How accurate are long-term forecasts?

Reports of snow have been aplenty, with lots of talk of snowfall in a month’s time. But these reports often do not come to fruition, so how accurate are long-term forecasts?

Despite significant technological and scientific breakthroughs, there are still limits to weather forecasting and some answers belong more to the realm of magic than to serious science.

Long-term forecasts are becoming increasingly popular in media. Sometimes you even get explicit forecasts of temperature, rain and weather for several months in advance. These supposed forecasts are based on different sources.

On one hand, there are weather models that calculate the weather weeks or even months into the future, but on the other hand there are also farmers' rules.

In truth, detailed forecasts of temperature, clouds, and rain are usually only possible three to seven days in advance. In complex weather conditions, an exact forecast even for the following day is sometimes very difficult, which makes meteorologists look lost.

With very stable high-pressure systems, accurate forecasts can stretch to eight, or in extreme cases, 10 days into the future. This is even tougher in the UK and Ireland where we are subject to six air masses creating our often fast-changing weather conditions.

After this point attempts to interpret developments in the weather situation and to derive a probability from them produces limited insights, generally suggesting a trend towards warmer, cooler, wetter or drier weather than before.

Precise statements about temperatures, rainfall amounts or cloud distribution cannot be made.