Blacksod, Blacksod calling ... Here is the weather report for June 3rd, 1944.

In the history of mankind, few weather forecasts have carried such import. As he cranked the telephone and delivered his news over a crackly line from Co Mayo's most westerly point, Irish Coast Guardsman and lighthouse keeper Ted Sweeney had no idea the lives of more than 150,000 Allied troops would hang on his words.

It was a fateful call. As he watched the barometer fall precipitously, Ted Sweeney's report from the Coast Guard station convinced General Dwight D Eisenhower to delay the D-Day invasion for 24 hours – a decision which averted a military catastrophe and changed the course of the Second World War.

This Friday, June 6, marks the 70th anniversary of the invasion of Normandy – codenamed Operation Overlord. The assault on Hitler's 'Fortress Europe' signalled the beginning of the end of the Nazi regime in Europe with the German surrender less than a year later.

Some 5,000 ships and over 11,000 aircraft carried approximately 156,000 Allied troops into battle on D-Day across a 60-mile beachfront and into the interior of Normandy's Cotentin peninsula.

Now, after a meticulous examination of the 70-year-old records conducted by Met Eireann forecasters, new details have emerged of how the Blacksod Bay forecast changed the course of history.

Years of planning for the Allied invasion came down to one crucial but uncontrollable factor in the tense days before D-Day – the weather.

Separate observations were taken at various locations by Royal Air Force, Royal Navy and United States Army Air Force meteorologists.

But an accurate forecast from the Irish Meteorological Service, based on the observations from Blacksod on the Mullet peninsula, was the most vital.

Blacksod's importance was due to the fact that it was the first land-based observation station in Europe where weather readings could be professionally taken on the prevailing European Atlantic westerly weather systems.

Despite Ireland's neutrality during 'The Emergency', the State did continue to send meteorological reports to Britain under an arrangement which had been agreed since Independence. Irish weather reports, however, were not passed on to Germany.

In August 1940, the future chief meteorologist for the Normandy Landings, Scotsman James Stagg, then representing the British Met Office, visited Irish met observation stations. Later in the war, in early 1944, and with planning well advanced for D-Day a Captain Robert Bundgaard, a meteorologist with the United States Army Air Force, reputedly visited Valencia Weather Station in Co Kerry.

D-Day was originally planned for June 5, but the following two days, June 6 and 7, were also pinpointed as possible dates because moon and tide conditions were deemed ideal for seaborne and airborne landings.

However, as the planned day of the greatest invasion in history approached, British and American forecasters could not agree on the likely weather for that date.

On June 2, the Americans were optimistic for a 'go', having faith in a ridge of high pressure dominating the original D-Day date of the 5th, whilst the British were "unmitigatedly pessimistic" according to Stagg's memoirs. Agreement could not be reached and Stagg was in a serious dilemma.

Then, just after 2am on June 3, Irish Coast Guardsman and lighthouse keeper Ted Sweeney sent his latest hourly weather observation report. It contained an ominous warning of "a Force 6 wind and a rapidly falling barometer" at Blacksod.

At Southwick House outside Portsmouth, Group Captain Stagg studied the Blacksod report and strongly advised General Dwight D Eisenhower to postpone for 24 hours. Eisenhower heeded the advice and postponed to Tuesday June 6. The postponement was due to the slowing down of the westerly weather system bringing in clear weather from the Atlantic. Two cold fronts were approaching Ireland in quick succession.

Eisenhower also strongly faced the possibility of having to postpone the invasion altogether until July when moon and tide conditions might once again be suitable but, as he later reminisced, that was an option "too bitter to contemplate".

As it turns out, history records that the weather for the following month was deplorable for a sea and air invasion.

At 11am on June 3, 1944, nine hours after Sweeney's report, the telephone rang at the remote Blacksod Coast Guard station in Co Mayo which also doubled as the area's post office which was run by Margaret Hughes. The call was taken by her daughter Maureen Sweeney, who worked in the post office. Today Maureen is 90 years old.

She recalls: "That call was from 'a lady with an English accent' and she requested a repeat of the last weather observations Ted had sent earlier. So I called my husband Ted to the phone and he checked and repeated the latest information we had sent by then."

An hour later, shortly after 12 noon, the telephone rang once more and again Maureen answered it.

"It was the same lady. The lady with the English accent and she asked if we could please check and repeat the very latest weather observations we had sent from Blacksod. So Ted repeated those ones again."

Ted Sweeney died in 2001, but in a 1994 interview he stated: "I was sending an hourly report 24 hours a day and night. It had to be phoned into London, (Dunstable). We got a query back.

"They asked for a check. 'Please check and repeat the whole report.' I was wondering what was wrong. I thought I had made some error or something like that. They sent a second message to me about an hour later to please check and repeat again. I thought this was a bit strange so I checked and repeated again. It never dawned on me that this was the weather for invading or anything like that. When I checked the report, I said: 'Thanks be to God, I was not at fault anyway.' I had done my job and sent over a correct reading to London."

Ted Sweeney's crucial Blacksod reports from June 3, 1944, in conjunction with observations from other met stations, indicated a cold front lying halfway across Ireland and moving rapidly south eastwards and that a deep depression lay between Iceland and Scotland. Stagg could now see that gale-force winds, low clouds and heavy showers would still be affecting the English Channel in the early hours of June 5, new Met Eireann analysis has confirmed.

Then, at 12pm on June 4, Ted sent the report that offered hope to Eisenhower and the Allied commanders at Southwick House to launch Overlord. It reported heavy rain and drizzle cleared, cloud at 900ft and visibility on land and sea very clear. A full clearance of the weather would actually arrive at Blacksod one hour later.

In the early hours of the following day, at Eisenhower's morning briefing, the latest report from Blacksod confirmed the passage of cold front at Blacksod at noon on June 4. A loud cheer went up in the room which broke the tension momentarily.

Complete confidence was restored. Eisenhower's long awaited weather clearance had finally arrived and he gave the order for Overlord to proceed. D-Day would definitely be June 6. As the meeting concluded, the rain stopped outside Southwick House and the clouds began to disappear. The clear weather reported from Blacksod had reached the English Channel.

Next Tuesday – 70 years to the day Ted Sweeney gave his weather report – the D-Day Museum in Portsmouth will be presented with high quality copies of the Mayo man's original Blacksod observation sheets, collated by the Irish Met Service. These contain the actual weather observations which proved so crucial to Group Captain Stagg and his team in correctly forecasting the D-Day weather for General Eisenhower.

The presentation will be made by Irish members of the Round Canopy Parachute Team (RCPT) who will fly in a formation of C47 wartime Dakota aircraft from Portsmouth to Normandy on Wednesday morning to parachute jump into Carentan, Normandy, as part of the 70th anniversary celebrations.

Sunday Independent