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Extended range products provide only a general overview of forecasts out to Day46; they focus mainly on the week-to-week changes.  They give a broad-brush outline of likely weather rather than day-to-day definition, and no confidence should be placed on any smaller scale detail that might be indicated, particularly at longer forecast ranges on a single forecast run.  Only if there is a consistent signal of an identified feature from one forecast run to the next may any cautious reliance be placed on the indicated change.  Equally the amplitude of a feature will not necessarily increase with each successive extended range forecast; it may become less evident with time.  A broad trend may sometimes be evident providing useful information on general conditions (e.g. a gradual increase in temperatures or a lessening of rainfall).  However the user should not be tempted to extrapolate such trends unless they have very good reasons so to do.  This will only be the case in exceptional circumstances.

Anomaly of forecast from model climatology

Some extended range ensemble products on Opencharts show the forecast as an anomaly.  This is the difference between the ensemble weekly mean of the real-time forecast and the corresponding ensemble weekly mean of the ER-M-climate.  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% are left blank (taken as not significantly different from ER-M-climate).

Over-statement of confidence

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. 

Circulation patterns

A major goal of Extended Range forecasting is prediction, well in advance, of persistent, anomalous large scale circulation patterns that themselves can lead to severe weather events:

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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).

The 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.