# Forecasting more than just weather

“Today we can expect a 50% chance of rain…” How many times have you heard these words on the TV weather forecast? Have you ever wondered why weather-people talk about percentages? Rain, winds, temperatures… all these phenomena come with their number attached: the chance they might occur. This is the result of complex mathematical formulas of the physics behind the meteorological processes, inserted in computational models that forecasters use to predict the way weather will behave.

Since all weather forecasts models are chaotic, tiny variations on the parameters on those models lead to different results of the forecasts. These results imply different scenarios in the real life: it may rain cats and dogs, it may be a gentle rain or it may not even rain at all, but how likely is each option? In the case of rain, they combine two different factors: the confidence that it will rain someplace in the forecast area, and the percentage of that area that will receive rain if it rains. This is what meteorologist call probability of precipitation.

And, if this is important for your day-to-day forecast (so you know whether to take your umbrella or your sunglasses), imagine how important it is when we talk about extreme weather-related disasters and how to prevent them. Emergency responders and decision makers need to have at their tables all the different possible scenarios and know how likely is each one of them to happen, so they can take the best possible decisions. That is why at I-REACT we are including weather-related data and models into our I-REACTOR, the system that will integrate this information altogether with satellite and UAVs images, crowdsourced information and many other data sources and technologies, to provide detailed disaster risk maps for Europe.

Forecasting extreme events (like high levels of precipitations or strong winds) is key for preventing disasters. And to do this, special forecasts, different from the weather forecasts you see on TV, must be designed. Our colleagues at the Finnish Meteorological Institute are in charge of providing the extreme weather-related data. This means that they feed different extreme weather scenarios to the system, each one of them accompanied by a number: the chance that that particular scenario may happen. By doing so, FMI is able to provide different thresholds for risks: a probability that may seem tiny for normal events can be of huge importance when associated with extreme weather events.

Instead of delivering a unique weather prediction, FMI runs several simulations with slightly different initial conditions, so we can know the different scenarios and know how likely is each one of them to happen. This is called the Ensemble method. To calculate these different scenarios, FMI uses complex numerical models that run on supercomputers. The accuracy of those models depends highly on the initial conditions: the starting points of the simulation, consisting on real data taken from satellite images, meteorological stations and other sources.

To provide the most reliable results, FMI is feeding their models the best available data at the moment: high resolution maps gathered from weather systems across Europe, with a resolution down to 7 kilometres on a European scale, and a 3 Km resolution on a national scale. a much higher resolution in comparison with the usual map in you TV weather forecast which resolutions usually goes down to 20 km.

By combining better resolution maps and more accurate probabilities for extreme events, I-REACT will be of great help in saving lives thanks to cutting-edge technological advances. Against disasters, we have a fighting chance. And now, we are better at calculating these chances.

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