British weather still confounds forecasting supercomputers

<span>Photograph: Jonathan Brady/PA</span>
Photograph: Jonathan Brady/PA

In the heady days of 1991 when the Met Office’s first supercomputer was installed there were predictions of accurate UK weather forecasts – days, weeks and even months in advance. Gradually this optimism was eroded by the realisation that despite the millions of calculations the computers had the capacity to make, the outcomes still varied wildly depending on the data that was fed in.

Short-term forecasts definitely improved, but in 2009 the Met Office predicted in April that there would be a barbecue summer and had to apologise in July when large parts of the country were flooded.

The Met Office now says its minimum temperature forecasts are 86.5% accurate, plus or minus 2C, on the first night after they are made, and daytime maximum temperature is 95% accurate.

Three- to five-day forecasts are a different matter. Comparisons with what is predicted on the fifth day and what actually happens show calculations can be wildly out – for example the recent cold spell in the south-east was supposed to be a short-lived affair but hung on for an extra five days. Temperatures dropped to -8C with none of the predicted frost-free nights. So 30 years on the British weather can still confound the most powerful computers.