How it works: from data to alert

1. We download the models to our own serverEvery 20 minutes we check for a new run of the ECMWF (the European centre: its classic IFS model, the AIFS artificial intelligence model and the 51 ENS scenarios), the American GFS, the German ICON-EU, the French ARPEGE and the British Met Office global model, all straight from their open data. They are downloaded once and served to everyone already processed, which is why the dashboard is fast. From the ECMWF we use all four daily runs (00, 06, 12 and 18 UTC), so the forecast is fresher in the afternoon.
2. We combine themWe calculate the consensus of the six models: the median, the range between the most optimistic and the most pessimistic, and how many exceed a threshold. If they agree, confidence is high; if they disagree, we tell you instead of hiding it.
3. We correct their quirksEvery day we compare what each model predicted with what the airport stations measured. That teaches us each model's bias in each place and how much weight it deserves. Near those stations, this correction has cut tomorrow's temperature error by 13.5 % compared with a plain average of the models.
4. We make our own forecast: the Atlas ForecastIn Catalonia we go further: we combine the six models with what the nearly 190 stations of the XEMA network (Meteocat) measure. Over months of data, the system has learned how wrong each model tends to be by time of day, season and place, and it corrects itself with the recent errors at nearby stations. Tested on stations it had never seen, it gets the temperature for the next 24 hours 17% more accurate than the median of the models (and 14% more than the best of them), and its chance of rain is 40% less wrong than the models'. It reaches 72 hours where it already beats the models, and it estimates the chance of snow and the snow level. Every morning we check what it said the day before: the card shows how far off it was near you yesterday. With the nearly 860 AEMET stations, it will switch on province by province across the rest of Spain as it proves more accurate than the models. Its accuracy, area by area and week by week, is on How accurate the Atlas Forecast is.
5. We cross-check with what is happeningRain radar and its extrapolation up to 90 minutes, the European MTG satellite and its lightning, stations, the official AEMET warnings for Spain's 233 areas, and what you tell us from where you are with “What's the weather like where you are?”.
6. We detect what matters and alert you straight awayCut-off lows, depressions, thunderstorms, heavy rain, gusts, heat or frost are detected automatically. Your alerts are checked with every new model run, and instant alerts every 5 minutes: rain less than an hour away (radar), lightning a few kilometres away (MTG satellite) and orange or red AEMET warnings. They reach your phone, Telegram if you link it, or your email.
7. We tell you, and we measure whether we were rightIn whichever view you choose, always with the source, the time and the uncertainty. And we keep score: the dashboard publishes each model's error against 32 airports over the last 30 days, and the Atlas Forecast's, area by area and day by day, on How accurate it is. And in the enthusiast view we tell you which model is most accurate in your area.

What it is not