ECMWF data can be split into 3 main categories: analyses, instaneous forecasts and accumulated forecasts. Analyses are produced by combining short-range forecast data with observations to produce the best fit to both. The data are available a few times per day. Instantaneous forecast data are produced by the forecast model, starting from an analysis, and are available at various forecast steps (hours) from the analysis date/time. (Note, forecasts are not initiated from all analyses.) These data are relevant to a particular verifying date/time (analysis date/time plus step). Accumulated forecast parameters are accumulated from the beginning of the forecast. You can divide values by the length of the forecast step to calculate averages over the accumulation period. Some parameters are only analysed (eg. model bathymetry), some are only forecast (e.g. radiative fluxes) and some are both analysed and forecast (e.g. temperature, winds and pressure).