D-Day Special: The Power of Observation

In the digital age, human and analog functions are being replaced at an exponential rate. Our world is now one of ultra-expedient computers, satellites, and communications—where machines teach other machines to perform better, human cognition is being captured and realistically replicated with code, and details of the earth that were once only visible under the iron fist of gravity can be seen from distances that merely brush the fingertips of gravity’s grasp. It’s astounding. But I’ll never be convinced that the human eye and mind aren’t capable of miraculous things.

The digital age has made the field of meteorology increasingly reliant on technology to churn out atmospheric predictions faster, with more accuracy, and farther into the future than before; however the future is nothing without the now. Numerical weather prediction (NWP, also called weather modeling) still requires the input of current weather conditions as a foundation before cranking through billions of calculations (I dunno, I’m not a modeler, call it a ballpark figure) and building a fancy graphical depiction (let’s be real, it’s just a map) that makes academic doctorates gaze at it in teary-eyed wonder (sorry not sorry Docs, ops for life!). Predicting the future must take the present into full account. This is a meteorological and existential concept that applies in 2023 as much as it did in 1944, and the Allies knew it.

The Allies needed very specific astronomical, meteorological, and oceanographic conditions for their complex amphibious and airborne invasion: a full moon, a low tide, a calm sea state, surface winds less than 20 mph, minimal clouds or a ceiling above roughly 10,000 feet, and no precipitation or fog. A big ask in the English Channel region. But these conditions ensured:

1) Obstacles on the beaches were illuminated and exposed.

2) Airborne troops and gliders could see and accurately hit their landing locations.

3) Amphibious landing craft and critical supply boats wouldn’t capsize.

4) Mortars could be accurately fired from ship to shore.

5) Troops could quickly maneuver without being bogged down by wet equipment.

6) Air support could effectively suppress enemy tanks, artillery, and infantry reserves.

These criteria were given to Royal Air Force Group Captain James Stagg, who supervised a team of six meteorologists. The six were further paired into three small teams: Lawrence Hogben and Geoffrey Wolfe from the Royal Navy, C.K.M Douglas and Sverre Pettersen from the U.K. Met Office, and Irving Krick and Ben Holtzman from the USAAF.

The astronomical predictions were the easiest to calculate well into the future, so these were run first. Invasion dates were narrowed down to 5-7 June or 18-20 June. General Dwight D. Eisenhower, the Allied Supreme Commander, wanted to launch the attack as soon as possible to ensure they maintained both elements of surprise—location and time. The location element was being covered by an elaborate deception plan known as Operation FORTITUDE designed to make the German High Command believe the invasion would occur across the narrowest portion of the English Channel into Calais, France. Eisenhower named 5 June as D-Day to secure the first available time element.

On 29 May, the timing of 5 June began to fall into question. Weather observations recorded from across the Atlantic, in Newfoundland, indicated weather in the English Channel on Eisenhower’s chosen date would not allow the invasion to succeed. This information was further validated between 30 May-3 Jun using the Allied weather observation network. The network consisted of fixed weather stations in Canada, Greenland, Iceland, Scotland, and—by secret agreement—neutral Ireland, as well as observations from ships and planes crossing the Atlantic.

Early on 4 Jun, with calm conditions overhead and mere hours until the start of the invasion, Stagg convinced Eisenhower to postpone for 24 hours. By mid-day conditions had turned into gale-force winds and driving rain, with wind speeds as high as 60 mph observed along the Scottish coast. On the evening of 4 Jun, based on a critical observation from a post office at Blacksod Point, Ireland, the team forecasted a short break in the conditions that afforded the Allies access to the beaches on 6 Jun, before another surge of poor weather on 7 Jun. Although it was still blustering and pouring outside, Stagg informed Eisenhower of the marginal window of opportunity, winds and waves would lull and the ceiling would rise. It wouldn’t be perfect, but a marginal go is still a go. Eisenhower agreed to stick with 6 Jun and slept for a few hours before the final weather call the next morning.

Surface analysis charts on 4 June 1944.

On 5 Jun, around 4:00am, Stagg confirmed high pressure was building in and would hold long enough for the Allies to safely cross the channel and insert forces. Eisenhower reviewed the information one last time, and issued the order: “Ok, we’ll go.” Shortly after, the massive air and sea convoys launched, made their 17-hour crossing, and landed in Normandy early on 6 Jun.

1) General Dwight D. Eisenhower's Order of the Day, issued 6 June 1944;

2) Aerial photo of launch, just off the Isle of Wight (a place my grandfather mentions!);

3) Aerial photo of Drop Zone N after gliders & airborne troops landed.

They certainly encountered less-than-ideal conditions. Cloud cover and winds made bombs and airborne troops miss their marks, some landed quite far from their intended targets. The marginal sea conditions caused some landing craft to capsize or made it impossible for them to reach as far up onto the beach as intended. It was not without its challenges and risks, but the risks associated with waiting were greater.

The most remarkable aspect of it, from a modern-day meteorologist's perspective, is Stagg and his team did this without computers, satellites, radar, models, upper air data, automated weather sensors, or Skew-T diagrams and any of the associated indices. It was done (almost) strictly with the combined observational power of human eyes, analog instruments, the trans-Atlantic assets on hand, seven meteorologically-trained brains, and the cooperation of no less than six Allied nations. And it successfully informed perhaps the most consequential decision of the 20th century. That is truly astounding.

Fun closing fact: From 1895-present, June 1944 was one of the windiest on record in southeastern England, beat only by June 1917. Weather on 19 Jun, the D-Day backup date, was atrocious. Hogben described it as “the biggest storm of the 20th century”. Weeks afterward, Stagg wrote to Eisenhower letting him know that 19 Jun had been “the worst weather in the English Channel in two decades.” Eisenhower replied, “I thank the Gods of War we went when we did."

Also, this awesome video: https://www.abmc.gov/multimedia/videos/ok-lets-go.

Also, also this whole thing for anyone who wants to see what happens when modern methods are applied to replicate the forecast... Warning: it will induce maximum squint, ultra nerd goggles activate! https://www.ecmwf.int/en/research/projects/era-clim/d-day-analyses

Tango Sierra out!