The Met Office will soon be able to issue detailed town-by-town forecasts that show precisely where extreme rain will fall, scientists said yesterday.

By 2011, new computers will allow forecasters to predict the exact path of downpours such as those that flooded Tewkesbury and Gloucester, giving communities much more accurate warnings of the risk they face.

A planned £120 million upgrade of the Met Office’s supercomputers in Exeter should mean that meteorologists will be able to say with confidence how extreme rain will affect areas of about 2sq miles (5sq km) rather than the current 23sq miles.

This means that forecasters will be able to tell residents of particular towns whether they are likely to face severe rainfall, rather than issuing broader regional forecasts.

The improved processing power should also provide earlier warnings of severe weather. Confident regional forecasts are now available about three hours in advance – as happened during last week’s storms – but the new computers should be able to increase this to six or even nine hours.

Brian Golding, head of forecasting research at the Met Office, said: “The research is progressing well, and we have done enough for the experts to be confident of very significant improvements. We are looking two years ahead for the initial implementation, though it depends on the next supercomputer upgrade. We have to be cautious about immediate improvements, but we are probably looking at 2011 before we gain the full benefits.”

Robert Napier, chairman of the Met Office, said that the work would cost between £100 million and £120 million. Recent upgrades to the Met Office’s computer systems have already delivered significant improvements that allowed forecasters to predict the impact of last week’s storms a few hours before they struck.

Today’s systems, for example, would have predicted the Great Storm of 1987, famously dismissed on air by the BBC’s former weather forecaster, Michael Fish. Ewen McCallum, the Met Office’s chief meteorologist, said: “The twentieth anniversary of the Great Storm is coming up, and we have done reruns. We would certainly have predicted it now.”

He said that the error in 1987 owed much to the unambiguous way in which weather information was presented then. Some models had predicted the storm, but others had suggested that it would not hit, and a decision was taken to release that message to the public with little nuance. “Now we would use more appropriate language about uncertainty,” Dr McCallum said. “Then it was very much yes or no.”

The Met Office also announced yesterday that the period of May, June and July this year is already the wettest in England and Wales since records began in 1766, even before July is over. Provisional figures show that 15in (387.6mm) of rain has already fallen, eclipsing the previous wettest May-to-July spell of 1789, when 14in was recorded.

“What we have here is what could be called an unprecedented event, which has not happened for 200 years,” Mr Napier said. Met Office scientists said that it was unclear whether such events would become more common because of global warming.

New research published this week in Nature indicates that climate change is contributing to greater precipitation at latitudes such as Britain’s, but most computer models suggest that the British climate will get wetter in the winter but drier in the summer.

Peter Stott, a senior climate scientist, said that warmer average temperatures meant that extreme summer rainfall was likely to become more common.

“In summer, we are generally speaking of a drying trend in the UK, but there is uncertainty,” he said. “Within that, climate change means that when it rains, it can rain harder. The atmosphere can contain more moisture in a warmer world.

“Even with the drying trend, when you do get rainfall events they can be very heavy, as potentially all the moisture available to form rain can fall out at the same time.”

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