Operation Overlord Weather Forecasts

March 20, 2019 – Talk about pressure. Everywhere James M. Stagg (1900-1975), Gen. Eisenhower’s chief meteorologist, looked in the days leading up to D-Day (June 6, 1944), there was pressure – on weather maps, on conference calls, and most of all, on his shoulders. Stagg remained calm, trusted his experience and colleagues, and deftly delivered to Gen. Eisenhower, Supreme Allied Commander; Gen. Bedell Smith, Eisenhower’s Chief of Staff; and other Allied commanders what is considered the most important weather forecast in history.

Student News Net interview with Frank Marsik, Ph.D.

Frank Marsik, Ph.D., Associate Research Scientist and Lecturer in the Department of Climate and Space Sciences and Engineering at the University of Michigan in Ann Arbor, explains J.M. Stagg’s D-Day forecasts. Student News Net first learned of these forecasts from Ron Drez, historian, while on tour with the Stephen Ambrose Tours during the 65th anniversary of D-Day in 2009.

Hindsight can be used to inform future decisions but when stakes are high, hindsight is irrelevant. Few meteorologists will ever face stakes as high as Stagg faced.

Student News Net photo of Southwick House taken on June 4, 2009 (65th Anniversary of D-Day) while on tour with Steven Ambrose Historical Tours.

Allied leaders had assembled and trained a force of 156,000 soldiers, 5,000 ships, and 11,000 planes to launch an invasion against Hitler’s fortified “Atlantic Wall” along the French coast hugging the English Channel.

The exact date for the invasion was a heavily guarded secret but spring 1944 seemed likely. Weather conditions had to be good enough for ships to navigate choppy English Channel waters, for soldiers to land on a 50-mile stretch of Normandy beaches from the sea, for airplanes to fly with visuals of the ground, and for 13,000 paratroopers to parachute from those airplanes to land behind enemy lines.

In sum, a successful D-Day invasion depended heavily on favorable or at least, acceptable weather.

By June 1944, they were ready to go. Allied leaders set June 5th for the invasion. Allied command headquarters moved to Southwick House, a few miles north of Portsmouth, England where thousands of troops and ships were staged for the invasion.

How did Stagg prepare for the high stakes forecasts he delivered in the days leading up to D-Day?
James M. Stagg, Gen. Eisenhower’s chief meteorologist during D-Day, wrote Forecast for Overlord in 1971 (Photo: Student News Net)

“He did the perfect thing if you’re going to start forecasting for an area you’ve not usually forecasted before. And that is that he went and he started looking at the climatology of the area,” Frank Marsik, Ph.D., Associate Research Scientist and Lecturer in the Department of Climate and Space Sciences and Engineering at the University of Michigan, said to Student News Net during a March 7th interview about Stagg’s D-Day forecast and Stagg’s 1971 book – Forecast for Overlord.

Dr. Marsik defines the difference between weather and climate.

Weather is “the day-to-day or short-term variation of things like clouds and precipitation and winds and temperature,” he said.

Climate is a set of weather conditions that represent an average of weather data collected and studied over many years for a particular area. In meteorological terms, these averages are called “climate normals,” according to Dr. Marsik.

For every rule there is an exception of course. Unbeknownst to Stagg and his team as they studied volumes of historical weather data in the weeks and months leading up to D-Day, spring 1944 in the British Isles would be an outlier to those “climate normals.” Weather there acted as though it was mid- January, not June.

With abnormal weather, the fact that Stagg and his team did not have computers, satellites, cell phones, or radar, and some surface observations were from ships moving to different spots on the water, his forecasts were brilliant, scientific exercises.

Arriving at the decision to postpone D-Day

Stagg managed a team of meteorologists who were in constant disagreement about forecasts.

Dr. Marsik explains why disagreements were inevitable. British and American meteorologists were emphasizing the use of different conceptual models to formulate forecasts. The Americans heavily relied on examining historical data to develop an analog to predict future weather conditions. In short, they would look at past weather events, find a situation that closely resembled the current situation, and then used information about how the past weather event evolved to predict the evolution of the current event.

British meteorologists used a conceptual model, pioneered at the Bergen School of Meteorology in Norway, that followed the principle that weather systems tended to follow consistent life-cycle with respect to the evolution of the associated low pressure center and surface frontal zones. Forecasters were also beginning to understand the important role that conditions in the upper atmosphere played in the evolution of surface weather systems, thus the British meteorologists began to rely on this information as well.

With this new focus on upper atmosphere conditions, in the days leading up to D-Day, Stagg became concerned about discrepancies he identified. “To my mind important localities of rising and falling barometer at the ground did not occur where the pattern of upper winds and temperatures predicted they should occur.” (p. 75)

This World War II era barometer was given to Frank Marsik by a former Univ. of Michigan meteorology professor. Note on the left side, the barometer is marked as “Signal Corps, U.S. Army.” (Photo: Student News Net)

On June 1, members of Stagg’s team disagreed about the weather outlook as the invasion was just four days away. American meteorologists were optimistic but others had doubts. After mentioning he was not as optimistic because of the inconsistencies between upper level conditions and surface events, Stagg describes what he saw on the charts that really had him concerned. He writes:

“But to me the dominating feature of the whole weather situation which no one seemed to be emphasizing enough was the great area of high pressure which extended a third of the way round the Arctic Circle from the Rocky Mountains to the White Sea. From this reservoir of high pressure there extended southward tongues of potential disruption, one towards the Great Lakes of North America and another between Greenland and Iceland. In my reckoning those extrusions of cold air from the north must sooner or later break through into the westerly flow of winds which now extended across the Atlantic from Newfoundland to the western seaboard of the British Isles. If and when that happened the chain of depressions L2, L3 and their successors would be re-invigorated and the result would be a period of stormy weather over the whole area of the British Isles and north France.” (p. 75)

Sunday, June 4 forecasts

Based on Stagg’s forecast that he delivered to Allied leaders at their 4:15 a.m. meeting on Sunday, June 4th, Eisenhower postponed the D-Day invasion. A cold front west of Ireland moved south and along with other factors, Stagg predicted clouds at 500 feet for Monday, a no-go condition for airplanes that were needed to support soldiers storming the beaches. Eisenhower said: “If the air cannot operate, we must postpone. Are there any dissentient voices?” (Forecast for Overlord by James Stagg, p. 102)

Stagg was instructed to come back at 9:30 p.m. with an update.

