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Extended Range Output

The output is rather different from the shorter period output of medium range ensemble products.  It provides a more general overview of the forecast up to Day46. It consists of an ensemble of 100 members plus one control member and is run every day based upon 00UTC data.

Output focuses mainly on the week-to-week changes in the weather rather than attempting to give unsupportable detail on individual days.  Output is mostly in the form of anomalies relative to ER-M-climate and is mainly shown as 7-day means for calendar weeks Monday-Sunday.   Specialised products for the extended range also include information on potential tropical cyclone activity and the Madden-Julian Oscillation (MJO) during the coming month.

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Graphical Products

For each point of the chart the atmospheric variables (e.g. 2m temperature, surface temperature, total precipitation, mean sea level pressure) have been averaged over week-long periods.  Extended range 7-day mean forecast chart products to day46.  

Correct use of charts - the difference between significance and confidence

The "anomaly", as shown on some of the extended range ensemble products, is the difference between the ensemble weekly mean of the real-time forecast and the corresponding ensemble weekly mean of the ER-M-climate.  On web charts, regions where the significance of this anomaly (difference) is:

• 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% (taken as not significantly different from ER-M-climate) are left blank.

On extended range web charts the confidence is "over-stated", because reliability diagrams consistently show that forecasts are over-confident.  Thus where a high probability of a particular type of event is predicted, the "true" chances of that event happening are actually less than the probability that is shown. 

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).  An explanation of reliability 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.

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.  For example if all ensemble members showed 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.

extend to day46.  

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

• View the ECMWF eLearning module on ECMWF Extended Range ForecastsCurrently Nil.

_{Updated/Amended 24/10/20 - amended chart links to open access.}