Page History

...

• greater than 99% are delimited by a dashed contour (blue: wetter, colder or lower pressure; red: drier, warmer or higher pressure).
• greater than 90% are coloured (blue: wetter, colder or lower pressure; red: drier, warmer or higher pressure).
• less than 90% are left blank (taken as not significantly different from ER-M-climate).

A particular consideration when using the precipitation charts is that the range of colours available for "below average" anomalies (red colours) is constrained by the local climatology in the ER-M-climate.  Even if all ensemble members forecast a dry week, the mean (-ve) anomaly could be no larger in magnitude than the mean in the ER-M-climate.  So in some locations the strongest dry signal one can ever see will only be in the first or the second of the red shades.

Over-statement of confidence

...

The extended range products should always be used with historical skill metrics in mind, as provided by verification probabilistic scores from the re-forecasts.

Some broad indications of ensemble performance and predictability in the extended range should be taken into account when considering the extended range output.  Users are strongly encouraged to consult verification information - e.g. monthly forecast reliability diagrams (for 2m temperature, surface temperature, total precipitation, mean sea level pressure) or monthly forecast relative operating characteristics (ROC) diagrams (for 2m temperature, surface temperature, total precipitation, mean sea level pressure).

Anomaly charts (500hPa N Hem anomaly and Global and Regional anomaly) show how past extended range forecasts performed in the lead up to a particular week.  The anomaly of one week of forecasts (relative to the ER-M-climate) is compared with the weekly mean anomalies (relative to the ER-M-climate) of recent extended range forecasts verifying over the same period.   They give information about the consistency between the monthly forecasts from one week to another.

An explanation of reliability diagrams and ROC diagrams is given in the annex to this guide).  Users can also reference metrics that describe how skill tends to vary in different scenarios - see General use of Extended Range Output section.