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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

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•  that typically last longer than about a week

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• (circulation patterns or regimes are described in this guide).  
• roughly indicate the timing of a change or transition 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:

• 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.
• Teleconnections, particularly those resulting from the Madden-Julian oscillation (MJO), have 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.

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