Taking account of previous ECMWF ENS solutions - Lagged Ensembles

An ensemble of NWP model runs starting from different data times is known as a lagged ensemble.   A combination of current and recent ensembles into a larger ensemble (as Fig6.1.1.2-1) can show consistency or divergence in forecasts.  They can also show alternative solutions.  Nevertheless, there can be some agreement between the spread and jumpiness. 

One shouldn't rely on the results of any individual ensemble member or ensemble run alone.  If there is any indication of extreme or hazardous weather in the ensembles then that threat should be passed to users, but with a very low probability.   

Fig6.1.1.2-1:  Schematic illustration of the relation between:

• several lagged previous ensemble forecasts (blue lines).  Some solutions show a large variation between regimes.
• the latest ensemble forecasts (green lines) show less extreme variability.
• two previous Ensemble Control Forecast (ex-HRES) solutions (red lines) show less extreme variability..

The latest solutions are rather more likely. This is because they benefit from up-to-date information and observations.  However, any solutions from earlier runs that show greater variability or are more extreme should not be ignored.  These results should be retained as a small probability.