Why good weather is hard to predict

Forecasters warned today of the pitfalls of predicting the weather a long way in advance after the British summer has so far failed to live up to expectations.

The warning came as the Met Office was set to issue a revised forecast following its claim in April that the UK was "odds-on for a barbecue summer".

Since a hot spell at the end of June and beginning of July, rainy conditions have been dominant and are set to continue.

Tom Tobler, forecaster at the MeteoGroup, the weather division of the Press Association, said it was difficult to forecast more than around two weeks ahead.

"The further you go into that period the more uncertain it is," he said. "We are usually confident about the general weather system for a number of days.

"As far as seasonal forecasting is concerned, it's only a very vague, general forecast."

The Met Office's summer predictions were based on percentage chances of it being warmer than average, although it did state Britain was unlikely to escape some downpours.

Mr Tobler said: "They will only say there is a certain percentage chance of it being warmer than average.

"If it was a 65 per cent chance it doesn't really tell you a lot as there is a 35 per cent chance that it could go the other way.

"It's a chaotic system. You are looking at the UK which is a fairly small area and it only takes a small change in the atmosphere to affect the weather.

"A small change one day leads to a bigger change the following day.

"Part of the problem with long-range forecasts is communicating with the public the uncertainty.

"When there is that much uncertainty it's not that useful to the general public but people are looking for the headlines like 'barbecue summer' and don't necessarily read on."

Revising forecasts is normal, Mr Tobler said.

"Whenever you do a long-range forecast you would update it at intervals because you are getting more information all the time," he said.

Mr Tobler said technology was being developed but long-term forecasting was likely to remain limited.

"It's something that is being worked on all the time," he said. "It's still in its infancy. Over time things will improve but there is that much unpredictability in the atmosphere that there will always be a limit to how far we will be able to predict with any accuracy other than a general trend."

The forecaster said temperatures should rise next week although there would still be rain around.

Ewen McCallum, chief meteorologist at the Met Office, said in April: "After two disappointingly wet summers, the signs are much more promising this year.

"We can expect times when temperatures will be above 30C, something we hardly saw at all last year."

But Mr McCallum warned that while there was a two in three chance the forecast would be right, there was a one in three chance it would be wrong.

He urged caution over seasonal forecasts and said the forecast was based on probabilities which showed there was a 50 per cent chance of the temperatures in June, July and August being above average, a 30 per cent chance they would be average and a 20 per cent chance they would be below average.

Philip Eden, vice president of the Royal Meteorological Society, criticised the Met Office for using the "barbecue summer" phrase.

He told BBC Radio 4's Today programme: "The key is that these are experimental forecasts. That's fine by me - it's exactly what they should be doing.

"They should be doing research into long-range forecasts.

"The problem is that we don't actually know very much about the forcing factors, the outside influences which affect the weather during a period of, say, a month or three months ahead.

"The big problem with these forecasts was the spin that was put on it by the Met Office's press office - the 'barbecue summer' bit."

He pointed out the wording of the forecast was "unambitious" and had a "very wide target".

"They simply predicted that temperatures for the summer quarter would be above average," he said.