How do 14-day forecasts work?

Question

I can see trends over the next day or two being predictable based on current weather… but how is it done for 14 days out?

In: 14

Answer 1

Badly.

The weather is a chaotic system, which means that unavoidable small errors we make in setting up our forecasting computer models — for instance not having observations at every single point on the entire globe — will get worse and worse over time, until our simulation is totally different than the real world We can minimize these errors by using very good models and observations, but that just pushes the problem down the road. You can’t beat exponential growth.

These graphs show some estimates of forecasting skill from modern weather simulations as a function of how many days into the future we’re trying to forecast. Two, three, four days into the future they’re great, but they get worse real quick. The horizontal line on the graphs is a rough estimate of when the computer simulation is no better than a random guess.

[https://www.researchgate.net/publication/321808794/figure/fig3/AS:633185247428610@1527974535940/HSS-values-based-on-ECMWF-reforecast-predictions-of-anomalous-AR-activity-as-a-function_W640.jpg](https://www.researchgate.net/publication/321808794/figure/fig3/AS:633185247428610@1527974535940/HSS-values-based-on-ECMWF-reforecast-predictions-of-anomalous-AR-activity-as-a-function_W640.jpg) [https://www.ecmwf.int/sites/default/files/AR2016-Delivering-regime-transition.jpg](https://www.ecmwf.int/sites/default/files/AR2016-Delivering-regime-transition.jpg)

So you can see that 14 days is about the limit. The forecast is right a bit more often than it’s wrong, so some people want to see it, but you definitely can’t count on it being at all accurate.

It’s also interesting how this has changed over time, as our weather observing network and computer simulations have improved. This graph shows the change in forecasting skill year after year:

[https://img.apmcdn.org/75e700b8a14453156922534a92480e00f7735f94/uncropped/3b27a9-20200102-european-model-ecmwf-forecast-accuracy-since-1980.jpg](https://img.apmcdn.org/75e700b8a14453156922534a92480e00f7735f94/uncropped/3b27a9-20200102-european-model-ecmwf-forecast-accuracy-since-1980.jpg)

You can see that despite huge improvements in technology, the gains are noticeable but not huge. Today’s forecasts 10 days into the future are about as good as the 7-day forecasts we were making in the 1980s, and our 5-day forecasts are a bit better than the 3-day forecasts from the 1980s.

Oh, and also, these graphs just show the predictability of the large-scale wind and pressure patterns. Predicting whether it’s going to rain at a specific spot at a specific time — which is what humans mostly care about — is even harder.

For more on chaos and how it affects future predictability, check this out: [https://www.aps.org/publications/apsnews/200301/history.cfm](https://www.aps.org/publications/apsnews/200301/history.cfm)