It's the third of June, 1944, the pitch dark of early Saturday morning. We're at a post office in County Mayo on the west Coast of Ireland, where a bleary-eyed Maureen Flavin is hard at work. It's a special day for Maureen, her 21st birthday. But right now, she's not thinking about celebrating. Instead, she's sat alone in a corner of the tiny Blacksod post office, hunched over a sheet of paper. Now and then she looks up, glances at the scientific instruments in front of her, mouths some numbers to herself, writes them down. She picks up the phone. Blacksod calling. Maureen reads aloud from the sheet in front of her. A weather report for the Met office in England. After a week of glorious sunshine, it looks like a tempest is heading for the British Isles. Although Ireland is officially neutral in the war, weather stations across the country have been supplying the Allies with crucial data. On the hour, every hour, Blacksod sends through the latest figures. Night duty is always the worst. Alone in the dark, Maureen's mind can't help but wonder, what if the Germans were sweep ashore now? It's a feverish notion. But then again, to the enemy, Maureen's data would be quite a prize.

At 3:00 AM, the phone rings, shattering the Ireland. Maureen picks up the receiver. There's an English woman on the line, brisk and business-like. The Met Office wants to double-check Blacksod's latest report. Maureen consults her records. There's no mistake. The numbers are correct. Like everyone, Maureen knows an invasion of Nazi occupied Europe is coming soon. It's a matter of when, not if. What she doesn't know is that British, American, and Canadian soldiers are already boarding their ships. In fact, some parts of the armada are on the move. The invasion is scheduled for dawn on Monday morning, less than 48 hours from now. Or rather, it was. Because across the Irish Sea, the latest news from Blacksod is causing a stir. D-day is about to be canceled. From the Noiser Network, this is D-Day.

Whether was to play a crucial role on D-Day.

Historian and author, Giles Milton.

Was the English channel going to be in a calm enough state to launch the biggest seaborne invasion in history? This was an invasion involving 7,000 ships and landing craft. It was essential to have calm seas.

At Southwick House, The 19th-century mansion just outside Portsmouth, that has recently become Allied Supreme headquarters, Group Captain James Stag is pouring over the latest meteorological data. At Dua Scott in his mid-forties, Stag enjoys few luxuries at Southwick. He sleeps in a tent beneath the trees of Sawyer's Wood, the same place where General Eisenhower has his trailer. His working resources are equally basic. As such, he relies heavily on his American Deputy, Colonel Donald Yates of the US Air Force. At first glance, they make an odd couple. Yates is 5'5, Stag, a willowy 6'2, but they work well together, an effective transatlantic partnership. This morning, Stag and Yates are absorbing Maureen's reports from the West Coast of Ireland, plotting the data on their weather maps. Stag prides himself on his calm demeanor. He's not one to be thrown off course by a sudden gust of wind, but the numbers from Blacksod have got him ruffled. This will have to go to the top, the Supreme Commander. General Eisenhower has already seen how the weather can shape this war. Just last year, unexpected high winds played havoc with the Allied invasion of Sicily. When it comes to Dede, he knows the right landing conditions may be the difference between success stress and failure, and he's relying on Stag to tell him what those conditions will be.

Fortunately, Stag's forecasting operation extends far into the Atlantic Ocean.

The Allies had one advantage over the Germans, and that is they had weather stations in the Atlantic, so they were able to forecast with some accuracy the weather that was coming. The Germans had also had weather stations, notably on the Coast of Greenland, but these had been captured by the Americans and knocked out. So the Germans were blind to what the weather was going to do.

For Stag, the weather station at Blacksod Post Office is a key source of information. As one of the most westerly spots in Ireland. It offers the Allies an early warning of what the Atlantic has in store for continental Europe. The data collected there is like gold dust. But gathering the information is only half the job. The real challenge is working out what it means. Jeremy Black is the author of Strategy and the Second World War.

