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. Combining the current and recent ensembles into a larger ensemble (as in Fig6.1.1.2-1) can show possible consistency in forecasts and identify any different evolution.
It is inappropriate to rely on the results of any individual ensemble member or ensemble run itself. But if there is some indication of extreme or hazardous weather the threat should be passed on to users, but with a very low probability. Nevertheless, there can be some agreement between the spread and jumpiness.
Fig6.1.1.2-1: Schematic illustration of the relation between the latest ensemble forecasts (green lines) and two previous Ensemble Control Forecast (ex-HRES) solutions (red lines) the several lagged previous ensemble forecasts (blue lines).
Some ensemble solutions from previous runs (blue) show a larger variation between regimes while the latest ensemble solutions (green) and the latest Ensemble Control Forecast (ex-HRES) solutions (red) all show less extreme variability. The latest solutions are rather more likely because they benefit from up-to-date information and observations, but the more variable or extreme solutions from earlier runs (blue) should not be ignored and the results may be retained as a small probability.
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