A wind power forecast corresponds to an estimate of the expected production of one or more wind turbines (referred to as a wind farm) in the near future, up to a year. Forecast are usually expressed in terms of the available power of the wind farm, occasionally in units of energy, indicating the power production potential over a time interval. Time scales of forecasts Forecasting of the wind power generation may be considered at different time scales, depending on the intended application: very short-term forecasts (from seconds up to minutes) are used for the real-time turbine control and electrical grid management, as well as for market clearing; short-term forecasts (from 30 minutes up to hours) are used for dispatch planning, intelligent load shedding decisions; medium-term forecasts (from 6 hours up to a day) are used for to make decisions for switching the turbine on or off for safety or conditions on the market; long-term forecasts (from a day up to a week or even a year) are used for long term planning (to schedule the maintenance or unit commitment, optimize the cost of operation). Maintenance of offshore wind farms may be particularly costly, so optimal planning of maintenance operations is of particular importance. For the last two possibilities, the temporal resolution of wind power predictions ranges between 10 minutes and a few hours (depending on the forecast length). Improvements of wind power forecasting has focused on using more data as input to the models involved, and on providing uncertainty estimates along with the traditionally provided predictions. Reason for wind power forecasts In the electricity grid at any moment balance must be maintained between electricity consumption and generation – otherwise disturbances in power quality or supply may occur. Wind generation is a direct function of wind speed and, in contrast to conventional generation systems, is not easily dispatchable, so fluctuations of wind generation require power substitution from other sources that might not be available on a short notice (it takes 6 hours to fire up a coal plant and 12 hours for a nuclear one). The problem gets more complex once the wind power starts providing more than a small percentage of the overall electricity supplied to the grid. Better forecasts allow utilities to deploy less spinning reserves, usually natural gas-based generators. The forecasts are typically requested by utilities on two separate time scales: short-term (minutes to 6 hours) forecast is