Brighter prospects ahead for seasonal forecasts

DON'T ask Dr Tony Hollingsworth what kind of weather to expect this summer. He has a good idea but he won't tell you. Not yet, in any case.

Dr Hollingsworth, a Dubliner, is head of research at the European Centre for Medium Range Weather Forecasts (ECMWF) at Reading, Britain. He and his team have been working for the past 2 1/2 years on a project aimed at delivering seasonal forecasts three to six months ahead.

Meteorological forecasters usually predict just two or three days in advance. Beyond that, they are really guessing, with longer forecasts based on trends.

To confidently predict the weather three or six months from now is a remarkable challenge but there are indications that progress is being made.

"There are encouraging signs that something useful might be possible," Dr Hollingsworth said yesterday. He was in Dublin to deliver a talk at the Royal Irish Academy on progress in long range forecasting.

He was quick to point out that the objective is not to predict, for example, whether it will rain on July 22nd or be sunny on the August Bank Holiday weekend. Rather, it involves making predictions about seasonal trends if it will be warmer or colder than average during a season, or wetter or dryer than usual.

Such information, he says, is commercially very valuable. French wine producers, for example, could tailor production if they expected a cool, wet season, and Irish grain farmers might plant extra acres if they expected a warm summer.

The Reading centre already provides Met Eireann - a contributor to the centre - with its "medium range" forecast. These are predictions looking two to 10 days in advance. The medium range techniques are being developed using satellite observations and the most powerful computers available

Confidence that seasonal predictions are possible comes from the intensive world research effort into the El Nino, a massive change in the Pacific Ocean that can alter weather patterns worldwide.

Prevailing easterlies usually push warm Pacific water up against Asia, but these easterlies periodically weaken. When this happens, thousands of cubic miles of warm water spread towards South America, causing significant changes to the normal weather patterns. These changes influence neighbouring weather.

The biggest El Nino of this century happened in 1982/83 but so little was known about it that meteorologists only became aware of it when the weather went awry, said Dr Hollingsworth. Subsequent events in 1987 and 1992 were much more closely observed, and scientists believe that a new El Nino has just got under way.

The Pacific is the "main heat engine for world weather", he said, so scientists have focused on what happens there. By understanding the big picture, it should be possible to understand similar, less powerful influences in other locations.