Ongoing research project |  -

Sub-seasonal to seasonal (S2S) prediction

A WWRP/THORPEX-WCRP joint research project

2m temperature anomalies for days 19-25 for forecast start date 11 June 2015 for different model centres and the verifying analysis   

Multi-model comparisons. A possible use of the S2S database is to make comparisons between the outputs of different forecasting centres. The image shows forecasts of 2-metre temperature anomalies from five S2S models and a verification panel based on observations. The forecast starting date is 11 June 2015 and the forecast range is days 19-25.


  • To improve forecast skill and understanding on the sub-seasonal to seasonal timescale with special emphasis on high-impact weather events
  • To promote the initiative's uptake by operational centres and exploitation by the application community
  • To capitalize on the expertise of the weather and research communities to address issues of importance to the Global Framework for Climate Services

The project will pay specific attention to the risk of extreme weather, including tropical cyclones, droughts, floods, heat waves and the waxing and waning of monsoon precipitation.

To achieve many of these goals, an extensive database of sub-seasonal (up to 60 days) forecasts and reforecasts (sometimes known as hindcasts) has been established, modelled in part on the THORPEX Interactive Grand Global Ensemble (TIGGE) database for medium-range forecasts (up to 15 days) and the Climate-System Historical Forecast project (CHFP) for seasonal forecasts.

Developing an extensive database for the sub-seasonal time scale is a challenging task, since consensus still needs to be reached on how to produce these forecasts (start dates, length of the forecasts, averaging periods, update frequency of the forecasts). For short- and medium-range forecasts, model error is not usually so dominant that a re-forecast set is needed, but for the sub-seasonal to seasonal range, model error is too large to be ignored. Therefore, an extensive re-forecast set spanning several years is needed to calculate model bias, which in some cases can also be used to evaluate skill. The S2S database contains ensemble forecasts and re-forecasts from 11 operational centres, which have been made available for scientific research via data archive portals at ECMWFCMA and IRI.

ECMWF’s role in S2S

  • ECMWF provides the extended-range forecasts to the S2S database
  • ECMWF hosts the S2S database

More information

This first phase of the project (S2S Phase 1) started in November 2013 and ran for 5 years. The project has then been extended for another 5 years (S2S Phase 2, 2019-2023). A project office has been established at the APEC Climate Center (APCC), Busan, Republic of Korea.

Although the project finished by the end of 2023,  another 5 year period (2024-2028) of the S2S database regular updates and support at ECMWF (and CMA) have been agreed and is currently underway.