Based on historical outturn data and detailed local wind forecasts, National Grid forecasts likely levels of wind generation for windfarms visible to National Grid, i.e. those that have operational metering and that are included in the latest forecast process. The forecasts are produced for the period from 21:00 on the current day (D) to 21:00 D+2.Wind Generation forecasts are produced by National Grid's own second generation windpower forecasting tool. The predictability of the wind varies with atmospheric conditions and so there may be periods where National Grid's forecast and outturn values differ significantly. Please note that the downloadable data will contain gaps for Original and Updated Forecast values in Settlement Periods that National Grid do not provide forecast values for.