After months of painstaking planning, the invasion was ready and military leaders were confident that it could be the beginning of the end of the war. The success, or otherwise, however of the entire operation was out of their hands. The responsibility was instead given to the chief meteorologist reporting to Allied Supreme Commander Dwight Eisenhower – a Scottish man named JM Stagg.
Along with his colleagues, it was his job to accurately forecast the weather at the invasion site of Normandy, as well as the English Channel, and give the vital all clear for the invasion to begin. If he got it wrong, he could have condemned thousands of men to death.
The original date for the invasion, 4 June, was abandoned when a storm raged in the English Channel. Strong winds and rough seas would be disastrous for the Allied forces. If it couldn’t be launched within a few days, it would have to be postponed until the next full Moon. This was needed to illuminate landing sites, and the tide would be low, exposing the German water defences.
Group Captain Stagg, however, predicted there would be a lull in the storm. He advised Eisenhower that the invasion could go ahead on 6 June.
While Eisenhower gave the green light, German meteorologists were sure that the storm would continue for days so the threat of invasion was small. Nazi commanders left the coastal defences, joking that even the enemy wouldn’t be so foolish as to attack in this weather.
But Stagg was right. The waters remained choppy and many parachutists missed their drop zones, but by noon on 6 June, the weather cleared. D-Day saw 160,000 troops enter Normandy, catching the Germans completely by surprise.
To mark 70 years since the D-Day landings and the success of Operation Overlord, have a look at how Stagg and the Met Office made their most important forecast.