English: The European Centre for Medium-Range Weather Forecasts (ECMWF) produces global numerical weather models. While national weather agencies use much higher-resolution processing to get local forecasts, these global models provide a crucial input into these systems.

The ECMWF publishes analyses of its errors over time. This is shown in the chart below.1 It shows the difference between the forecast and the actual weather outcome for forecasts 3, 5, 7, and 10 days in advance. The metric used here is the “500 hPa geopotential height”, a commonly used meteorological measure of air pressure (which dictates weather patterns).

The solid line is for the Northern Hemisphere, and the dashed line is for the Southern.

Three-day forecasts — shown in blue — have been pretty accurate since the 1980s, and have still gotten a lot better over time. Today the accuracy is around 97%.

The biggest improvements we’ve seen are for longer timeframes. By the early 2000s, 5-day forecasts were “highly accurate” and 7-day forecasts are reaching that threshold today. 10-day forecasts aren’t quite there yet but are getting better.