The Rising Accuracy of Weather Forecasts
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 above. 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.
Why have weather forecasts improved?
A few key developments explain these improvements.
The first big change is that the data has improved. More extensive and higher-resolution observations can be used as inputs into the weather models. This is because we have more and better satellite data, and because land-based stations are covering many more areas around the globe, and at a higher density. The precision of these instruments has improved, too.
These observations are then fed into numerical prediction models to forecast the weather. That brings us to the next two developments. The computers on which these models are run have gotten much faster. Faster speeds are crucial: the Met Office now chunks the world into grids of smaller and smaller squares. While they once modeled the world in 90-kilometer-wide squares, they are now down to a grid of 1.5-kilometer squares. That means many more calculations need to be run to get this high-resolution map. The methods to turn the observations into model outputs have also improved. We’ve gone from very simple visions of the world to methods that can capture the complexity of these systems in detail.
The final crucial factor is how these forecasts are communicated. Not long ago, you could only get daily updates in the daily newspaper. With the rise of radio and TV, you could get a few notices per day. Now, we can get minute-by-minute updates online or on our smartphones.
Dataset
Accuracy (1981, Northern Hemisphere) | Accuracy (2018) | |
---|---|---|
3-day forecast | 86% | 98% |
5-day forecast | 67% | 92% |
7-day forecast | 46% | 77% |
10-day forecast | 23% | 49% |