Weather forecasting is extremely difficult. I live in Exeter, which is where the British Meteorological Office is, and our weather forecast is often episodic, shall we say, in its accuracy. And that is with the advantage of modern computers and the computer crunching of vast amounts of information. In 1944, weather forecasting was, to use the term primitive, would suggest that these people were in some way defective. That's not the case. They were operating within the parameters of the information available. But essentially, you are trying to predict what the weather is going to be by looking at what it is out in the Atlantic Unfortunately, not everyone working for STAG agrees on how to interpret the data.

Dr. Katherine Ross of the National Meteorological Archive.

In the run up to D-Day, there were three forecasting teams working together, trying to provide the most accurate information they could. That was a team from the US, a team from the UK Navy, the admiraltry, and a team from the Met Office. Stag's basically appointed because he was a great administrator. He was able to pull that together into one relatively coherent forecast and piece of advice which he could then take forward.

Pretty much the only thing Stag's team seem to agree on is that he is the wrong man for the job. His forecasting experience is mainly in the deserts of Iraq. Not exactly ideal preparation for the Blustry English channel. The other forecasters see Stag as a gloryhound, a good organizer, but a second-rate meteorologist. On almost every other issue, the three teams are divided, and that includes the most basic principles of how to predict the weather.

The Met Office used a method of forecasting called synoptic forecasting. They would take all of the available observations to them, plot them onto a chart, and then six hours later, they do it again, or sometimes hourly, they would do it again, and they would look at how those observations are changed, use that change to be able to plot onto the chart, isobars and fronts, and then decide what the weather was going to do. When it came to the American team, they used a totally different approach. Particularly, Irving Crick, who was leading up that team, His approach was to look at historical weather charts for the same period, 10, 20, 30 years ago, and look for patterns. When you get to June 1944, when the weather is not doing anything like it normally does at that time of year, in Evitably, there was significant clash between the two organizations.

The differences between the rival teams are personal as much as professional, and the Americans haven't exactly endeared themselves to their new British colleagues. If stag is all stiff upper lip, US meteorologist Irving Crick is his polar opposite. A former radio station manager from San Francisco, he exudes slickness, breo, and self-confidence. Crick is adamant that he can produce reliable five-day weather forecasts, perfect for the needs of the D-day planners. The Brits roll their eyes. In California, maybe, but not in the channel.

The British were rather dismissive of the American meteorologist, and they said, Well, where you come from, the weather hardly ever changes. Whereas in the English channel, it changes not only every day, but hour by hour it changes. So they tended to rubbish everything that the Americans said.

At least Stag's orders from General Eisenhower are crystal clear. Find me a day with low winds, clear skies, a full moon, and a low tide. But in the channel, that's a tricky combination to pin down. A month ago, Stag identified six dates in June as potential D-Days. Eisenhower plumped for the earliest, Monday the fifth. The upbeat radiomane Irving Crick, is confident in the boss's choice. His 50 years of weather maps auger well for Monday morning. Based on previous form, he expects the fifth of June to provide the exact conditions the Supreme Commander needs. But with less than a week to go, the Met Office have spotted something unexpected, a band of low pressure drifting across the Atlantic.

When you collect weather observations, you collect a raft of different information. Pressure, temperature, wind direction, cloud amount, rainfall. One of the most important is pressure. If you see pressure starting to drop, It's not a good sign. A low pressure means rainfall, strong winds, basically not the conditions you need to be trying to cross the English channel in with flat-bottom boats.

Crick scoffs at the Met Office doomongers. As far as he's concerned, the historic data is clear. A storm on the fifth of June is not going to happen. Caught between the two forecasting teams, Stag is in a quandry. What if he advises Eisenhower to call off the invasion and then the storm never comes? He will be the man who canceled D-Day for nothing. And then on Saturday morning, with just two days to go, Maureen's data arrives from Blacksod post office. Her figures laid out in black and white, steadily falling pressure. There's bad weather on the way. Throughout the rest of the day, Stag continues to monitor the pressure readings, looking for any sign of improvement, but everything continues to point in the same direction. In the early evening, he confers with all three forecasting teams. The Americans are still breesily confident the pressure will rise again before it reaches Normandy. But the team from the admiralty, who were previously sitting on the fence, have now decided they agree with the Met Office. It's time for Stag to deliver his forecast to the Allied commanders. At 9:30 that evening, Stag sets off for the library at Southerk House.

