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Long Range Output - Seasonal

Long-Range (Seasonal) Structure

The latest ECMWF seasonal forecast system, SEAS5, has its own detailed SEAS5 user guide.     Here we discuss a few of the products and some of the verification material that are available for forecasters and briefly discuss some ways in which they might best be used.

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Re-forecasts provide a Seasonal model climate (S-M-climate) against which to compare the actual forecasts.  The re-forecasts for a given system (now SEAS5) are all created before the System comes into operation.  In this sense the re-forecast strategy differs from the update-in-real-time approach used for shorter ranges (ER-M-climate and M-climate).  The distributions of the seasonal ensemble forecasts can be compared with the corresponding S-M-Climate distributions to give information regarding forecast anomalies.

Availability

Seasonal forecast output is available:

• at 12UTC on 5th of the months Jan, Mar, Apr, Jun, Jul, Sep, Oct, Dec for forecasts of months 1-7.
• at 12UTC on 5th of the months Feb, May, Aug, Nov for forecasts of months 1-13.

Forecast ranges & resolution

Graphical Products

For each point of the chart the atmospheric variables (e.g. 2m temperature, surface temperature, total precipitation, mean sea level pressure) are averaged values over month-long periods.  

Use of the Long-Range (Seasonal) Output

Seasonal forecasting is the attempt to provide useful information about the "climate" that can be expected in the coming months, or a "statistical summary" of the weather events possible in a given season.  It is not and can never be a weather forecast, though long-range forecasts have proved very useful for El Niño predictions.  Seasonal predictions, particularly over mid-latitudes, should never be used without some reference to the expected forecast skill, which includes the reliability (i.e. typically there is over-confidence) in the forecasts, and the ability of the forecast system to discriminate when certain categories of event are more likely.  The forecasts themselves and the supporting verification information are all available as web-based products.

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

• More information on products available from the atmospheric model ensemble, long range forecast (Set V - SEAS) are available on the ECMNWF website.
• ECMWF SEAS5 User Guide
• ECMWF Seasonal Forecast Graphical Products (Forecast products and Verification measures).

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