At this time, the World API does not store previous forecasts. At each Rapid Update Cycle, the previous forecast data is overwritten with the latest predictions.
This limitation exists due to the incredible volume of World API forecasts we generate - approximately 600 million each hour! It is simply not cost-effective to keep them nor to make such data available.
If you are interested in this topic because you would like to validate the forecasting data - we do provide running 7-day calculations of the Mean Absolute Percent Error (MAPE) and Root Mean Square (RMSE) for hour-ahead and day-ahead forecasts in the API Toolkit!
You can also interact with our forecast validation tool on our website at this link, although this particular feature is focused on large scale solar generation.