The true story of a weather forecast that changed the world on D-Day

That decision is the subject of a new movie: "Pressure" starring Brendan Fraser as Ike, now out in theaters.

The real-life D-Day was a herculean effort to reclaim a critical part of Europe from the Nazis and turn the tide of World War II.

Stagg, a geophysicist by training, and his fellow British and American meteorologists operated without the technology and equipment that today's forecasters take for granted, such as satellites, weather radar, computer modeling and instant communications.

Relying mainly on surface observations from military and civilian weather observers in the British Isles and in western Europe and a few military observers at sea, predicting the weather more than a day or two in advance in that era was unrealistic.

Additionally, weather prediction in England and Europe — especially before satellites — was much trickier than forecasting in the United States. Weather systems in the U.S. could be tracked for days after hitting the West Coast and moving east, while European forecasters often operated "blind" with the empty ocean to the west.

In the days leading up to D-Day, meteorologists were troubled by a parade of storms that barreled across the Atlantic and into the British Isles, any one of which would have stirred up the dangerous waters of the English Channel where the fleet was gathering, and provided unwelcome cloud cover for the aerial assault of Normandy.

Predicting the exact timing, track and strength of these storms put Stagg and his colleagues under almost unimaginable pressure and conflict, according to the book "The Forecast for D-Day." Author John Ross said that if the invasion failed, the secrecy about when and where the Allies would land would be lost, victory in Europe would have been delayed for a year, and the Soviet Union might have taken control of the continent.

Why was the forecast so crucial?

According to meteorologist and weather historian Sean Potter, the success of the Normandy landings on D-Day hinged heavily upon the weather and each component of the invasion — naval, air and land forces — had its own weather requirements.

"A forecast that might mean ideal conditions for the bombers, for example, might not take into account the needs of the seaborne invasion," Potter told USA Today. "The invasion also depended on a narrow combination of tides, moonlight, cloud cover, wind, and sea conditions that occurred only during a limited window of opportunity."

While weather forecasting saw vast improvements in the two decades before D-Day — most notably with the development of the polar front theory of how storms develop — there were still limitations that made forecasting a real challenge, Potter explained.

"Forecasters today have advantages of radar, satellites, and other advanced technologies, including numerical weather prediction, to help them make accurate predictions. None of these were available to the D-Day forecasters," he added.

One of the three Allied forecasting teams that contributed to the D-Day forecast — led by American Irving Krick — relied primarily on a technique known as analog forecasting, comparing the weather situation at hand with similar scenarios in the past to determine what might happen.

"By 1944, analog forecasting was viewed with skepticism by many meteorologists and was largely being replaced by other, more modern methods," Potter said.

How accurate was it?

The forecasters identified a crucial break in the weather that allowed the invasion to take place, though conditions remained rough and far from ideal, and not entirely as predicted.

Potter explains: A 2020 paper published in the Bulletin of the American Meteorological Society suggests the forecast for D-Day was essentially “right for the wrong reason.”

The paper's author, Swedish meteorologist Anders Persson, reviewed transcripts of telephone discussions between the Allied forecasting teams and concluded that, while forecasters accurately predicted a break in the weather June 6, they misunderstood why. They believed the storm system that delayed the invasion would move away and be replaced by higher pressure and improving conditions.

Instead, the storm lingered but weakened slightly, producing weather that was acceptable for Eisenhower’s decision to proceed.

Years later, President-elect John F. Kennedy asked President Eisenhower why the Normandy invasion was so successful.

Ike's answer: "Because we had better meteorologists than the Germans!"

How accurate is the new movie?

Overall, the movie is very accurate in its portrayal of the role weather played in planning for D-Day and the challenges forecasters faced in predicting it, Potter said.

"It did take some liberties, such as suggesting that James Stagg, Eisenhower’s chief meteorological adviser (played by Andrew Scott), didn’t get involved in the forecasting for D-Day until several days prior to the invasion. In fact, he had been involved for months. It also likely overdramatizes the tension and conflict between Stagg and his American counterpart, Irving Krick," Potter said.

"There were disagreements, but they were kept mainly professional. One thing the film does a very good job of is conveying the sense of uncertainty that Stagg had in the forecasts he presented to Eisenhower," he added. "Understanding forecast uncertainty is still an important part of making decisions when weather is a factor."