Computer models which meteorologists use to forecast the weather are made up of complex mathematical equations.
Different models, and therefore different equations, determine the forecast period. The further out a computer model runs, the less dependable it becomes (due to the "chaos theory").
We always emphasize that long-term models should be used primarily for the trends they exhibit, not specifics.
They may give us some insight on global patterns or trends that may be developing, like the patterns and trends of El Niño and La Niña years.
We always, always take long-term forecasting with a "grain of salt".