James Martin Stagg was born on 30 June 1900 in Musselburgh, Scotland. He studied at the University of Edinburgh and began his professional life as a science teacher before joining the British Meteorological Office in 1924. Before the Second World War, he worked as a senior technical officer and was appointed superintendent of Kew Observatory in 1939.

During the war, Stagg became attached to the Royal Air Force Volunteer Reserve and served as chief meteorological officer at the Supreme Headquarters Allied Expeditionary Force (SHAEF). In this position, he advised senior Allied commanders during the planning of Operation Overlord. His task was to coordinate and interpret weather information from different British and American forecasting teams, including specialists from the Royal Navy, the Met Office and the United States Army Air Forces.

The weather forecast for D-Day was particularly important because the landings depended on several conditions coming together. Planners had to consider tides, natural light, moon phase, wind, cloud cover and visibility. The original date chosen for the invasion was 5 June 1944, but the weather situation over the Channel and the North Atlantic deteriorated in the days before the operation. Forecasting was difficult, as it relied on weather ships, reconnaissance flights and reports from places including the west coast of Ireland, in a period before satellite technology.

At Southwick House, Stagg presented the weather situation to General Dwight D. Eisenhower and the Allied command. On 4 June, he advised that conditions on 5 June would be too poor for the invasion, but that a temporary improvement was expected on 6 June. Eisenhower accepted this advice and postponed the operation by twenty-four hours. The next suitable period, taking tides and moonlight into account, would not have come for about two weeks.

On 6 June 1944, Allied forces landed by sea and air on five beaches in Normandy. The weather was not ideal, but it was better than on the previous day, and the forecast allowed the operation to proceed. For his work during the D-Day period, Stagg later received the United States Legion of Merit. After the war, he returned to the Met Office, where he worked as director of services until 1960. He published Forecast for Overlord in 1971 and died on 23 June 1975.