D-Day: Weather and History Intersect

This coming Sunday, we will celebrate the 77th anniversary of the D-Day invasion, which was a major turning point in World War II. If you are familiar with history, you may have heard the many tales of how crucial the weather forecasts were for this operation. I personally knew the basics facts of how the invasion was delayed a day due to weather, and that it was an extremely tense and high-pressure decision when General Dwight D. Eisenhower made the ‘go’ call on June 6, 1944. However, I learned some really interesting information as I dove into this topic more deeply for this blog. I hope that you find it as interesting as I did!

Courtesy: National World War II Museum.

The D-Day invasion: key facts

Allied invasion plan. Courtesy: Vox.com

D-Day is considered the onset of the Battle of Normandy (June-August 1944). Plans for an invasion into German-occupied France across the English Channel had been in the works for a long time. Thus, the Germans were building what is known as The Atlantic Wall– a 2000+ mile network of fortifications, landmines, and other obstacles along the Normandy coastline. General Eisenhower was commander of the Allied operation, known as Operation Overlord.

A bunker from the German Atlantic Wall.

Key to the success of the operation were a near-full or full moon (for visibility) and low tide at dawn (to expose underwater obstacles placed by the Germans). As such, there were precious few potential dates to begin with- June 5-7 and June 18-20. Add to that the need for clear, dry weather and calm winds and you can see how critical the weather forecast was to the success of the operation. The team was made up of meteorologists from the British Royal Navy, the British Meteorological Office, and the U.S. Strategic and Tactical Air Force (source: history.com). Secrecy was critical to maintain the element of surprise.

Courtesy: Military Times.

Weather and logistics

The English Channel can be characterized by difficult weather conditions at any time of the year. In the Air Force publication The English Channel: Meteorology and Climatology (1943), it is noted that ‘extended periods of weather suitable for specific military operations are possible in all seasons; conversely, extremely undesirable weather conditions may be encountered at any time”. For example, this week in the Channel, sea surface temperatures are in the 50s (Fahrenheit), daily high temperatures are forecast to be in the 50s and 60s, and winds are between 15 and 25 miles per hour.

English Channel weather forecast for D-Day Anniversary 2021. Courtesy: weather-now.co.uk.

In particular, it is noted that bad conditions can occur during frontal pasages, and that visibility issues may occur about 1/7 of the time during any season. Further, winds which were light enough to support parachute landings were only expected about half of the time, and, perhaps most telling: “favorable periods…are frequent in no season but are possible in any season”. Doesn’t sound like a place I would like to be forecasting for a multi-faceted invasion with only a few day window on either side! Keep in mind, this just considers the meteorological logistics- compound that with the logistics of (secretly) positioning the hundreds of thousands of troops in Portsmouth, England and the actual military planning that was involved in the operation, and one can see what a feat this was.

Courtesy: BBC/Getty Images.

A deeper look

The oft-told story is that of meteorologist Group Captain James Stagg, who was a UK Met Office Meteorologist with the Royal Air Force and appointed the Chief Meteorological Officer for Operation Overlord. When Captain Stagg met with General Eisenhower on June 4, a low pressure system was causing poor conditions in the channel.

Group Captain James Stagg. Courtesy: History.com

British forecasters were relying on their knowledge and observations of the English Channel and surrounding areas, while American forecasters were relying on a historical analog approach (looking for similar past weather events). As is often the case, differing methods resulted in slightly different forecasts- and this was before a time of computer forecast models! There were differing opinions even as to what month to conduct the invasion – the Americans were in favor of May 1944, while the British wanted to wait until August. When the time came to decide ‘go’ or ‘no-go’ on June 5, there was still dissention. While the Americans were in favor of continuing with the plans for invasion on June 5, British forecasters were pushing for a delay of one day. It fell on Group Captain Stagg to make the final recommendation to General Eisenhower, and in the end he recommended a delay of one day, to June 6.

German meteorologists, across the Channel were at the same time trying to forecast the weather in the region, knowing that an invasion was possible at some point. They, too, were constructing weather maps using observations- from on land, from listening in to British communications, and from U-Boats scattered around the Atlantic. Their forecasts for this time periods were that gale-force winds would arrive on June 5 and continue for several days, which likely contributed to their lack of anticipation of the attack.

German surface analysis for June 5, 1944. Courtesy: UK Met Office.

At issue was a low pressure system which tracked over Ireland and Scotland, causing wind gusts over 60 miles per hour. The break in the weather which was forecast for June 6 did not materialize as early in the day as was forecast, and so when the invasion began the first paratroopers were still dealing with clouds, rough seas and winds. The conditions meant difficult travels for many of the ships crossing the Channel, with soldiers enduring hours and hours of seasickness. However, the effect of surprise was achieved, and the poor weather likely helped with that.

Courtesy: BBC.

An interesting tale relating to the weather forecast is that of Maureen Flavin (Sweeney), who was taking weather observations at an Irish lighthouse in Blacksod on her 21st birthday, June 3, 1944. She, along with the lighhouse keeper, Ted Sweeney (whom she would later marry), submitted weather reports to the UK met office which attested to the incoming area of low pressure which would impact the Channel near the target dates. These observations were key to the forecast- and as someone who relies on volunteer meteorological observations to do my work today, I have great appreciation for the diligence of Ms. Flavin, Mr. Sweeney, and the many others who helped the Allied cause in this way.

Maureen Sweeney. Courtesy: The Times.


In my research for this blog, I came across an interesting article in the Bulletin of the American Meteorological Society, in which the authors call into question some of the commonly held beliefs, some of which I discuss here (e.g. the analog forecast system used by the Americans, the ability of the Germans to construct weather maps of the Channel, and whether Stagg’s recommendation to delay was actually ‘right for the wrong reason’. It is an excellent read, even for non-meteorologists.

It is striking to me that these forecasts were made without the benefit of technology that we as modern-day meteorologists take for granted: radar, satellite, and instantaneous weather observations.

In all, by June 11 (less than one week after the initial invasion), the beaches were secured, and over 300,000 troops, plus equipment, were onshore in Normandy. The story of course continues, that the surprised Germans were forced into a messy retreat, and the Allied troops made steady progress over the coming months to retake France from German control. Although the loss of life was staggering, the contribution of these brave soldiers, in addition to the countless men and women who aided in the planning and coordination that an attack of this magnitude required, helped to turn the tide of the war.

Normandy is definitely on my ‘bucket list’ of places to visit someday. Researching this blog has only made me more curious, and I plan to add this topic to my summer reading books this summer! I hope that you find this narrative inspiring as well!

Courtesy: Rick Steves/MaryAnn Cameron.

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