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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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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.  

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• Whether the circulation applies to the whole region or whether a different flow is likely near the edge(s).  Confident winter-time forecast circulation can have a large spread in forecasts of 2m temperature due to atmospheric thermal gradients shifted away from their climatological positions.  
• Atmospheric dynamical processes (Rossby wave propagations, weather regimes, etc).
• Land Surface conditions: Snow cover, Soil moisture.
• Ocean conditions: Sea-surface temperature, Sea ice.
• Stratospheric initial conditions.
• The Madden-Julian oscillation (MJO) - has a significant impact on the forecast skill scores.

Verification

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.