When pressure is low over Iceland and high over the Azores, westerly winds sweep across the Atlantic and give Britain a warm and wet winter. But if Iceland's pressure rises and that of the Azores dips, we catch cold air from the Continent.
Over the past 54 years, scientists have observed that when the NAO was in a particular state in May, a very cold winter often followed in Britain. A high-pressure system tended to block the prevailing warm moist airflow from the west, allowing cold air from the north and east to cover the country.
The Met Office spotted signs of the NAO pattern in May, when areas of unusually cool water appeared in several places in the Atlantic - these kinds of sea temperatures are thought to contribute to the NAO effect.
Richard Graham, from the Met Office's seasonal forecasting unit, said: "We get the sign of the NAO correct in two years out of three, and it has worked for more than 50 years of data."
Ewen McCallum, chief meteorologist at the Met Office, said there is the potential for long periodsof up to two weeks "where the temperature may not rise much above freezing".
Mayor of London Ken Livingstone warned that London and the South-east could face the worst winter since 1962-63, when Britain was frozen from Boxing Day until April with an average temperature of 0.2C (32.3F). He said: "We could see quite severe loss of life."Reuse content