This image taken from family video in June 2015, shows...

This image taken from family video in June 2015, shows Maureen Sweeney. Sweeney was a postal clerk at Blacksod Point on the northwest coast of Ireland, where one of her duties was to record data that fed into weather forecasts for the British Isles. In early June 1944, Sweeney sent a series of readings that helped persuade Gen. Dwight D. Eisenhower, the supreme commander of Allied forces in Europe, to delay D-Day and avoid potentially disastrous weather that could have wrecked the landings. She didn’t learn of her role in history for more than 10 years. Credit: AP/Fergus Sweeney

VER-SUR MER, France — Along with the generals and the paratroopers, the pilots and the infantrymen, spare a thought for the young Irish woman who may have played the most important role of all in making the D-Day landings a success.

Maureen Sweeney was a postal clerk at Blacksod Point on the northwest coast of Ireland, where one of her duties was to record data that fed into weather forecasts for the British Isles.

In early June 1944, Sweeney sent a series of readings that helped persuade Gen. Dwight D. Eisenhower, the supreme commander of Allied forces in Europe, to delay D-Day and avoid potentially disastrous weather that could have wrecked the landings. She didn’t learn of her role in history for more than 10 years.

“It’s something to remember for a lifetime,” Sweeney told her grandson in an interview filmed before she died last December. “It’s the only time they ever noticed our forecasts. The one that counted. And set the world alight.”

As D-Day loomed, Eisenhower faced a dilemma.

Almost 160,000 troops had gathered on the south coast of England in preparation for the long-awaited invasion that was scheduled for the early hours of June 5. The ships that would deliver them to the beaches were already warming up their engines. And 12,000 aircraft were ready to pound the Nazi defenses and provide air cover for the landings.

But the success of Operation Overlord depended as much on the elements as military might.

This undated family handout photo shows Maureen Sweeney. Sweeney was...

This undated family handout photo shows Maureen Sweeney. Sweeney was a postal clerk at Blacksod Point on the northwest coast of Ireland, where one of her duties was to record data that fed into weather forecasts for the British Isles. In early June 1944, Sweeney sent a series of readings that helped persuade Gen. Dwight D. Eisenhower, the supreme commander of Allied forces in Europe, to delay D-Day and avoid potentially disastrous weather that could have wrecked the landings. She didn’t learn of her role in history for more than 10 years. Credit: AP

D-Day had been set for June 5 because it offered the right combination of low tides, full moon and, Eisenhower hoped, good weather to give Allied forces the best chance of smashing through the Nazi’s “Atlantic Wall” with a minimum of casualties.

As the appointed hour approached, however, Allied meteorologists were still arguing about the weather.

While U.S. Army Air Force experts forecast that good weather would continue, Britain’s Meteorological Office predicted high winds that could swamp landing craft and thick cloud cover that would hamper air operations.

Relying on readings Sweeney took at Blacksod Point, the Allies’ chief meteorologist, a Scot named James Martin Stagg, finally told Allied commanders that the weather would be unfavorable on June 5.

Eisenhower delayed the landings.

“It was the weather that worried the Supreme Commander most,” author John Ross wrote in his book “The Forecast for D-Day,” published in 2014.

“If he gave the word to ‘go,’ and the weather turned sour, the lives of thousands of men and massive amounts of equipment and supplies would be lost,” Ross added. “Worse yet, the Germans would have learned beyond any doubt where the Allies planned to invade,” eliminating the advantage of surprise.

Operating in an era before Doppler radar and high-speed super computers, Allied meteorologists had to rely on hand-drawn maps, historical data, and spotty weather observations to put together their forecasts.

That’s why Blacksod Point, about 500 miles (800 kilometers) from Normandy on the extreme northwestern edge of Ireland, was so important.

While Ireland had been an independent country since 1922 and remained neutral throughout the war, it continued to share weather readings with Britain’s Met Office, which used the data to produce forecasts needed by Irish farmers and fishermen. But after war broke out, British authorities asked for the readings to be taken every hour, instead of every six hours.

Sweeney was on the midnight to 4 a.m. shift on June 3, her 21st birthday, when she recorded a drop in the barometric pressure. She telegraphed the readings to Dublin, which sent them on to London, then didn’t think much more about it.

But a few hours later, the phone rang and a “squeaky voiced Englishwoman” asked whether the readings were correct. She read off the data and hung up, only to get two more calls seeking confirmation of her readings.

For Stagg, the data from Blacksod confirmed his forecast that a low pressure system would move in from the Atlantic, bringing high winds and thick clouds to the Normandy coast on the night of June 4 and into June 5.

But Sweeney still had another part to play in D-Day.

At 1 p.m. on June 4, she recorded a slight increase in barometric pressure.

That helped Stagg forecast another change in the weather, and later that day, he told Eisenhower that he expected the winds to die down and the clouds to abate in time for a landing on June 6.

The invasion was a go.

“Well, Stagg, we’re putting it back on again,” Eisenhower told his chief forecaster, according to Stagg's book, “Forecast for Overlord,” Ross said. "For heaven’s sake, hold the weather to what you’ve told us and don’t bring us any more bad news.”

Sweeney didn’t learn about the part she played in history until 1956, when Ireland’s meteorological service gave her a copy of the data that informed the D-Day weather forecasts, her grandson, Fergus Sweeney, said in an interview with The Associated Press.

She died on Dec. 17 at a nursing home near Blacksod. She was 100.

“I think she she would be very proud that she did her job diligently that night because of what followed, and I think she would maybe try and remind us all that if we don’t stop the madness, we could be back at another Normandy,” Fergus Sweeney said.

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