# One forecast says the storm misses. Run it 50 times. How many say it hits?

- Series: [The Better Guess](https://miu.fm/thebetterguess.md)
- Published: 2026-11-04
- Page: https://miu.fm/thebetterguess/one-forecast-says-the-storm-misses-run-it-50-times-how-many

## Story

One forecast says the storm misses. Run it 50 times. How many say it hits?

On 15 October 1987 the forecast for southern England said a storm would pass to the south, just skimming the coast. That night it crossed England, with a gust of 115 miles an hour at Shoreham and millions of trees down. Days earlier the models had shown a storm in the right place; on the day, the last runs sent it south.

One computer run gives one answer. But the starting weather is never measured perfectly, and a tiny nudge at the start can send a storm down another road. In 1974 Cecil Leith proposed running a forecast many times from slightly nudged starts, and in December 1992 Europe's weather centre began running forecasts in crowds of 33.

Our toy storm, its drift steered by Lorenz's chaotic equations, run 50 times: 12 runs cross England. Not "it misses", but about a one-in-four chance. One run is one guess; a crowd of runs tells you how sure to be. Forecasters call it an ensemble.

When the European centre re-ran the 1987 storm in 2003 with 51 runs, about one run in ten showed gusts over 29 metres a second over the Channel three and a half days ahead. The toy's 12 of 50 shows the method, not the 1987 odds.

So, how many did you guess?

Sources: Met Office, "Lessons and legacy of the Great Storm of 1987"; T. Jung, E. Klinker and S. Uppala, "Reanalysis and Reforecast of Three Major European Storms of the 20th Century Using the ECMWF Forecasting System", ECMWF, 2003; ECMWF, "30 years of ensemble forecasting at ECMWF", 2022; C. E. Leith, "Theoretical Skill of Monte Carlo Forecasts", Monthly Weather Review, 1974.
Sounds: our own, made in code (earshot, CC0).

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