An atmospheric prediction model is a system of simulating the atmospheric physical processes. The atmosphere is simulated with a discrete number of grid points in specific levels, that begin from the ground, and reach the upper atmospheric layers. By this way, a three dimensional grid is produced, which is used for all the necessary mathematical calculations (Figure 1).

1. INTRODUCTION TO WEATHER PREDICTION MODELS An atmospheric prediction model is a system of simulating the atmospheric physical processes. The atmosphere is simulated with a discrete number of grid points in specific levels, that begin from the ground, and reach the upper atmospheric layers. By this way, a three dimensional grid is produced, which is used for all the necessary mathematical calculations (Figure 1). Figure 1: A three dimensional grid of a numerical weather prediction model The time is procceding with very small time steps of the order of few seconds. The continuously repetition of estimations in each time step leads to the weather prediction for next day or next week. The denser the grid the more realistic the atmospheric simulation by the models. The weather prediction models are divided in two categories:

2. Global models In those models the grid points extended to the whole earth and the equations are integrated in the three dimentional atmosphere (north and south hemisphere).

3. Limited Area Models (LAM) The limited area models are used for the prediction of the small scale disturbances. These models are based in almost the same equations as the global models . The main differences is that LAM are executed in a limited area, their grid is denser, and they are cabable of simulating accurately the small scale disturbances. Whereas in global models the equations are integrated in the whole earth, with a sparse grid. In limited area models the attention is focused in a small, specific area (eg Europe or Greece) and the grid is much denser. The limited area models are separated in two main categories: the hydrostatic and the non hydrostatic models. The non-hydrostatic models are used for the simulation of atmospheric procedures with a smaller scale. The execution of numerical weather models requires internationally the use of supercomputers for two main reasons: Weather prediction models are generally large code and requires an execution of a large amount of data. The results of operational prediction model has to be available in an operational time. The HNMS (Hellenic Meteorological Service) is a member of ECMWF (European Center for Range Weather Forecasts), so we use ECMWF`s products operationally. As well as the products of GM (Global Model) of the German Meteorological Service. The ECMWF and GM products are used as boundary conditions for our Limited Area Models.