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Seasonal Forecasts

The seasonal range forecast provides a broad overview of the atmospheric evolution for s 7-month or 12-month forecast period.    It is not just an extension of the medium-range or sub-seasonal forecasts but is a completely separate system.  

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 (extended to month 13 every quarter).
    • is based on an older version of IFS and therefore uses slightly different model physics.
    • 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.
    • he products are available on the 5th of each month.
    • the dissemination schedule:

Section 8.3 Seasonal range output gives a description of sub-seasonal forecast products.

The perturbed members are similar to the unperturbed control forecast member but differ only that initial states and model physics have been perturbed.   This allows exploration of the currently understood range of uncertainty in the observations and the model forecasts. 

A set of sub-seasonal range re-forecasts provide a basis for the seasonal range model climate (S-M-climate).  

Standard output products focus mainly on broad monthly weather conditions.   Chart products generally are presented in terms of anomalies relative to the sub-seasonal range model climate (SUBS-M-climate).  The anomalies are mainly shown as 7-day means for calendar weeks Monday to Sunday.   From these one can infer if weather conditions (e.g. temperature, rainfall, etc) are likely to deviate from the average for a location and time of year, and whether such an anomaly could be extreme.

Sub-seasonal range forecasts are coupled to the Wave model (ECWAM) with resolution of 36km and the Dynamic Ocean model (NEMO) run with resolution of ~28km.

Information of available seasonal range ensemble forecast output is given in greater detail.  Some are additional products are only available by arrangement.

The seasonal model is based on an older version of IFS and therefore uses slightly different model physics.  Standard output products focus mainly on the month-to-month trends in the weather.  Chart products 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 weather 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 seasonal 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). Nino plumes and Climagrams (a type of meteogram) give a month-by-month picture of the probability of appearance of these patterns or indicators, 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.

Section 8.3 Seasonal range output gives a description of sub-seasonal forecast products.


 Additional Sources of Information

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


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