D-Day: The weather forecast that changed history

Every move was strategically mapped out, but the one thing masterminds of D-Day – the largest amphibious invasion in history – couldn’t control was the weather.

As war raged across Europe in 1944, the Allies were finalizing plans to establish a beachhead in France after two years of meticulous planning. It was a massive undertaking and a mission that could change the course of history. But what the masterminds of D-Day – the largest amphibious invasion in history – couldn’t control was the weather.

It was early June 1944. One of the most crucial weather forecasts in history was about to be made. Allied meteorologists were tasked with delivering the final word for exact date of the highly anticipated D-Day invasion of Normandy.

"You had 5,000 ships crossing the English Channel, you had thousands of planes involved, you had millions of troops supporting, ultimately, and the element of surprise was so important," AccuWeather founder and chairman, Dr. Joel N. Myers told AccuWeather Network's Chief Meteorologist Bernie Rayno in a peek at the Invisible Iceberg episode, “Forecasting D-Day."

Planners needed very specific conditions for a successful outcome, Dr. Myers also noted in his book Invisible Iceberg: When Climate and Weather Shaped History. Wind, visibility and cloud cover were the key weather factors that would influence the invasion. "It would need to happen at low tide with calm seas and 3 miles (5 km) visibility. It would have to remain that way for at least 36 hours to give many of the forces time to land."

With thousands of lives and the outcome of the war hanging in the balance, the timing of the launch was critical. Six Allied meteorologists—two each from the Met Office, the Royal Navy and the United States Air Force—would guide the launch in what has been called "the most important weather forecast in history."

As Dr. Myers notes in his book, the Allied forecasters had advantages over the Germans. Because weather typically travels west to east and the Allies had more control over the Atlantic, they could better observe the weather moving in from the ocean. Additionally, they had cracked the German code used to encrypt communications, meaning they could use German weather observations as well as their own.

Only a few dates were possible for the invasion requiring both a full moon for illumination and a low tide at dawn to expose underwater German defenses. June 5 was the first date in a narrow three-day window. On June 4, conditions were stormy. But the two U.S. meteorologists believed conditions would improve and the mission should go forward on June 5. The Royal Navy and UK Met office teams disagreed. With just hours before D-Day operations were set to launch amid the approaching storm, British Group Captain James Stagg advised General Eisenhower for a last-minute delay by one day.

Stagg and the British team were proven right the following morning when 25- to 30-mph winds created rough seas in the English Channel. But would the weather improve enough for the mission the next day? It was a very difficult call amid incredibly high stakes. If the weather wasn't favorable, the next window of opportunity would not happen for another two weeks.

Once again, the Allied weather teams could still not reach a consensus, but, with two of the three teams saying conditions would be favorable, Eisenhower decided to go with the majority. "It was a fateful decision," Dr. Myers wrote. "The storms on June 4 and 5 had kept German reconnaissance planes on the ground and had persuaded them that the Allies would not invade at that time."

At 6:30 a.m. on June 6, more than 150,000 Allied forces stormed the beaches to liberate France from Nazi control. Although the worst of the storms had subsided, conditions were still far from ideal. High winds tossed ships around, causing soldiers to become wet and seasick. In spite of the setbacks, Americans were still able to take the beaches, advance to the cliffs and move inland.

"Had Stagg not recommended the operation go forward on June 6, it would have been delayed so long that the Germans would have learned of the plan. As it turned out, on the backup dates of June 18 and 19, a storm pummeled the coast of Normandy which would have caused another postponement," Dr. Myers wrote in his book. "It was the advice of meteorologists correctly forecasting a change to favorable weather that gave the Western Allies their most significant victory of World War II."

Discover how a last-minute change in the forecast helped to achieve one of the greatest military victories of all time. Tune in to this must-watch series, "Invisible Iceberg" Mondays and Thursdays at 12 p.m. ET on the AccuWeather Network and AccuWeather NOW to gain a deeper appreciation for the invisible iceberg that has silently steered the course of history.

The AccuWeather Network is accessible via cable TV while AccuWeather NOW offers a comprehensive streaming service available on Roku, Zumo, STIRR, LG and Plex. Enjoy 24/7 programming dedicated to weather phenomena, extreme natural events, global forecasts and a variety of climate and weather-related documentaries from around the globe.

You can also find "Invisible Iceberg" on YouTube.

You can purchase Dr. Myers' book, Invisible Iceberg: When Climate and Weather Shaped History, from Barnes & Noble or on Amazon.com and wherever good books are sold.

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