One of the most crucial aspects of the D-Day landing on June 6, 1944, was how the weather affected the invasion’s naval and air operations on and above the English Channel.
Weather had the ability to turn the invasion into a disaster, costing thousands of lives and prolonging the war—perhaps for years.
Also Read: The first American on the beach at D-Day served through the Vietnam War
Group Capt. James Stagg, a British Royal Air Force meteorologist, should know. He became a trusted adviser to Supreme Allied Commander Gen. Dwight Eisenhower. Stagg had the unenviable job of trying to predict the notoriously unpredictable weather over Britain and the English Channel for the invasion.
As it happened, Allied access to trans-Atlantic weather data—denied to German Luftwaffe military weather personnel after 1941—became a deciding factor in the invasion’s success. As the initial June 5 assault date approached, a low-pressure system developed in the North Atlantic Ocean near maritime Canada, which consists of Nova Scotia, New Brunswick, and Prince Edward Island. That seemed destined to cancel the invasion during a period of otherwise favorable conditions.
Delaying the Invasion
Continuing to monitor the weather data, Stagg recommended canceling the originally planned date.
Holding out for a possible short break in stormy conditions, Stagg favored landing on the Normandy coast on June 6. At the same time, with the sparse information available to the Luftwaffe weather personnel, the Germans forecasted a two-week period of continuous stormy weather along the French coast.
As observations flowed into Stagg’s meteorological team, a prediction emerged for a ridge of higher pressure, forming off the west Irish coast. It was expected to cross over southern England in time to launch the massive flight operations needed to land glider troops and paratroops, enable fighter and attack planes to cover the invasion, and allow the huge invasion fleet to set sail.
Now a waiting game began to see whether further observations verified the forecast.
Although technically neutral, Ireland belonged to the British Commonwealth during World War II. It routinely provided reports of weather conditions to the British meteorological system.
Maureen Sweeney and her future husband, Ted, were lighthouse keepers, and weather observers on the coast of County Mayo, jutting out into the Atlantic Ocean in the far west of Ireland. The young couple’s weather observation and recording duties came under the Irish Met Éireann, the country’s National Meteorological Service.
Maureen Sweeney provided round-the-clock hourly observations starting on June 3, but she received no explanation for the unusual request. The weather began to improve on June 4, with the skies clearing to the west.
With a potential break in the stormy weather, Stagg provided a favorable forecast to Eisenhower on the evening of June 4, 1944. Based on that forecast, Ike gave the order to commence Operation Overlord.
Invading Tanks Struggle to Come Ashore
Three Days in June: The Story of the D-Day Forecast| BBC Select
While the weather cooperated for the Allied invaders, it was rough going in the English Channel for the men in the ships and landing craft on June 6.
The Allies modified some tanks with dual drives and canvas “skirts” to keep water from getting into the lower driver’s and gunner’s hatches, which were intended to aid in river crossings.
Now these tanks were assigned to “swim” to the invasion beaches from up to three miles out. Very few made it to the beaches; the rough water swamped most of them, causing them to sink. They still reside on the sea bottom off of the Normandy coast today.
Meanwhile, a top-secret scheme provided an artificial harbor for offloading cargo ships. The stormy weather that resumed soon after June 6, though, took a heavy toll on these arrangements.
The largest military amphibious invasion ever owed some of its success to people like Maureen Sweeney. The Irishwoman was 98 years old and in a nursing home in 2021 when she received the rare U.S. House of Representatives Medal, a small token for the part she played on D-Day.
Sweeney died on December 17, 2023.
“Pressure” tells the story of how Stagg advised Eisenhower about when to launch the D-Day invasion. The movie will premiere in theaters on Friday, May 29, 2026.
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