Updated 22 November 2007
BackgroundMet Office winter forecasting methods are based primarily on the influence of global ocean temperatures on European winter climate. The nature of these influences for Winter 2007/8 has been assessed using statistical methods and a number of global forecasting models. Forecasts are expressed as variations from 1971-2000 averages. Seasonal temperatures over Europe/UK are now often warmer than 1971-2000 averages as a result of recent warming, and the underlying influence of such trends is included in this forecast.
Variations from seasonal averages usually affect quite large geographic areas, so the forecast for the UK is cast in the broader picture for Europe as a whole.
Winter 2006/7 was the second warmest on record for the UK, with well-above-average rainfall.
This year, conditions in the North Atlantic Ocean favour a near-neutral North Atlantic Oscillation (NAO), rather than the positive phase predicted and observed last winter. See the hyperlink for details of the impact of NAO on winter weather in Europe. However, La Niña conditions – which have widespread impacts across the globe - are now well established in the tropical Pacific Ocean and are expected to persist through the winter period. There is evidence that La Niña has a weak influence on European winter climate, favouring positive NAO in the latter part of winter.
Winter, in this context, is defined as the months of December, January and February although it is recognised that winter weather can extend beyond this period, especially in northern Britain.
Forecast for Winter 2007/8
Temperature
We continue to predict that above-normal winter temperatures are more likely than below-normal temperatures over much of the European region. However, this winter is likely to be less mild in most regions than last winter, when exceptionally mild conditions were widespread across Europe.
For the UK as a whole, winter-mean temperatures are more likely to be above normal than either near or below normal. Although a winter milder than the 1971-2000 average is favoured, temperatures are likely to be lower than those experienced in the very mild winter last year.
Precipitation
Latest indications suggest that, for northern Europe, above-average winter rainfall is more likely than below-average rainfall. In contrast, for southern Europe below-average rainfall is more likely than above-average.
For the UK as a whole, winter rainfall is slightly more likely to be near, or above average, than below average.
Forecast uncertainties
Seasonal forecasts indicate how slowly-varying large-scale climate influences make particular seasonal conditions more likely than others. Random, unpredictable factors ('chaos') also partly determine year-to-year variations, and these will sometimes override large-scale influences. Such uncertainties make a probabilistic format, as used here, advisable for seasonal forecasts.
There is evidence that the influence of La Niña on winter over northern Europe changes during the season, favouring above-normal temperatures for most of the winter but slightly increasing the likelihood of cold spells in early winter.
