The AMS webinar Forecasting D-Day: The Science Behind History and the Story Behind Pressure explored one of the most consequential weather forecasts in history: the prediction that helped determine the timing of the Allied invasion of Normandy on June 6, 1944.
The discussion generated more questions than time allowed and AMS Headlines followed up with moderator Sean Potter and panelists Jim Galvin, James Fleming, Martin Young, and Anthony Maras–as well as meteorologist Stephen Burt who was a consultant on Pressure–to help answer some of the audience’s most intriguing follow-ups.
What were the criteria that had to be met for the Allied invasion of Normandy, and what was their window of time? What might have happened if they delayed the invasion further than they did?
The invasion required a rare combination of astronomical, oceanographic, and meteorological conditions. Planners needed moonlight for airborne operations, an extreme low tide at dawn so beach obstacles and defenses would be exposed, and sufficient visibility for ships and aircraft. Wind speeds, wave heights, cloud cover and visibility also needed to remain within acceptable limits, not only on D-Day itself but for several days afterward to support the continued movement of troops and supplies.
Military planners had identified a narrow window in late May and early June when these conditions were most likely to align. By June 1944, however, logistics were not ready for an earlier launch.
When deteriorating weather forced commanders to postpone the invasion from June 5 to June 6, they faced a difficult choice. A 24-hour delay was disruptive but manageable. A 48-hour delay, however, would likely have required troops to return to port, resupply, and wait approximately two more weeks for the next favorable combination of tides and moonlight.
That delay could have dramatically altered history. A powerful storm struck the English Channel later in June, producing conditions significantly worse than those encountered on D-Day. Had the invasion been attempted during that period, losses at sea could have been severe and the operation itself may have been jeopardized.
Interestingly, not all of the desired weather criteria were fully met on June 6. Winds, waves, and cloud cover exceeded preferred limits in some areas. The success of the invasion demonstrated that the ideal forecast did not need to be achieved—only conditions good enough to allow the operation to proceed.
What were the Allies’ sources of weather information to feed their predictions? What role did wave forecasting play in the invasion?
Unlike modern meteorologists, forecasters in 1944 had no satellites, weather radar, or computer models. Their forecasts relied on a limited network of observations collected from ships, aircraft, weather stations, and other reporting sources spread across the Atlantic and Europe.
Because observations from the ocean were sparse and often delayed, forecasting depended heavily on the expertise of meteorologists interpreting incomplete information and identifying developing weather patterns.
Wave forecasting also played a critical role. The success of an amphibious invasion depended not only on atmospheric conditions but also on sea state, surf, and swell. Oceanographers and meteorologists worked together to develop methods for predicting waves and landing conditions, helping determine whether troops and equipment could safely reach the beaches. These efforts helped lay the foundation for modern wave forecasting and operational oceanography.
What were the main sources of disagreement about the forecast? How well does the film encapsulate those disagreements?
The disagreements reflected a broader debate within meteorology during the 1940s.
One forecasting approach relied heavily on analog techniques—comparing current weather patterns to similar situations from the historical record and using those precedents to predict future conditions. This method was favored by the American team and formed the basis of some forecasts favoring an invasion on June 5.
Other forecasters, in particular the British team, relied on synoptic meteorology, using evolving weather patterns, fronts, upper-air observations, and conceptual understanding developed by the Bergen School of meteorology, and how those dynamical and physical processes would influence future conditions. This approach represented a more modern view of forecasting, though it was still in its developmental stages.
Neither method was perfect, particularly given the limited observational data available at the time. The forecasting challenge lay in determining which interpretation of the evolving Atlantic weather pattern was most likely to prove correct.
The film captures the tension between these competing approaches and the uncertainty involved in making a forecast whose consequences would affect thousands of lives. While dramatic adaptations inevitably simplify some details, the central disagreements portrayed in the film reflect real scientific debates that took place among the forecasting teams.
What do we know about the Nazi weather forecasters, their sources of intelligence, their predictions, and their fates?
