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A comprehensive set of post-processed ensemble products use a feature-based approach to represent objectively the location and behaviour of near-surface, synoptic-scale features typically associated with adverse weather (eg fronts, frontal waves, cyclonic features).  Co-location masking, a feature-type hierarchy and a minimum separation threshold, are all used together to help keep all cyclonic features 300km or more apart.  Mean sea level pressure, estimated from 1000hPa geopotential height and temperature, is shown as a reference point on many charts.  A tracking algorithm is used to follow the cyclonic features as they evolve in each ensemble member.  As a severe weather event approaches, the products can:

• indicate an increasing risk of a major storm in the area of interest,
• highlight the track that the storm is likely to take,
• suggest the degree of confidence that can be placed in that track (see Fig8.1.15-8).
  
  

Fig8.1.15-1: To view extratropical cyclone forecasts:

1. On charts page, click on extratropical cyclones.
2. On extratropical cyclones page select the extratropical cyclones diagram.
3. Select nominal data time of the forecast of interest.
4. Select feature of interest - in this case a barotropic low (black).
5. Display of product (in this case ensemble member tracks, 1km wind speeds, 300hPa wind speeds, central pressures and vorticity).
6. Step to next or previous nominal data time of interest.
7. Select alternative presentation of forecast features (e.g.fronts, dalmation dalmatian charts etc) from drop-down menu.
   
   

Example Charts

The extratropical 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.

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.

Fig8.1.15-5: An example of a "Dalmation 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.

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.