How to read a 30-day forecast

Thursday, 15 October · Zefita

No forecast in the world knows exactly what the weather will be three weeks from now. The atmosphere is a chaotic system: a tiny error at the start grows fast. That is why the 30-day forecast on Zefita has three levels.

Days 1–3. High-resolution models (ICON, ECMWF) give the temperature with an average error of 1–2 °C and get rain right on about 85 % of days. You can plan a trip or outdoor work.

Days 4–10. Still useful for the trend: cooling, warming, a rainy spell. The exact hour of rain is no longer reliable. Check again the day before.

Days 11–30. Here we show a trend, not a forecast: the same model is run 31 times with small differences (an “ensemble”) and we show the average and the range most scenarios fall in. A narrow range means the scenarios agree; a wide one means high uncertainty. Use it to see whether the month will be warmer or colder than normal, not to pick a wedding day.

Every page shows the source, the model and the time of the last update. No value is modified by artificial intelligence.