The largest amphibious military operation in history is known as Operation Overlord, the invasion of Normandy on June 6, 1944.
Few military operations in history were as dependent on the weather. The new movie “Pressure,” which premiered on May 29, takes you to the nerve center of one of the most important weather forecasts issued, one that would change the course of history one way or another — the weather forecast that gave General Dwight D. Eisenhower the narrow window needed to launch the D-Day invasion and change the outcome of the war.
Set in a military base in the Southwick House in Hampshire, England, right near the southern coast in the days leading up to the invasion, the focus is primarily on Scottish meteorologist James Stagg (Andrew Scott), the chief weather adviser to Eisenhower (Brendan Fraser), and Stagg’s efforts to provide guidance on whether the forecast would cooperate with a massive, ocean-based invasion.
I will steer away from most spoilers, but I’ll tell you one thing: The requests, or orders, from Eisenhower, as depicted in the film, made Stagg’s job exceptionally difficult, if not impossible. One line stuck with me in particular in Stagg’s response to his meteorology team — “If we can measure it, I want it.”
At the time, forecasting technology was primitive compared to what meteorologists have access to today. There were little radars, no satellites or computer models, and certainly no ensemble guidance, which blends all of our current technology to provide possible forecasts. Meteorologists back then relied on sparse weather balloon data, ship observations, and surface observations to create handmade surface maps to interpret atmospheric patterns. I mention all that to say this: Their job was exceptionally difficult.
And yet, the forecast had to be nearly perfect. Months of preparation, draining resources with extensive troop and equipment mobilization, depended on it. Plus, the Allies faced a race against time as fears increased that the Germans were getting hints that an attack was imminent.
The original invasion date was planned for June 5, 1944. But there were signals of a couple of low-pressure systems crossing the Northern Atlantic that concerned Stagg in giving the green light for invasion.
The movie does an excellent job of showing the differing approaches in meteorology at the time, which led to major rifts within the forecasting team in the hours leading up to D-Day. Back in the ’40s, meteorology depended on pattern recognition: comparing previous examples of weather conditions on a specific date and weighing the odds for what type of weather was likely for a particular date. Irving Krick (Chris Messina in the film), a cocky American meteorologist, leaned into these “analog years” and remained steadfast that the weather would be near perfect for the planned June 5 D-Day.
However, Stagg became known for his modern (at the time) forecasting methods and leaned heavily into upper-level measurements to identify weather features like the jet stream and areas of low and high pressure. Plus, Stagg knew the region and understood how the weather behaved locally, which was what led to his appointment to lead the weather team.
Simply put, the conversation centered on fair or poor weather on the morning of June 5.
Watching it as a meteorologist, I thought the movie did an excellent job showing how weather forecasting is often about uncertainty management rather than guarantees. The skies were blue and sunny on the days Stagg was urging Eisenhower to postpone, creating an upheaval of disbelief across most military branch leaders. The mounting pressure was intense as he insisted on the accuracy of his forecast while attempting to manage the response by military members and his colleague and fellow meteorologist, Krick. Stagg was even approached privately and practically threatened if he did not give the green light for the June 5 invasion.
Under immense pressure to give the go-ahead for an invasion on June 5 when the military wanted to take advantage of a full moon, Stagg instead warned Eisenhower that conditions would be too dangerous for a successful amphibious landing and airborne assault. The data he gathered, which came from over 1,000 weather stations within a more than 2,000-mile radius, predicted rough seas, strong winds, and widespread low cloud cover over the English Channel and the French coast that would have doomed the invasion and cost countless lives. Eisenhower reluctantly called it off.
Then, perhaps the most important phone call in history came into the weather office. Twenty-one-year-old Maureen Sweeney noticed an uptick in pressure, meaning better weather west of the English Channel and Normandy. This presented a window to Eisenhower to invade the next morning, on June 6. Otherwise, the invasion would have had to wait another two weeks for prime moonlight and ideal tide heights. By then, the Germans would definitely know the attack was coming.
I actually wish they featured Sweeney a bit more in the movie, because it was also her station on the west coast of Ireland that spotted the worsening weather to influence Stagg’s decision to defer the June 5 invasion. Much of the work by Stagg, Krick, and their squad of researchers rang true — the era-appropriate weather balloons, the frantic telegrams, Morse code, and phone calls to weather stations throughout the United Kingdom — but it was a shame not to see Sweeney’s contributions depicted, when it was her work that changed the course of the plan. There should have been more to highlight Sweeney in this film, but that really is my only qualm.
Anyway, remember how I mentioned two storms earlier in this article? Well, the data Sweeney reported revealed a temporary sliver of high pressure between the two storms, giving a tiny window to launch the invasion on the early morning of June 6 — and the rest is history.
If the invasion had been called off, an even stronger storm would have ruined plans two weeks later.
Here are a few specific weather elements that were critical for D-Day:
- Moonlight: A moonlit night was necessary for airborne paratroopers to have sufficient light to drop in the night before the beach landings began.
- Tides: The invasion needed a low enough tide shortly after dawn, exposing German beach obstacles while having enough water depth to approach the shore.
- Visibility: The invasion required synchronized operations on an enormous scale with at least 3 miles of visibility. Low clouds, fog, and rain would certainly disrupt coordination between naval, air, and ground forces. The Allies needed to see with thousands of ships and aircraft.
- Wind speeds and wave heights: The window was far from perfect. The most impactful weather was the strong winds and rough seas. This made aerial bombardment accuracy drop, and left many soldiers ill from motion sickness. The choppy waves also made it extremely challenging to unload tanks and other supplies from the boats.
All in all, it’s pretty wild to think the forecast was largely correct. Conditions on D-Day were far from ideal — the soldiers still faced choppy seas and wind gusts — but they were manageable enough for the operation to proceed. And talk about pressure — I can’t imagine what it must have been like to be a soldier on D-Day.
“Pressure” is a reminder that forecasts have always carried enormous weight. Sure, forecasting an impending landfalling hurricane has more impact than telling you to pack an umbrella — but the pressure to get things right certainly remains.
As meteorologists, we’ve taken an oath to do our best to protect lives and property through accurate forecasting and likely implications. The troposphere, where weather happens, is about 40,000 feet high in the sky. There are so many pieces to the puzzle. We sort through these pieces multiple times a day to deliver the best possible forecast.
I can’t imagine the array of emotions and pressure Stagg and the team of meteorologists felt during the hours leading up to D-Day, but they got it right — and that’s what matters.
When John F. Kennedy asked Eisenhower what led to the success of Operation Overlord, Eisenhower replied, “We had better meteorologists than the Germans.”
Ken Mahan can be reached at ken.mahan@globe.com. Follow him on Instagram @kenmahantheweatherman.