If Gen. Dwight D. Eisenhower had had his way, D-Day would have unfolded on the beaches of Normandy on June 5, 1944.

But no military commander can control the one element that always proves crucial in the timing of any invasion: Mother Nature.

So, in the face of a nasty forecast that winds would be too high and clouds too low over the English Channel and the French coast, Operation Overlord, the Allied assault to reclaim Europe from the Nazis, was ultimately put off until June 6.

In the end, the invasion that will be commemorated with 70th anniversary ceremonies in France on Friday unfolded after meteorologists using a controversial forecasting technique sensed there would be an unexpected break in the weather. [To see how D-Day unfolded in real-time, follow CBC's live D-Day Twitter feed June 4 to 6 showing what troops were doing at different points during those days in 1944.]

"Weather is always a great weapon if you can get it right," says David Phillips, Environment Canada's senior climatologist.

Canadian troops cross the English Channel for the D-Day landings in northern France on June 6, 1944. (National Archives of Canada/The Canadian Press)

In the 70 years since 150,000 Canadian, British and American troops stormed those French beaches, some have called the D-Day forecast the most important ever made by meteorologists.

Phillips won't go that far. "But when you [think] what was at stake, it was clearly a turning point in the war," he says.

"The forecast was just right enough. Things did unfold as scripted by the forecast, and while conditions were marginal and certainly losses were there because of the weather in the sea and the swell and waves, it provided that opportunity" for eventual success.

Low tides and good visibility

Operation Overlord had been months in the planning, and Eisenhower, the supreme Allied commander, had his sights set on June 5 to launch the offensive.

A view from the 2nd Canadian Flotilla shows ships en route to France on D-Day. (National Archives of Canada/Canadian Press)

Ideally, the Allies wanted to hit the beaches when the tides were low, winds from the northwest were light and visibility was good. June 5-7 was a prime time from the tidal point of view, as was a three-day window two weeks later.

But meteorologists advising the Allied forces couldn't ignore what their data was telling them about June 5: a cold weather front would likely be rolling in over the English Channel that day, bringing with it winds of up to nearly 50 km/h and a lot of low cloud cover.

"Strong winds and high waves would make the naval landing impossible, and low-level clouds would prevent pilots from seeing targets for dropping paratroopers and bombing enemy lines," says CBC News meteorologist Emilie Benoit.

So, in the early hours of June 4, Eisenhower scrubbed the mission for the 5th.

"If they had gone on the 5th, it would have been catastrophic," says Phillips.

No satellites and no computers

The forecasters making the D-Day predictions seven decades ago didn't have anything close to the scientific and technical resources available today.

Look out the window Weather forecasting may have been crucial for the D-Day landings, but in Canada during the Second World War, such predictions weren't generally available. "Public forecasting, the kind of forecasts you and I get today, were prohibited," says David Phillips, Environment Canada's senior climatologist. "It was felt that would give the enemy an advantage to know what conditions were like ... in Canada and the United States." So people had to look out the window to see what the weather might be like. There were exceptions: frost warnings were given, and some information was available for fishermen heading out on the water.

"There were no computers or satellites to aid in forecasting. The radar technology that we use today to track storms simply wasn't used for this purpose in the 1940s," says CBC News meteorologist Jay Scotland, who along with staff in the CBC Weather Centre did some historical research into the weather and forecasting of June 6, 1944, for the purpose of this article.

"In fact, rain or snow was seen as a nuisance by radar operators because it made it difficult to identify enemy aircraft."

But forecasters did have some advantages beyond their observations and intuition.

"In June of 1944, the Allied naval presence in the Atlantic at least allowed for some ability to track storms at sea heading towards Europe. This gave forecasters at least a bit of an edge on Mother Nature," says Scotland.

And some of the forecasters who were advising the Allied forces were favouring a newer, synoptic forecasting technique that was controversial at the time. That technique, now widely used today, relied on gathering more data, particularly from reconnaissance planes that flew higher in the atmosphere.

On the afternoon of June 4, Phillips says, meteorologists noticed a change in the weather seemed to be afoot. Rising air pressure was observed west of Northern Ireland.

"It threw the forecasters into confusion," says Phillips. "They went back to the drawing board."

'Window of possibility'

Three teams of two meteorologists each were advising the Allies: one American team, one from the Royal Navy and one from the British weather office. The British weather office team included Sverre Petterssen, a Norwegian air force officer who had been seconded to the U.K. service and who favoured the newer, synoptic approach to forecasting.

Canadian soldiers land on a Normandy beach during the D-Day invasion on June 6, 1944. (Department of National Defence/The Canadian Press)

Petterssen, Phillips says, thought that a storm to the west would slow down, creating a "window of possibility" for Operation Overlord.

"He was the one that saw the pressure change, that reorganization of the atmosphere," says Phillips.

Late on June 4, Group Capt. James Stagg, Eisenhower's chief meteorologist, met with the commander and said the weather was improving.

At 4:30 a.m. on the 5th, Stagg was back with Eisenhower, telling him the high pressure was holding and the decision was made to go on the 6th, Phillips says.

"The forecast was I think accurate. The weather was not ideal, but it was good enough, so to speak."

Off the English coast, the temperature was 12 C at 7 a.m. on Tuesday, June 6. Temperatures were 15 C on the Normandy beaches at 1 p.m. Clouds cleared over the coast by the afternoon.

Rough seas

That's not to say, however, getting over the channel was easy for the forces.

Personnel of the 9th Canadian Infantry Brigade land at Bernieres-sur-Mer, France, on D-Day, June 6, 1944. (National Archives of Canada-Gilbert Alexander Milne PA137012/Canadian Press)

"The naval fleet endured a challenging crossing, with winds from the northwest at 25 km/h and one- to two-metre waves," says the CBC's Benoit.

The assault seemed in ways to catch the Germans off guard.

"It's quite likely that little window of opportunity was not recognized by the Germans," says Phillips, noting how some of their troops had stood down, planes had been grounded and some officers were on leave.

"I think that was a big factor in the outcome."

The Germans were also lacking whatever information they might have gleaned from weather stations they had set up years earlier across the Atlantic Ocean in Labrador. Those stations had been dismantled or their batteries had failed, said Phillips.

Now, Phillips says, when people look at the work done by the D-Day forecasters and model it using today's means, "they wouldn't have come up with a more accurate forecast."

"These guys were heroes in a way."