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Overview

Sub-seasonal 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 sub-seasonal 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.

Significance and confidence

Significance

The anomaly, as shown on some of the sub-seasonal 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, the 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).

Circulation patterns

A major goal of sub-seasonal 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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• 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.

Confidence

On sub-seasonal 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. 

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Users can also reference metrics that describe how skill tends to vary in different scenarios.

Verification

The sub-seasonal range products should always be used with historical skill metrics in mind.

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An explanation of reliability diagrams and ROC diagrams is given in the annex to this guide).

(FUG Associated with Cy49r1)