New D-Day weather reconstruction highlights key driver of military success and security | Copernicus

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D Day – a weather forecast that shaped history

New D-Day weather reconstruction highlights key driver of military success and security

Date: 4th June 2026

On 6 June 1944, the world bore witness to the largest amphibious invasion in history, D-Day. Also known as the Normandy landings, the operation saw the air, sea and land forces of the allied armies descend upon northwest France, laying the foundations for the liberation of Western Europe. However, stormy conditions in the run up to that fateful day put the whole mission at risk. Weather Replay , the latest app released by the Copernicus Climate Change Service (C3S), allows us to recreate the weather on this poignant day. Doing so reminds us of the importance of forecasting for safety, health, military success and global security, historically and in the present day.

03 Jun 2026Download (10.56 MB)

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Recreation of the weather on 6 June 1944, D-Day, using the Weather Replay App. Credit: C3S

Saturday 3 June 1944 was a Saturday unlike any other for meteorologists. On that day, meteorological advisor Group Captain J.M. Stagg presented his overall assessment of weather forecasts from three teams of scientists1 to the Supreme Commander of the Allied Forces in Europe, General Dwight Eisenhower, which led to the postponement of the landings and aerial assault, key parts of Operation Overlord. That forecast, confirmed in the early hours of 4 June, predicted that stormy conditions in the English Channel would have put the entire operation under threat on 5 June, the day the mission was initially planned for. C3S’ modern tools enable us to see that their information was accurate, with a threatening storm forming west of Britain.<br>As is often the case with military operations, favourable weather was not just important for D-Day's success – it was a fundamental prerequisite. The allies required little cloud cover, light wind, low seas and a low tide. However, the forecast teams predicted the next occasion when tides were favourable was almost two weeks later. Postponing until then would have likely had hugely damaging consequences for the war effort.<br>With the fate of around 160,000 allied troops on their shoulders, the teams of scientists regrouped later on 4 June to make another forecast. Within that prediction, they identified an alternative window of quieter weather on 6 June that would allow the operation to proceed, which led to military orders for it to go ahead on the following day – 24 hours later than originally planned – being issued, and resulted in a key military success for the allies.<br>Released late last month in the US, the film ‘Pressure,’ directed by Anthony Maras, shines a light on the courageous scientists tasked with forecasting conditions for the Normandy landings in the critical 72 hours leading up to the operation. The film has raised awareness of operational weather forecasting all over the world, with expert scientists at the American Meteorological Society recently hosting a webinar to discuss the science behind it with the Director.<br>In 2014, the European Centre for Medium-Range Weather Forecasts (ECMWF) scientist Adrian Simmons wrote an in-depth article for the ECMWF website analysing D-Day weather based on the European Reanalysis of Global Climate Observations (ERA-CLIM) collaborative research project. Many of its findings are still valid, despite subsequent improvements in reanalysis and collections of additional historic observations.

Recreating historical days

The Weather Replay app enables anyone to easily access and use Copernicus’ newer ERA5 data to reconstruct the weather experienced by those in the air, at sea and on the beaches during D-Day, see the conditions they faced, and better understand the importance of forecasting for the outcomes of that operation. It demonstrates the power of the ERA5 reanalysis dataset and the meteorological archive of the ECMWF.

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Recreation of the weather on 6 June 1944, D-Day, using the Weather Replay App. Credit: C3S

"Weather Replay makes...