24-hour maximum wind gusts (shade) and MSLP valid 10 August 12UTC (contour). The forecast from  8 August 12UTC gave much strong winds, and the next set of plots focus on 9 August.


The plot below show observed 24-hour wind gusts (first plot) valid for 9 August 12UTC to 10 August 12UTC and HRES forecasts for 24-hour maximum wind gusts (shade) and MSLP valid 10 August 00UTC (contour).



3.3 ENS

The plots below shows EFI and SOT for wind gusts valid 10 August.

The plots below show cyclone feature maps for 1 km maximum wind within 300 km radius, valid for 10 August 12UTC.



The plot below shows the ensemble evolution for 4-hour maximum wind gusts valid  10 August for a point on the sea between Denmark and Norway. Ensemble distribution (blue), HRES (red dot) and model climate distribution (red).



Regarding the false alarm for Holland, the plot below show the ensemble evolution (blue) and HRES (red dot) for 24-hour maximum wind gusts valid 12UTC 9 August to 12UTC 10 August, for a point on the Dutch coast near Leeuwarden. It is abvious that the most extreme forecast was 8 August 12UTC but also the jump to the last forecast is clear. The observed gusts were in the range 20-25 m/s.



3.4 Monthly forecasts


3.5 Comparison with other centres


4. Experience from general performance/other cases


5. Good and bad aspects of the forecasts for the event

  • False alarm for the windstorm to hit the Netherlands.
  • Big jump in signal from 6 to 7 August

6. Additional material