How the success of D-Day hinged on a weather forecast

Had it not been for a crucial weather forecast, D-Day—the largest seaborne invasion ever—would have unfolded on June 5 as initially planned. Such an occurrence would have likely led to catastrophe, with thousands of soldiers engulfed by storm-driven waves. Instead, the Allied forces postponed by a day, significantly altering the course of history.

The tale of this decisive World War II moment, which secured the Allies a strategic position in mainland Europe and marked the beginning of the end for Adolf Hitler’s regime, has been retold in numerous books, films, and miniseries. Yet, the vital role of the weather forecast in the operation’s success remains relatively unknown to the general public.

This transformative prediction is the focus of Pressure, a new film debuting today. Adapted from a play of the same name, the movie delves into the critical forecasting and decision-making processes that took place in the 72 hours leading up to the Normandy landings. While it dramatizes events, the film highlights the often-overlooked efforts to gather weather data, the importance of evidence-based decision-making, and the fine line between triumph and failure on that pivotal day.

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“D-Day hinged on the weather, and some individuals faced incredibly tough decisions with what we would now consider limited data points,” explains Catherine Ross, library and archive manager at the U.K.’s Met Office. “They held the fate of thousands of lives in their hands.”

A War of Data

Both the Allies and the Germans recognized the critical importance of weather forecasting in their military strategies. Each side employed meteorologists within their ranks to provide forecasts for operations ranging from prolonged bombing raids to precise artillery targeting.

In the race to gather weather data, both sides used every available resource, including aircraft, military and merchant vessels, meteorological units near front lines, and civilian readings. Later in the war, after breaking the Enigma code, the Allies even integrated German weather data. “They understood that the data was paramount,” Ross adds. As James Stagg, the film’s protagonist portrayed by Andrew Scott, emphasizes, “Get me the data; that’s what counts. If we’ve measured it, then I want it.”

A crucial tool was the radiosonde, a box of instruments attached to weather balloons that measure temperature, pressure, and other parameters. The movie features colorized archival footage of actual wartime balloon launches. Radiosondes remain in use today, but at that time, forecasters lacked computers to analyze the data and project future scenarios. Instead, meteorologists plotted data on hand-drawn maps, connecting points to indicate areas of high and low pressure. They based their predictions on how these areas changed over several hours, making forecasts beyond a day or two largely speculative.

Thus, the forecasters advising Gen. Dwight D. Eisenhower, who led the Allied forces in Europe, had far fewer resources compared to modern meteorologists. “They didn’t have a lot of data, but they did a lot with it,” notes weather historian James Fleming, who has studied the D-Day forecasts.

“I Need a Forecast”

In early June 1944, over 150,000 men were poised to cross the English Channel on a vast array of ships and planes. The complexity of the invasion demanded specific conditions: a full moon for visibility, low tide to bypass coastal defenses, and favorable weather. As depicted in the film, Brendan Fraser’s Eisenhower tells Stagg, “I need a forecast.”

Both the Allies and the Germans were aware that the moon and tide would align from June 4 to June 6. The Allies had created decoy armies to mislead the Germans. If the invasion had been delayed for the next moon-tide alignment later that month, this deception could have been compromised. The pressure was intense for both generals and forecasters.

While the film simplifies by keeping all key players together, in reality, there were three separate forecast teams—one American and two British—in different locations to prevent being entirely knocked out by bombing.

Lt. Col. Irving Krick and his American team used “analogs,” or weather charts from previous periods with similar meteorological setups, to predict good weather. However, as Stagg warns Krick in the film, “The weather never replicates its own history.” Small differences in conditions can escalate over time.

The British teams, on the other hand, predicted stormy weather. They were correct, leading to the cancellation of the planned June 5 invasion. They initially believed the entire June 4–6 period was unsuitable. But, as shown in the movie, they eventually identified a brief lull in the storms, just enough to allow the invasion to proceed. Stagg’s line in the film, “The weather won’t be perfect, but it’ll do,” captures this pivotal decision. Consequently, at 6:30 A.M. on June 6, tens of thousands of Allied troops landed on six beaches along the French coast, catching the German forces off guard.

There is some indication that German meteorologists recognized the same weather break, as Heinz Lettau, a wartime German Weather Service meteorologist, mentioned in a 2002 interview. “The Germans had very good talent,” Fleming says. Despite this, German commanders either ignored the forecast or underestimated its likelihood, leaving them unprepared and disorganized.

After the war, Krick, who later pursued a questionable cloud seeding project, claimed the Americans had made the successful forecast. Stagg, however, wrote a letter criticizing that office’s work and credited the correct prediction to the team led by Norwegian Sverre Petterssen. Petterssen preferred that all forecasters receive collective recognition, as Fleming notes: “Petterssen said we should just give credit to everybody that contributed. We shouldn’t try to take credit for the heroic forecast that saved the world.”