(WFRV) – It is one of the most important days in world history. D-Day or codename mission “Operation Overlord” changed the course of World War II as the Allies invaded the beaches of Normandy and claimed victory over the Nazis. The next year in 1945, World War II would come to an end.

What a lot of people don’t realize is the forecast was difficult and crucial in the invasion. Meteorologists had no satellites to check and heavily relied on surface observations and prior knowledge back in the 1940s. Most forecasts made more than a day out were considered an educated guess.

On June 4, 1944, a strong area of low pressure tracked over Ireland and Scotland. This storm system raised questions about the D-Day invasion.

The weather set-up for the original invasion date on June 5, 1944, had a cold front right near the beaches of Normandy. This created low-level clouds and winds over 25 mph in the English Channel which would have made the invasion very difficult.

Thanks to the great work of the meteorologists for the Allied forces, forecasters found a very short window where the invasion could happen the next day. On June 6, the cold front moved further away, while a stronger system lingered just off to the west.

The Germans forecasters did not anticipate this break in the weather which made the invasion even more surprising. They did not have access to some of the surface observations that the Allies used to help determine brief windows of calmer weather.

The weather pattern past June 6 would have delayed the invasion multiple weeks, and by then the Germans may have realized an attack was imminent.

According to the History Channel, American and British forecasters had conflicting opinions about the forecast. Captain James Stagg, who was the meteorologist in charge for the British Royal Air Force, advised General Dwight D. Eisenhower to postpone the invasion to June 6. The weather wasn’t perfect to start June 6, but the rest is history when conditions lightened up.