That report was instrumental in persuading General Dwight D. Eisenhower to delay the D-Day invasion by 24 hours, a decision that may have averted disaster and changed the course of World War II.

Seventy years later, on June 3, 2014, the D-Day Museum in Portsmouth received Sweeney’s original weather observation sheets, collected by the Irish Meteorological Service. These records contained the crucial weather data that influenced the invasion.

According to the Irish Independent, evidence from Met Éireann meteorologists shows how the forecast from Blacksod altered the outcome of history. Despite extensive planning, the success of the Allied invasion ultimately hinged on one unpredictable factor: the weather.

Although weather observations were being made at various other locations by meteorologists from the Royal Air Force, Royal Navy, and U.S. Army Air Force, the Irish Meteorological Service’s report from Blacksod on the Mullet Peninsula proved most vital. Despite Ireland’s neutrality in the war, the country continued sharing weather reports with Britain under a long-standing agreement. Blacksod was Europe’s first land-based station where weather readings of the Atlantic systems could be professionally monitored.

The Normandy invasion had initially been scheduled for June 5, with plans involving nearly 5,000 ships, 11,000 aircraft, and 156,000 troops to be deployed across a 60-mile stretch of beach. The 6th and 7th of June were also considered, due to ideal moon and tide conditions for amphibious landings and airborne assaults. However, British and American forecasters were divided on the weather forecast for June 5.

James Stagg, chief meteorologist for the invasion, recalled in his memoirs that by June 2, U.S. meteorologists were optimistic about proceeding, while their British counterparts were strongly against it. No consensus could be reached.

In the early hours of June 3, Ted Sweeney sent his routine weather report, warning of strong winds and a falling barometer at Blacksod. Group Captain Stagg, based at Southwick House near Portsmouth, analysed the report and advised Eisenhower to delay the invasion. The D-Day operation was postponed to June 6.

In a 1994 interview, Sweeney, who passed away in 2001, recalled the event: “I was sending hourly reports 24 hours a day. It had to be phoned into London. They queried the report and asked me to check and repeat it. I thought I had made an error, but I hadn’t. I had no idea it was for the invasion.”

Met Éireann’s analysis confirmed that the Sweeneys’ June 3 report indicated a cold front moving southeast across Ireland, along with gale-force winds, low clouds, and heavy showers affecting the English Channel on June 5. A follow-up report from Blacksod on June 4 showed improved conditions, with rain clearing and visibility increasing. By June 6, the cold front had passed, and weather conditions were clear enough for the invasion.

On June 6, with the weather clearance from Blacksod confirmed, Eisenhower gave the order for Operation Overlord to proceed, launching the historic D-Day invasion.