Past observations are used to deduce significant correlations between the weather in the last three months (up to the beginning of the month) and the weather over the next season (from the end of the month). The main predictors are El Niño / La Niña and the trends due to global warming. Overfitting is avoided as much as possible. The system has been documented in Eden et al, 2015.
The development of this forecast system was supported by the EU-project SPECS.