Weather forecasts with details of weather developments in individual places are currently, and for the foreseeable future, only possible a few days in advance. This is due to the chaotic nature of the atmosphere. The fact that the smallest, unpredictable perturbations can quickly spread means that weather forecasts are restricted to around ten days ahead, at the most. Forecasts for the upcoming season do not make predictions on individual weather occurrences or on the weather conditions of particular days. It is only possible to predict trends regarding average weather conditions. Forecasting in this way removes the chaotic processes from the equation, thus allowing a longer forecasting period. At the same time, however, a whole range of other factors take on a greater significance, such as the soil moisture content and snow cover of the continents, and, above all, the oceanic conditions. When these factors can be taken into consideration, it becomes possible to provide forecasts on weather trends over longer periods of time.
Model simulations on supercomputers
The long-range forecasts produced by MeteoSwiss are based on a coupled ocean-atmosphere-land model. This enables the development of conditions in the oceans and the atmosphere to be simulated with the help of complex comparisons. As in the process for monthly outlooks, all measurements from around the world available at the starting point are fed into the calculation to produce the seasonal outlook. To estimate the uncertainty level of the forecast, numerous such model simulations are carried out. This enables the probability distributions for possible climate conditions to be quantified. The final step is the calibration of the forecasts with past measurements. These comprehensive model simulations are conducted at the European Centre for Medium-Range Weather Forecasts (ECMWF). The ECMWF is jointly run by 34 countries, including Switzerland, to ensure optimum pooling of the member states' individual resources for this highly complex and costly work.