Page History

...

The ensemble median is is more suited to parameters like wind speeds and precipitation because these usually have skewed distributions.  

Ensemble spread

The ensemble spread is a measure of the difference between the members and is represented by the standard deviation (Std) with respect to the ensemble mean (EM).  Theoretically, on average, small spread indicates high forecast accuracy, large spread indicates low forecast accuracy.   The ensemble spread should reflect the diversity of all possible outcomes.  

The ensemble spread is flow-dependent and varies for different parameters.  It usually increases with the forecast range, but there can be cases when the spread is larger at shorter forecast lead times than at longer.  This might happen when the first days are characterized by strong synoptic systems with complex structures but are followed by large-scale fair weather high-pressure systems.

Two similar-looking forecast charts may display large differences in geopotential if they contain systems with strong gradients that are slightly out of phase.  Conversely, two synoptically rather different forecast charts will display small differences if the gradients are weak.  The spread refers to the uncertainty of the values of mean sea level pressure, geopotential height, wind or temperature, but not necessarily to the flow patterns.  Also “jumpy”, deterministic forecasts might indicate that very different weather developments are possible.   Such aspects are reflected in charts of ensemble spread.   

Relationship of ensemble mean and ensemble spread against forecast lead-time

...