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Example Charts

The extra-tropical cyclone diagrams provide a comprehensive display of the variation between the forecasts of each member of the ensemble regarding positions of fronts, depth of depressions, and strength of winds at 1km altitude. In the examples the features near Brittany relate to an extreme windstorm which in terms of European losses was the major windstorm of the 2016-17 winter.  For interpretation of the charts see a guide to using cyclone database products.

Fig8.1.15-3: An example of a chart showing positions of fronts diagnosed from ensemble members (see legend below chart for details) illustrating the variation in positions.  DT 00UTC 03 March 2017, T+84 VT 12UTC 06 March 2017.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Fig8.1.15-4: An example of a "Dalmatian Plot" showing the centres of cyclonic features coloured to show an analysis of the cyclone class as derived from ensemble members (see legend below chart for details) showing the variation in forecast positions.  DT 00UTC 03 March 2017, T+84 VT 12UTC 06 March 2017.  Note that not all the spots denote genuine low pressure centres; it is only the barotropic lows (black spots) that are guaranteed to be.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Fig8.1.15-5: An example of a "Dalmatian Plot" showing the centres of cyclonic features, coloured to show an analysis of the forecast maximum wind strength, at 1km altitude, within 300km of each centre derived from ensemble members (see legend below chart for details).  Chart highlights show the variation in positions and intensity.  DT 00UTC 03 March 2017, T+84 VT 12UTC 06 March 2017.  Note several members suggest a maximum wind of 65-85kn in the vicinity of northwest France.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Fig8.1.15-6: An example of a chart showing the percentage of ensemble members predicting a cyclonic feature point will track within 300km in a 24-hour period T+72 to T+96 (i.e. 00UTC 06 March to 00UTC 07 March 2017).  DT 00UTC 03 March 2017, T+84 VT 12UTC 06 March 2017.  Only cyclonic features with a maximum wind speed exceeding 60kn at 1km altitude within 300km of the centre at some point in the 24h period are included.  A probability greater than 60% (darker orange) is shown over the western English Channel and NW France.  For a cyclonic feature moving west-to-east in this part of the world the strongest winds will ordinarily be found to the south of the low track.  This needs to be taken into account - indeed it is important for the user to not misinterpret the shading on these strike probability charts as being like a simple wind gust probability chart.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

Clicking on the frontal wave depression (orange) at the point shown by the red arrow in Fig8.1.15-7 will display the subsequent forecast movement and development as shown in Fig8.1.15-8 (albeit that plumes for some parameters have been omitted on this figure).

Fig8.1.15-8(top): Forecast tracks of frontal wave (arrowed in Fig8.1.15-G) from ensemble members.  

Fig8.1.15-8(left): Forecast central pressure (hPa) of each depression developing from the frontal wave as identified by the ensemble members.

Fig8.1.15-8(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.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

 Fig8.1.15-9: 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.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

 Fig8.1.15-10: 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.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.

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.

Note: HRES and Ensemble Control Forecast (ex-HRES) are scientifically, structurally and computationally identical.  With effect from Cy49r1, Ensemble Control Forecast (ex-HRES) output is equivalent to HRES output shown in the diagrams.   At the time of the diagrams, HRES had resolution of 9km and ensemble members had a resolution of 18km.