On his way, he passes through the Grand Hall, a majestic example of Georgian architecture. High ceilings, marble pillars, a sweeping staircase, an elegant but imposing place. Inside the library, he finds General Eisenhower, flanked by Air Chief Marshal Arthur Tedder and Chief of Staff, General Walter Beddell-Smith. Seated opposite them on rows of armchairs are the rest of the military chiefs. Despite the amount of gold braid on display, the atmosphere in the room is relaxed. You'd better have good news to share one of the general's teases stag. He smiles weekly. In his measured, emotionless way, he begins his briefing. Low pressure drifting eastwards, heading for the Normandy Coast. High winds, thick, low cloud, rough seas, poor visibility. Worst of all, it looks set to hit the beaches just as D-day is scheduled to begin. Eisenhower sits perfectly still in his chair. Could there be a mistake in the readings, he asks. Perhaps tomorrow the picture will look different. No, says Stag. There's nothing to suggest that. The beach landings have been months in the planning. The manual for Operation Neptune runs to over a thousand pages. The logistics are dizzying. But at its heart, This mission is about getting men ashore, trusting their lives to purpose-built landing craft.

Now, a pen-pushing scientist is saying that that plan might not work after all.

Large ships with a deep draft could clearly sail in a storm. Landing vessels are a completely different situation because landing vessels do not have a deep draft. They can be easily swamped in high seas. In other words, to deploy a large fleet in the channel would have been possible even if the weather had been awful. But to actually deploy a landing force would have been a different matter.

It's not just the Seaborne invasion that's affected by Stag's forecast. Operation Overlord relies on airborne missions, too, and they need clear skies.

In 1943, in the landing in Sicily, it had gone wrong, and the airborne troops were scattered over an enormous area. They didn't want that. They wanted a much more tight landing on target. It's not going to help you if you're landing in the wrong place.

For General Eisenhower, it's a stark choice, stick or twist. He knows his men have already started to embark. Warships from Scotland and Northern Ireland have begun their long voyage towards the channel. The first troop convoys will set sail early in the morning. This is pretty much the last chance saloon. James Stag can see the Supreme Commander is on edge. His anxiety is palpable. Eisenhower decides to defer the decision by a few hours at least. They'll meet again tomorrow morning at 4:15, punishingly early for a Sunday. But that's the latest he can push it, the last possible moment when he can still hold back the ships. Stag doesn't sleep a wink that night. He doesn't even try to. Instead, he writes a lengthy account explaining the reasoning behind his forecast. He knows the data isn't conclusive. If his prediction turns out to be wrong, he needs the world to know why he called it the way that he did.

He's writing from the point of view of how he wants to be remembered. We can tell that because he's written on the front of it, Keep, historical document. He was a very stressed individual, and I can understand why. Eisenhower, of course, makes the final decision, but it's based solely on the weather advice. Everything else is in place. It all depends on the weather.

Before sunrise, everyone is back at Southerners House. If there is a storm on the way, it hasn't reached the south of England yet. The night sky is crystal clear. The air is still. But when he returns to the library, Stag delivers the news nobody wants to hear. The forecast for D-Day hasn't changed. A deflated Eisenhower goes around the room. One by one, he asks the military chiefs for their advice. Go or no go. Field Marshal Montgomery is bullish. Go, he regardless of casualties. Admiral Ramsey is on the fence. Stag's forecast certainly isn't ideal, but the beach landings might still be possible. But Air Chief Marshal Tedder is adamant. With poor visibility, his operation won't work. He believes there's no option but to postpone. Reluctedly, Eisenhower calls off the invasion, but it's not canceled yet. Instead, he decides to delay by 24 hours, hoping against hope for a last minute break. Aside from the right weather conditions, the Allies need a full bright moon for the initial airborne phase of the invasion, plus a low tide to protect the landing craft from the mines, spikes, and other obstacles the Germans have placed on the beaches. The window of opportunity is closing fast.

