D-Day landings and the most crucial weather forecast in history

by Kirsty McCabe, FRMetS

In June 1944, a team of meteorologists spotted a suitable weather window for the Allied invasion of France. But how did they do it without modern technology?

Over 80 years ago, meteorologists didn't have the satellites, weather radar, computer models and instant communications that today’s forecasters take for granted. Instead, they relied mainly on surface observations from military and civilian weather observers in Britain and western Europe, as well as a few military observers at sea. Predicting the weather more than a day or two in advance was unrealistic, let alone predicting the exact timing, track and strength of developing storms out in the Atlantic.

For the invasion to have any chance of success, the conditions had to be right — a full moon, low tide, little cloud cover, light winds and low seas. In June 1944, a full moon and low tide coincided on the 5th, 6th and 7th. General Eisenhower, Supreme Commander of the Allied Forces, scheduled the invasion for 5 June, 1944.

However, weather observations taken on the morning of 3 June 1944 alerted the Allies to an approaching storm, throwing Eisenhower's strategy into chaos. The pressure was on to find a new suitable weather window. As meteorologists drew up the charts on 4 June, they received an observation from a single ship stationed six hundred miles west of Ireland reporting a rise in the barometric pressure. 

Chief Meteorologist Group Captain James Stagg deduced that a brief ridge of high pressure would build after the passage of the storm, and told Eisenhower that they thought the weather would improve on 6 June. Not quite the conditions desired, but hopefully good enough it might catch the Germans off guard.

That forecast was a pivotal moment in world history. If the forecast was wrong, the lives of thousands of men and massive amounts of equipment would be lost. But, on the other hand, if the unsettled weather forecast for 5 June had not happened and the weather had been good, the Germans might have spied the massive build-up of forces along the coast of southern England.

The decision was taken to invade and convoys set out on 5 June in force 5 winds. After a seventeen-hour crossing, they arrived on 6 June to find that a brief weather window had opened, but conditions were rather more challenging than expected. The forecast may not have been perfect, but the decision to invade paid off — shortening the war and saving thousands of lives.

In a memorandum accompanying an official report to Eisenhower, Stagg noted that had the invasion been delayed until the next suitable tides two weeks later, the troops would have run into the worst Channel weather for 20 years. Eisenhower wrote across the top of the memorandum: "Thanks, and thank the Gods of War we went when we did".

Behind the Archives

The National Meteorological Archive includes the historic observations of the Met Office, private weather diaries, and climate observations from around the world. Catherine Ross, Library and Archive Manager at the Met Office, explains how these historic documents are cared for and the human stories they reveal.

As the Library and Archive Manager at the Met Office, and an archivist by profession, my role is to care for the globally important collections held in the National Meteorological Archive. 

Caring for these records includes ensuring their permanent preservation, storage in archival packaging such as acid-free boxing, and ensuring we maintain appropriate temperature and humidity conditions in our strongrooms. It also includes sharing the stories and providing access to the documents whilst ensuring they are not overhandled. One of the ways we do this is by providing scans of many of our most useful and most important records on our Digital Library and Archive, which includes the key D-Day records.

Whilst the data in the archive is hugely important we are also keen to share the human stories. Great care was taken over the drawing of charts and apart from the occasional coffee ring, blotted red ink from an errant plotting pen and corrected front which tells of the forecaster reworking their analysis, we don’t often see past the data to the people behind it. 

But we do perhaps get a hint of the men and women who spent hour upon hour plotting observations onto charts from the only chart which bears a title other than the date and time. We have found only one in the entire chart series and it is for 8 May 1945 at 0900. After so long plotting observations and understanding at least in part how those would go on to direct decisions which would decide the fate of allied soldiers, sailors and airmen, I can only imagine the joy with which a plotter broke with convention to ink the words ‘VE Day’ onto the top of that chart (notably 6 hours before it was announced to the nation by Winston Churchill).

If you'd like to know more about the D-Day landings and the role of weather forecasting in the military operation then watch Paul Gross's documentary "Forecast: Overlord", which was created on the 50th anniversary for WDIV ClickOnDetroit.