Numerical weather and wave forecasts

The production of a numerical weather forecast can be divided into three steps:

  • Weather observations are collected and analysed in a three-dimensional grid. This defines the initial state of the atmosphere (temperature, humidity, wind etc.)
  • Future changes of this initial state are calculated step by step, using physical and dynamical laws
  • Some weather parameters, such as mean sea level pressure or wind and temperature near the surface, are calculated by post-processing of the final result and presented on a map

Such maps are often called “numerical forecasts”. But it must be kept in mind that such “forecasts” can be very different from the formal weather forecast which is the result of a forecaster’s work, based on the output from several numerical models, the latest weather observations, satellite images and other data. Numerical forecasts are only updated at regular time intervals (usually every 6 or 12 hours) while the forecaster keeps his forecast under continuous watch and issues amendments and warnings as necessary. Numerical forecasts should therefore always be checked against the official forecast and used mainly as a clarification and an addition to the official forecast rather than a replacement.

The main goal of the European Centre of Medium Range Weather Forecasts (ECMWF) is to produce reliable weather and wave forecasts in the “medium range”, up to 10 days ahead, twice a day. For this purpose it is very important to collect all available data from the entire globe for each run of the models. This takes a lot of time and up to 12 hours delay must be expected from the time of the observations until the final forecast products can be made available on the web. Verifications show clearly that the models used by ECMWF are the best in the world for medium range forecasts, but due to the long cut-off time some other models, which run more frequently for limited areas, might sometimes give better guidance for short range forecasts.