Thunderstorms like popcorn: why forecasts stay uncertain

A bulletin giving a sixty or seventy per cent chance of rain is not a piece of bureaucratic caution. It is the correct way to describe a phenomenon that models can only capture in terms of probability. So says Gaudenz Flury, meteorologist and new head of forecasting at SRF.

To make the point, Flury uses the image of popcorn in a pan. You know the kernels will pop and you know roughly when the process will start. "But you don't know exactly which kernel will pop, or when," he says. Summer storm cells work the same way: the unstable air mass covers a whole region, while an individual storm forms over a few square kilometres and lasts a few dozen minutes.

That explains the difference between two neighbouring valleys, one receiving thirty millimetres of rain in half an hour while the other stays dry. The forecast is correct in both cases, because it gave the probability of the event, not its exact location.

The comparison with 2003

Flury also dwells on the comparison with the summer of 2003, the one that was called the summer of the century. In Basel the thermometer reached 39.7 degrees. "What is incredible is that only 23 years after what was called the summer of the century, we already have to ask again whether this summer is even more extreme," he notes.

The comparison applies above all to the northern side of the Alps, where a persistent high-pressure system and the absence of Atlantic disturbances produced a long run of hot days. On the southern side the dynamic is different: southerly flows bring moisture from the Mediterranean and evening storms are more frequent, but they often fail to make up the accumulated water deficit.

Two summers described as exceptional occurring so close together is the fact climatologists most often use to explain a shift in the mean. When the central value of the distribution changes, what used to be a rare extreme becomes an ordinary event, and the vocabulary of records stops working.