Only a few invasion dates were possible because of the need for a full moon to gain enough light for the landing site and troops. Also, Allied forces needed a low tide at dawn to expose underwater German defenses. So June 5 was the date chosen, as it was the first date in a narrow three-day window.
But by June 3, the forecasting team of six meteorologists working on three different teams determined the June 5 date would be a problem for the invasion, as high pressure over France and low pressure northwest of Ireland would maintain strong southwesterly winds in the English Channel, meaning seas would likely be too rough for landings and cloud coverage too thick for bombing operations.
While years of preparation were at stake, on June 4, 1944, British Group Captain James Stagg urged General Eisenhower to delay the attack only hours before the launch of D-Day operations as a storm approached. The high winds and rough seas could put the amphibious assault at serious risk and low clouds could block air support.
The American team of meteorologists did not agree with the severe weather forecast. But the team used an analogue forecast method that compared current weather conditions with past conditions to determine a forecast. Their forecast was overly optimistic and would have likely resulted in disaster on June 5, 1944 according to historians.
At the last minute, following the advice of Captain Stagg and other British forecasters, President Eisenhower postponed the invasion. After sifting through dozens of weather reports, Eisenhower ultimately decided that the best opening for an invasion would be on June 6, although the weather was not expected to be ideal.
Interestingly, the German forecasters had also predicted the rough weather conditions, but they were not expecting conditions to improve for at least another week.
The German forecasters did not have the same amount of forecast information as the Allied forces. The German Navy had few remaining vessels nearby and their weather stations in Greenland had been shut down. This would prove to be a huge miscalculation, as many Nazi commanders left their defenses.
On Tuesday, June 6, 1944, under barely tolerable conditions, the largest amphibious landing force ever assembled landed on the beaches of Normandy. More than 150,000 Allied forces led the charge to liberate France from the Nazi’s control, leading to the death of nearly 2,500 Americans in one of the bloodiest days of the war, according to NPR.
With the more accurate weather forecast from Allied forces, Eisenhower commenced the D-Day operations, setting a historic shift in World War II.
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