Page History

Fig8.1.9.8A(top): Forecast tracks of frontal wave (arrowed in Fig8.1.9.7) from ensemble members.  

Fig8.1.9.8B(left): Forecast central pressure (hPa) of each depression developing from the frontal wave as identified by the ensemble members.

Fig8.1.9.8C(right): Forecast wind strengths (kn) at 1km altitude within 300km of each depression developing from the frontal wave as identified by the ensemble members.

The ensemble CNTL is shown by thin  green lines.  Most ensemble members forecast the track of the selected cyclonic feature to curve towards Britain before moving SE into northwest France.   Almost all ensemble members deepen the low, some to below 995hPa with winds at 1km altitude reaching more than 60kn and a few greater than 70kn.  The threat of severe weather is clearly shown but it is necessary to inspect the ensemble members, meteograms, EFI charts etc. to identify the associated risk.  DT 12UTC 04 March 2017.

 Fig8.1.9.9A(top): Ensemble Extreme Forecast Index (EFI) and Shift of Tails (SOT) charts for mean daily 10m wind speed (left) and M-climate for this (right) at 99th quantile (typically 1 in 100 occasions in the ensemble realises more than the values shown).  DT 00UTC 03 March 2017, T+48 to 72. 

 Fig8.1.9.9B(bottom): Ensemble Extreme Forecast Index (EFI) and Shift of Tails (SOT) charts for maximum 10m wind gusts (left) and M-climate for this (right) at 99th quantile (typically 1 in 100 occasions in the ensemble realises more than the values shown).  DT 00UTC 03 March 2017, T+48 to 72. 

 EFI exceeding 0.7 in much of France suggests unusual winds, and 0.8 in some places suggesting very unusual winds for those locations.  Meanwhile areas of non-zero SOT suggest a genuinely extreme event is possible. 

Considerations when dealing with small cyclones

The spatial resolution utilised in generating the extra-tropical cyclone charts (~50km) is rather larger than the resolution intrinsic to the ensemble, and is primarily applicable to monitoring mid-latitude depressions and their associated features. This is partly by design, in that we are trying to capture "synoptic scale features", and not every minor nuance in the model fields. Mid latitude depressions typically have a length scale of order 1000km and the program can extrapolate realistic central pressures from the surface pressure pattern.  As the feature is followed through the forecast period, feature specific plumes of central pressure, upper and lower altitude winds, and vorticity are plotted.  These are presented in the subsidiary diagrams.  However, resolution of the input data means a reduced capacity to correctly represent certain aspects (such as depth or maximum 1km winds) of small, deep vigorous cyclones, where the length scale is < ~200 km, say.  Nevertheless, the forecast positions of small vigorous centres are normally well captured.