The European Centre for Medium-Range Weather Forecasts (ECMWF) is the internationally recognized leader in Numerical Weather Prediction (NWP). They are the best in the world right now.

Its mission: “We are an international organisation supported by 31 States. We provide operational medium- and extended-range forecasts and a state-of-the-art super-computing facility for scientific research. We pursue scientific and technical collaboration with satellite agencies and with the European Commission.”

ECMWF has re-examined the weather conditions on D-day in a strictly scientific manner.

“Weather forecasts critical to the success of the D-Day landings of 6 June 1944 were made on the nights of 3/4 and 4/5 June 1944:
- forecasts for conditions on 5 June made on the evening of 3 June and confirmed early in morning of 4 June;
- forecasts for conditions on 6 June made on the evening of 4 June and confirmed early in the morning of 5 June.

The first of these two forecasts, presented to General Eisenhower by his meteorological advisor, Group Captain J.M. Stagg, led to the postponement of the invasion planned for 5 June; the second enabled Eisenhower to make the decision to go ahead on the following day.

One of the forecasters involved, Lawrence Hogben, writing in the Royal Meteorological Society’s magazine Weather in June 1994 recalled how three separate teams, from the Met Office, the Royal Navy and the US Air Force, first made separate forecasts and then sought consensus – an early example of what today we refer to as ensemble forecasting. On the evening of 3 June the teams initially split two-to-one in favour of conditions leading to postponement; the following evening it was initially a two-to-one split in favour of conditions that would allow the invasion to proceed. Demanding military requirements, stormy weather in the Atlantic and associated fronts moving up the English Channel combined to make forecasting far from easy, and decisions were finely balanced.

Today, the science and technology of weather forecasting is far removed from that of sixty years ago. Forecasting beyond a few hours ahead nowadays relies on automated procedures for processing digitized observations to give a snapshot of the state of the atmosphere. This is used to initiate a computerised model of the atmosphere that projects the information forward in time to form the forecast. The initial snapshot, known as the “analysis”, itself depends on the atmospheric model; it is constructed by blending the latest observations with a “background” forecast initiated from the analysis made a few hours earlier. As time goes by, this process of “data assimilation” produces a picture of how the weather has evolved, built up by the succession of analyses.”

Full details on the re-analysis and forecast on D-day at ECMWF.

Photo from  America’s Library.