Long-range (Seasonal) Forecasts

Other names: System5 Forecast, SEAS5

The long-range forecast provides a broad overview of the atmospheric evolution for the 7-month or 12-month forecast period.  

The seasonal range ensemble:

    • is run once monthly, base time 00UTC on 1st of each month and run on the 5th of each month covering the forecast period  Month 0 to month 7 month forecast (extended to month 13 every quarter).
    • has horizontal resolution of 36 Km and vertical resolution of 91 model levels.
    • has land-atmosphere and ocean-atmosphere coupling.
    • has 50 ensemble members plus an unperturbed control member.
    • the dissemination schedule is given in Section 3.1:

The long-range forecast is a land and sea coupled.  It is based on an older version of the IFS and that therefore uses slightly different model physics.  It runs at 36km resolution, similar to the sub-seasonal range ENS but lower than the resolution typical of shorter range forecasts.  The chart products generally are presented in terms of anomalies relative to the seasonal model climate (S-M-climate) and are mainly shown as 3 month mean anomalies and probability of greater or lesser deviations.   From these one can infer whether conditions (e.g. temperature, rainfall, etc.) are likely to deviate from the average for the location and time of year, and whether such an anomaly could be extreme.

An important aspect of long-range forecasts is the identification of broad scale indicators or indices that represent important aspects of the global weather patterns (e.g. El Niño Southern Oscillation (ENSO) index). 

Climagrams (a type of meteogram) give a month-by-month picture of the probability of appearance of these patterns, and of their possible amplitude.  The typical skill level of seasonal forecasts, as assessed using re-forecasts, is generally much lower than that of short range forecasts.  Skill levels are typically lower in the extra-tropics than in the tropics. Indeed in some regions, for some forecast variables, skill levels are vanishingly small. Users should familiarise themselves with verification figures from the outset to avoid over-interpretation of and over-reliance on the seasonal forecast charts.

Long-range (Seasonal) Products

Guidance on the Interpretation of the seasonal products is discussed in Section 8.3 Long Range Output.



 

Additional Sources of Information

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


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