If there is no let up in the weather before the seventh of June, D-day will have to be postponed by at least two weeks. The element of surprise may be lost altogether. With the decision taken, those ships that have already begun their voyage must be recalled. A code word, Bosprit, is sent out to all vessels, but not all of them get the message. Already on their way to Utah Beach, an entire convoy of ships either doesn't receive or doesn't understand the code. In the nerve center of Suddic house, alarm sets in. An attack by one group of ships alone would be suicidal, but worse, it could blow the entire plan for D-Day, tipping the Allies hand to the Germans. An American destroyer, the USS Forest, is dispatched to round up the errant ships. Mercifully, it catches them in time. The convoy turns around, and the Germans are none the wiser. For the D-day planners, The last 24 hours have been a crushing disappointment. But the fick British weather has one more surprise in store. Just hours after Eisenhower delays the invasion, new data arrives from the Atlantic weather stations, including Maureen Flavin's post office in Blacksard.

Once again, Stag feels his pulse quicken.

On the morning of the fourth of June, they started to see from the data some hopeful signs. Again, it all comes down to pressure. So this time around, rather than the pressure dropping and indicating bad weather, the pressure started to rise. High pressure brings settled conditions. It brings lower winds, less rain fall, the conditions that they would have wanted to send boats across the channel in.

If this data can be trusted, it could change everything.

In Stag's diary, he's asking himself the question, Is it enough? Is this enough of a weather window?

On Sunday evening, Stag delivers his new forecast to Eisenhower and the other military commanders. There will be a brief period of improved weather from Monday afternoon, he predicts, meaning a landing on Tuesday morning might just be possible. The room is a buzz. Eisenhower's chief of staff is astonished. Looks to me like we've gotten a break we could hardly hope for, he tells his boss. It's a hell of a gamble, but it's the best possible gamble. Admiral Ramsey urges a quick decision. If the word is go, the Navy must be informed within half an hour. There is so little time, so little certainty. But Stag has done his part. All he can do is bring the latest data and do his best to read the tea leaves correctly. Eisenhower sits in silence for a moment. He then asks General Montgomery for his opinion. Monty's reply is predictable. I would say, go. It's approaching 10:00 PM on Sunday evening. A decision must be made. The Supreme Commander makes up his mind. We must give the order, he declares. I don't like it, but there it is. Even now, though, there's a chance of one more reversal. A final conference is scheduled for tomorrow morning, at half past 3:00 this time.

That will be the true point of no return. As Eisenhower leaves the library, he has some parting words for Stag. For heaven's sake, hold the weather to what you've told us, and don't bring any more bad news. For the second night in a row, Stag doesn't sleep a wink. Slowly, slowly, 3:30 AM rolls around. The early hours of Monday morning find Southwick house under assault from dreadful weather. Wind that bends the trees, rain that flies through the air in horizontal streaks. Stag seems almost relieved. If the invasion force had arrived in this, the Germans would have driven them straight back into the sea. The storm he forecast for today has arrived on cue. But what about tomorrow? As he walks into the library, Stag sees those same familiar faces. The military commanders were splendent in their braided uniforms. There's no teasing or joking today, though. There's too much at stake. Eisenhow is on his feet, pacing. The rain and wind off at the windows. Stag delivers the latest forecast, based on data collected overnight. It's good news. That pocket of high pressure is still there. It looks like the storm will break by tomorrow morning.

Eisenhower thinks and paces, paces and thinks. He is a man of warfare, of action. This peculiar, capricious science is not his territory, but he trusts Stag. He admires his precision, his attention to detail, his lack of fuss. Okay, says the Supreme Commander. Let's go.

I think no one was confident that the weather would hold. They were all praying that it would.

Totally total uncertainty. I think there's possibly more confidence from the Joint Chiefs than there is from the forecasters themselves.

James Stag felt that the window would last for maybe 9, 10 hours on D-day, which would be enough time for the ships to cross the channel, get to the beaches, and land the men. Thereafter, they had no idea what the weather was going to do.

In the next episode, a mysterious radio broadcast summons help from behind enemy lines. British spies work with French resistance operatives to sabotage key German targets, and a wave of brutal reprisals horrifies the people of France. That's next time on D-Day.