German meteorologists faced many of the same forecasting challenges as their Allied counterparts. They relied on observations from weather stations, ships, U-boats, and broader European weather networks to monitor conditions across the Atlantic and Europe.
Available evidence suggests that German forecasters generally anticipated poor weather during the period surrounding June 5 and June 6. In broad terms, their forecasts were largely correct.
Ironically, that expectation may have contributed to a strategic blind spot. Because weather conditions appeared unfavorable for a major amphibious assault, German commanders considered an Allied invasion less likely during that period. The brief improvement in conditions identified by Allied forecasters created an opportunity that the Germans did not fully anticipate.
The film can only cover so many people. Who were the key figures on the Allied forecasting team, including those who didn’t have as much of a role in the movie? When did they join the team, and how long had they worked together?
The D-Day forecast was the product of a large collaborative effort involving meteorologists, oceanographers, and military weather specialists from Britain, the United States, and other Allied nations.
Among the most important contributors were experts from the British Meteorological Office, the Royal Navy, and the U.S. Army Air Forces. Their responsibilities ranged from forecasting cloud cover and visibility to predicting winds, sea state, and surf conditions. Some specialized in atmospheric dynamics, while others focused on climatology, oceanography, or operational forecasting.
Several individuals played particularly important roles in analyzing North Atlantic weather systems, coordinating forecasts among the Allied services, and helping military leaders interpret conflicting guidance. Others were responsible for forecasting waves and sea conditions that would directly affect the landings.
Although the various forecasting groups had collaborated during the war, Operation Overlord required an unprecedented level of coordination. Bringing together multiple forecasting teams, reconciling differing opinions, and presenting a single recommendation to military leadership became one of the greatest forecasting challenges of the twentieth century.
In addition to Stagg and Krick, there were several key members of the Allied forecasting teams who were not featured in the film. They include:
Donald Norton Yates—U.S. Army Air Forces (and later U.S. Air Force) officer who served as Stagg’s American counterpart. In February 1944, he was appointed Director of Weather Service for the U.S. Strategic Air Forces in Europe. Although he is not featured (at least not by name) in the film, he played a major role in forecasting for Operation Overlord, including helping reconcile differences between the American and British forecasting teams, and worked directly with Stagg in advising June 6 as the optimal date for the invasion.
Sverre Petterssen—A Norwegian meteorologist who was part of the British Met Office team. Having studied under pioneers such as Vilhelm and Jakob Bjerknes and Tor Bergeron at the famed Bergen School of Meteorology, his synoptic forecasting skills are credited with identifying the Atlantic disturbance and deterioration/improvement sequence that were crucial in allowing the invasion to move forward on June 6.
Charles Douglas—Douglas was in close contact with Stagg from Dunstable (Meteorological Office headquarters) as chief forecaster and led in the advisory capacity, as regards all aspects of weather expected. In doing this, he liaised closely with Petterssen. Never a man to seek the limelight, Douglas was widely recognized in the UK to be the greatest synoptic forecaster of his era.
Ben Holtzman—Former chief meteorologist for American Airlines, he worked alongside Krick as part of the American forecasting team that argued the attack should take place on June 5, based on forecasts produced using analog techniques.
Geoffrey Wolfe—Commander in the Royal Navy, led the British Admiralty forecast team, focusing on forecasts for waves and sea state.
Lawrence Hogben—New Zealand-born Royal Navy lieutenant commander who worked with Wolfe on the British Admiralty team. He later received the Bronze Star Medal for his role in the D-Day forecasts.
The success of the D-Day forecast ultimately depended not on a single individual, but on the collective efforts of a diverse team working under extraordinary pressure and uncertainty.
For further reading, Toby Smith, of the Association of American Universities and speaker at the AMS Science Policy Colloquium, wrote an article about Pressure and U.S. investment in science for AAU Leading Research Universities Report (LRUR): The Forecast That Changed D‑Day and Why Weather Science Still Matters | Association of American Universities (AAU).
