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.
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:
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.
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:
Extended range, sub-seasonal forecasts cannot be expected to accurately represent day-to-day weather variations. However, they should be able to capture large-scale circulation patterns that typically last longer than about a week, and also be able to roughly indicate the timing of a change from one circulation type to another.
High confidence in forecasts of a circulation pattern over the fairly large region may not imply high confidence in forecasts of the associated surface weather in all parts of the area. Factors that need to be considered are:
The extended range products should always be used with historical skill metrics in mind, as provided by verification probabilistic scores from the re-forecasts.
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.