Weather can often change our plans, but there was one weather event that changed the course of history forever.

The D-Day invasion was the start of Allied operations that ultimately liberated Western Europe, defeating Nazi Germany and ending the Second World War. However, this would have not been possible without one key and perfectly executed weather forecast.

In 1944, there we no sophisticated tools such as weather models, radar, or satellites. So it was up to military weather forecasters and their old-school weather projections to determine conditions over the next few days.

Despite the deficiencies of weather forecasting at the time, one specifically was so accurate that it changed the course of the world forever.

The invasion in Normandy, France (known as D-Day), was originally scheduled for June 5, but bad weather conditions made the troops delay the plans.

The invasion was planned for years. Meteorologists from the British Royal Navy, the British Meteorological Office, and the U.S. Strategic and Tactical Air Force worked together to develop a precise forecast for invasion day.

General Dwight Eisenhower wanted the most accurate forecast, with high confidence and no room for errors. Captain James Martin Stagg, chief meteorologist of the British Royal Air Force briefed General Eisenhower about the rough seas and lashing rain affecting the shore on June 5. Capitan Stagg provisioned the small window of better weather for early morning June 6.

The window of improved weather came thanks to a high-pressure system that developed over Greenland and the Azores, driving a low-pressure system to the north-northeast across the Atlantic.

Over these latitudes of the world, it is common to have parades of low-pressure systems, which bring constant periods of unsettled weather and, in between, very brief instances of tolerable conditions.

On June 6, a low pressure had moved east of England, and a stronger low-pressure was just off the southeast coast of Greenland. Helping even further the invasion was the full moon and an early morning low tide.

There were partly cloudy skies allowing the full moon’s light to help soldiers during the overnight invasion. The extreme low tides would ensure extreme low sea level so that the landing crafts and soldiers could spot all mined obstacles scattered on the beaches.

Furthermore, overcast skies inland helped disguise the incoming allied aircraft which would drop more than 13,000 bombs over German bases.

In charge of defending german-occupied beaches was Field Marshal Erwin Rommel, who was certain there would be no invasion between June 5 and 8 due to unfavorable weather and tides. General Rommel had even returned home during those days, and the Nazi commanders gave the green light to coastal defenses to leave their post and participate in nearby war games.

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If the invasion had been postponed til mid-June, soldiers would have been in the path of a potent storm that lasted four days, from June 19 to 22. Churchill described this storm as the “worst channel storm in 40 years.”

Until this date, there has been no other weather forecast that has had such an impact on the outcome for so many people in the world and in history.