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70 Years After D-Day: The Forecast That Changed the Tide of War

One of the most important weather forecasts in world history would occur in early June 1944, as Allied meteorologists prepared to deliver the final word for the long awaited D-Day invasion of Normandy.

Thousands of lives and the tide of the war depended entirely on teams of Allied meteorologists who determined what constituted suitable weather conditions for the invasion in a small time window.

"The Allies had decided that they wanted to go in at low tide on the landing beaches and that the airborne needed basically a full moon to have the proper dropping conditions," Historian and Author John McManus said on the Diane Rehm Show.

High winds and rough seas could impede the amphibious assault and low clouds could block vital air support. The weather factors that would play a significant role in the invasion would be wind, visibility and cloud cover according to a MET article.

"On the Allied side, six meteorologists working in three different teams were responsible for the D-Day forecasts," according to a report by James R. Fleming, president of the International Commission on History of Meteorology.

By June 3, the forecasting team determined the June 5 would not be an ideal day 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 too rough for landings and cloud coverage too thick for bombing operations, according to MET.

Years of preparation were at stake, but on June 4, hours before the launch of D-Day operations amid an approaching storm, British Group Captain James Stagg urged General Eisenhower for a last-minute delay, according to the History Channel.

According to History Channel, only a few invasion dates were possible because of the need for a full moon for illumination and for a low tide at dawn to expose underwater German defenses; June 5 was the first date in a narrow three-day window.

"The American team used an analogue method that compared the current weather with past conditions. Their forecast was overly optimistic and would have resulted in disaster on June 5, 1944," Fleming said.

At the last minute, following Stagg's advice and the other British forecasters', Eisenhower postponed the invasion.

"June 5 becomes quickly off the table because of a terrible storm that is coming in and it's going to make any invasion basically impossible," McManus said. "So, Ike has to postpone it a day and then he has to sift through dozens of weather reports to ultimately decide on June 6 as a kind of an opening in the system that allows weather that's at least good enough, while nowhere near ideal."

German forecasters also predicted the hostile weather conditions; however, they did not expect the high winds or rough seas to weaken until mid-June.

The German forecasters did not have the same amount of forecast information as the Allied forces. The German Navy had few remaining vessels in the Atlantic and their weather stations in Greenland had been closed down, according to MET.

This would prove folly, as many Nazi commanders left their defenses.

"German Field Marshal Erwin Rommel even returned home to personally present a pair of Parisian shoes to his wife as a birthday present," the History Channel reported.

With a more accurate forecast from Allied forces, Eisenhower would commence the D-Day operations, setting a historic shift in the war.

"On Tuesday, June 6, 1944, under barely tolerable conditions, the largest amphibious landing force ever assembled landed on the beaches of Normandy," Fleming said.

More than 150,000 Allied forces would lead 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.