This year, for the first time, the Met. Office is publishing an experimental long range forecast for the average Central England Temperature (CET). The CET is an indicator of the average temperature and, according to the predication, it's going to be a fairly average Summer feeling a bit cooler than the last few years.
Day-to-day forecasting depends on mathematical models of atmospheric changes. The air in our atmosphere moves in chaotic swirls resulting in the high and low pressure zones familiar from the weather map. However, predicting these changes requires immense computation and is usually only accurate for a few days at a time.
To produce a long range forecast statisticians look for patterns between different weather variables and use these patterns to forecast future trends. The 1997 forecast is based on correlating the sea surface temperature for January and February with the CET for July and August each year.
This forecasting technique is still experimental and is being developed at the Hadley Centre for Climate Predication and Research as part of the Met. Office's on-going research and development operations.