Many were involved in the planning of the invasion for many years. There were three teams of weather forecasters, which included the British Royal Navy, British Meteorological Office, and the U.S. Strategic and Tactical Air Force.
Gen. Dwight “Ike” Eisenhower wanted the most accurate forecast, with high confidence and with no room for errors. Capt. James Martin Stagg was the chief meteorologist of the British Royal Air Force and was the one to brief Eisenhower about the rough seas and lashing rain affecting the shore on June 5. Stagg previsioned the small window of better weather for early morning June 6. A high-pressure system was located 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 and 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. Overcast skies dominated inland, where aircraft would drop more than 13,000 bombs.
The Germans did not foresee this brief break in the bad weather. In fact, they forecast unsettled weather until mid-June. In charge with the defense of the invasion of beaches with the Germans, Field Marshal Erwin Rommel was certain there would be no invasion between June 5 and 8 because the tides would not be favorable. Rommel had even returned home during those days and the Nazi commanders gave the green light to coastal defenses to leave their posts and participate in nearby war games.
If the invasion would have been postponed for mid-June, soldiers would have been in the path of a potent storm that lasted four days, June 19 - 22. Churchill described this storm as the “worst channel storm in 40 years.”
Although there may be other "D-Days" in the military, there has been no other where weather forecasting has had such an impact in the outcome for so many people in the world and in history.