No radar, no satellite, no computer models. Just gut instinct and hand drawn maps.
A forecast made by hand, in 3 separate buildings in the United Kingdom able to communicate only by telephone.
Oh, and the largest invasion by land and sea was resting on the idea the forecast was right.
Quite the tall order but for 3 small forecast teams the liberation of Europe relied on exactly that.
To many meteorologists the forecast for Operation Overlord, the codename given to the invasion by land and sea in Normandy was the forecast to beat all forecasts.
Did you know the 6th was not supposed to be the invasion date? It was delayed, from June 5th to the 6th.
Low clouds obscured the landing zones from the air making it impossible for paratroopers to drop accurately.
Strong winds churned the seas to the point landing craft, the infamous Higgins boats with their square shape and large ramps, bucked about with no way to drive for the beaches.
A storm off the western coast of England was sweeping down the eastern Atlantic and was expected to continue obscuring the French coast for the "original" D-Day on June 5th.
3 forecast centers, a U.S. office codenamed "WIDEWING", and 2 U.K. offices codenamed "DUNSTABLE" and "ADMIRALITY" were set with the task of forecasting the invasion.
The downside was their locations...nowhere near each other.
"WIDEWING" was located on the western outskirts of London.
"DUNSTABLE" was north of the city while "ADMIRALITY" was smack dab in the middle of downtown London.
The 3 forecast centers could communicate only by telephone and would never meet in person as they prepared the most important forecast ever created.
And no, there was a point they did not agree.
The offices were split 2-1 on postponing the invasion when they convened by phone on June 3rd.
Group Captain J. M. Stagg was the meteorological advisor to General Dwight Eisenhower and gave him the news he had been dreading, the forecast was grim and the invasion would take place without air cover or accurate ranging for cannons on battleships in the English Channel .
So the invasion was postponed, in theory at the time, until the 6th unless conditions stayed terrible.
It was a 24 hour wait-and-see approach for the largest invasion in history, not exactly ideal when attempting to liberate a continent.
The allied commanders met with the meteorological team again after 4am on the morning of the 4th.
The forecast was still ugly so the invasion was officially postponed to June 6th.
The meteorological staff continued to coordinate, or attempt to, through the day on the 4th and another meeting of the commanding staff advised by Captain Stagg was held that night.
No sleep for the forecasters because the morning of the 5th held yet another meeting, and this one had big results that would go down in history: The storm was moving and the skies would clear enough to begin the invasion on the morning of the 6th.
D-Day, troops from 3 countries, and liberation would come to Europe.
General Erwin Rommel, who believed and had been advised by German forecasters that the weather was too ugly for the invasion to take place, left France for Germany to attend his wifeâ??s birthday.
He was supposed to return to Normandy on June 8th after the Happy Birthdays had been said but by the 8th the allies had punched through the main German line and held Normandy.
On the morning of the 6th the forecast was right and the invasion moved forward, but it was not as smooth as expected.
The skies still held low clouds for the massive air-force dropping troops over Normandy, except they were more scattered giving the pilots some ability to see.
This environment allowed planes to drop out of the clouds and lend surprise to their side, but also made it difficult to plan their drops in advance thus scattering their troopers across Normandy in some areas miles away from their intended landing zones.
The beach landings got a wild ride to shore, winds still bringing waves high enough to pitch the boats about to the point many soldiers were sea-sick when they landed.
However, the clouds cleared as the assault began so while the seas were rough the ships in the channel were able to target more accurately.
This is a forecast that could have gone drastically wrong.
The 3 teams were accurate with far less equipment and technology than we use today.
This is a great example of the first instance of ensemble forecasting, using multiple forecast methods and blending them together to come out with likely outcomes.
This method would eventually be used in Ensemble forecast models we use today.
The war would eventually lead to advances in radar technology which lead to our weather radars we use.
So 70 years ago, on the 1,738th day of World War II, the largest invasion began thanks in part to some of the first modern meteorologists and their determination to get it right.