Even with today’s advanced forecasting technology, the decision to launch the D-Day invasion on June 6, 1944, would still have been a challenging one, according to a new study. Meteorologist James Stagg provided a critical weather forecast that influenced the fate of more than 150,000 Allied troops preparing to land on the Normandy beaches. The success or failure of the invasion hinged on Stagg’s forecast, which General Dwight D. Eisenhower awaited in a briefing room near Portsmouth. His decision to proceed was based on the accuracy of these predictions. A new film, Pressure, has sparked renewed interest in Stagg’s forecast, portraying a meteorologist as a wartime hero. This has led to discussions about whether modern forecasting systems would have predicted the D-Day weather more accurately. Researchers combined newly digitized historical weather data with modern meteorological science and an AI forecasting system to recreate the forecasting challenge Stagg faced. Their findings suggest that even with today’s technology, the decision to delay the invasion on June 5 would likely have been supported, but the severity of the conditions on June 6 might have been underestimated. Modern weather forecasting starts with the current state of the atmosphere, but today’s scientists have fewer historical observations available than Stagg did in 1944. Many of those observations were recorded on paper and have not yet been digitized. However, progress is being made, and making this data accessible is helping scientists better understand past weather patterns and climate trends. By combining observational data with advanced forecasting models using a technique called data assimilation, researchers can create a more accurate picture of the weather conditions around the D-Day landings. If weather satellites and modern forecasting models had been available during World War II, Stagg and his team might have questioned how a low-pressure system in the North Sea would develop and whether cloud cover from its weather fronts would have hindered the invasion. The film highlights two teams using different forecasting methods: one focusing on the physical development of weather systems, and the other using historical examples to make predictions. Stagg had to combine these often conflicting forecasts to provide clear guidance to Eisenhower. This mirrors the evolution of meteorology in the 20th century and current debates on the best ways to predict the weather. Today’s mobile phone weather apps use detailed physical models of the atmosphere, run on supercomputers, which stem from the techniques developed by meteorologists like Sverre Pettersen. These models have improved by one day per decade since the 1980s. American meteorologist Ed Lorenz discovered that the atmosphere is a chaotic system, making weather forecasting inherently difficult. Modern forecasters use multiple predictions, or ensemble forecasts, to anticipate different possible weather outcomes. More than a dozen weather models now use AI to make predictions, essentially using "analogue" forecasts. A neural network is trained to recognize patterns in historical weather maps and make predictions based on how those patterns evolved in the past. Researchers tested one of these AI models, the AIFS developed by the European Centre for Medium Range Weather Forecasts, using D-Day weather conditions. The model identified the storm in the North Sea but showed significant uncertainty about the Normandy beaches. The storm moved more quickly in the model's forecasts than it actually did in reality. Even with these advanced tools, the complexity of the D-Day decision remains clear. Would Stagg still have advised Eisenhower to proceed on June 6 if forecasts on June 5 had shown a 30% chance of strong winds and significant cloud cover? While the future of weather forecasting may involve a mix of physical models and AI, the role of a human expert in interpreting and challenging these forecasts remains crucial. Today’s forecasters have tools unimaginable to Stagg, but someone like Eisenhower still needs the experience and intuition of an expert to interpret forecasts of extreme weather.