Stagg and Yates slept for a couple of hours. When they awoke Sunday morning, they were relieved to see worsening weather as they had predicted.

At 1p.m. Sunday, Stagg studied new weather charts that confirmed Portsmouth, where thousands of soldiers and ships were staged, would have a stormy night with clouds, wind, and rain extending into Monday.

But Stagg noticed something else on the 1 p.m. charts. A low pressure system (a depression) had intensified over the North Atlantic likely leading to its delay in reaching the European continent. A barometric reading of 993 millibars was recorded in the center of the depression. The cold front had already moved across Ireland. With the now expected delay of an approaching low pressure system, there would be a brief period of improving weather. It might be just enough time to launch the invasion, Stagg thought. But Gen. Eisenhower would make that decision. Stagg made a conscious decision to never include in his forecasts an opinion about launching the invasion.

Although lengthy, the following is Stagg’s forecast he delivered at the Sunday evening (June 4) meeting after which Gen. Eisenhower said the invasion was on again. It would be confirmed in the wee hours of Monday, June 5th.

“Gentlemen, since I presented the forecast last evening some rapid and unexpected developments have occurred over the north Atlantic. In particular a vigorous front – a cold front – from one of the depressions has been pushed more quickly and much farther south than could have been foreseen. The front is approaching Portsmouth now and will pass through all Channel areas tonight or early tomorrow. After the strong winds and low clouds associated with that front have moved through there will be a brief period of improved weather from Monday afternoon. For most of the time the sky will then be not more than half covered with cloud and its base should not often be below 2,000 – 3,000 feet. Winds will decrease substantially from what they are now. Those conditions will last over Monday night and into Tuesday. Behind that fair interlude cloud will probably increase again later on Tuesday-Wednesday. From early Wednesday until at least Friday weather will continue unsettled – variable skies with cloud 10/10ths and base height down to 1,000 feet for periods of not more than 4 to 6 hours at a time but interspersed with considerable fair intervals. Wind will be westerly throughout; force 4 to 5 along English coasts, force 3 to 4 on French coasts and probably less, force 2 to 3, along sheltered stretches on that side of the Channel.” (p. 112)

June 5 forecast
Weather chart from June 5, 1944 – The rectangle is the English Channel and the area of the invasion. In his book, Stagg notes that the original weather charts were lost but these closely represent those original maps. (Photo: Student News Net at Southwick House in 2009)

Stagg and Yates went back to their tent in the woods behind Southwick House. “Two or three hours’ rest mainly spent thinking of what it all meant, then out again at 2 a.m. (Monday, June 5th) and back to Southwick House for a quick wash, shave and coffee.” (p. 115)

Stagg and Yates held a conference call with the rest of their team at 3 a.m. to discuss the next forecast.

Allied leaders, except British General Montgomery, were in battle-dress uniform for the 3:30 a.m. meeting. Stagg was told to deliver his forecast.

“Gentlemen, no substantial change has taken place since last time but as I see it the little that has changed is in the direction of optimism. The fair interval that has now reached here and will extend through all southern England during the night will probably last into the later forenoon or afternoon of Tuesday. In this interval cloud amount will generally be 3/10th or less, with height of base 2,000 – 3,000 feet. Winds along the assault coasts will not exceed force 4 or maybe force 5 locally and should be no stronger than force 3 for much of the time. Visibility will be good. Later on Tuesday cloudiness will increase to 10/10ths for a period and its base will be down to 1,000 feet locally. From Wednesday through to Friday weather will be variable – periods of completely overcast sky, with cloud base around 1,000 feet accompanied by strong winds up to force 5 or even 6 at times, will be interspersed with considerable fair to fine periods: in these good periods wind will be below force 4 and visibility will be good throughout.”

In his book, Stagg writes about the reaction to his forecast.

“The relief that statement brought into the room was a joy to behold,” Stagg said. (p. 118)

At 3:30 a.m. (0330 hours), Gen. Eisenhower said: “Okay, let’s go.” The D-Day invasion was on.

In the hall after the meeting, Gen. Smith talked with Stagg while Yates was talking with someone else. “You’ve given us a helluva break, Stagg: hold on to it and then you can go off on a week’s leave to get rid of those hollows under your eyes,” Gen. Bedell Smith said. (p. 119)

The weather chart from June 5th at 1 p.m. recorded a barometric pressure in the L6 low pressure system of 978 millibars, the lowest pressure ever recorded in June in the British Isles for the last 100 years.

When he wrote Forecast for Overlord in 1971, Stagg said he believed the team approach was the correct choice even though it was a process filled with heated discussions. His team members were all experts who contributed to the overall effort.

Stagg handled the highs, lows, and pressure on the maps with confidence because of his own meteorological might and handled the pressure on his shoulders with aplomb, a crucial combination Allied leaders recognized in Stagg, Dr. Marsik noted.

Maps with concentric circles, triangles, semicircles, and numbers tell a fascinating aspect of the D-Day invasion. Two forecasts derived from those maps were pressure-packed exercises by experts that probably saved thousands of soldiers’ lives and ultimately liberated Europe.