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The Most Important Weather Forecast Of All Time: D-Day, 6 June 1944

Meteorologist Paul Dorian, Vencore, Inc.

June 6th marks the 75rd anniversary of the D-Day invasion in Normandy, France during World War II and the weather forecast for that historic event makes for quite an interesting story in what turned out to be a pivotal moment in world history.

Years of detailed planning went into the D-Day invasion on June 6, 1944, but success hinged on one element that no military commander could control — the weather.  Defying his colleagues, Captain James Martin Stagg advised General Dwight “Ike” Eisenhower to postpone the invasion of Normandy by one day from June 5th to June 6th because of uncertain weather conditions in a weather forecast that was arguably the most important of all-time.

There were no computer forecast models, no satellites, radar was in its infancy and being used primarily for military purposes only, and yet General Dwight “Ike” Eisenhower wanted a definitive weather forecast for the planned invasion of Normandy, France with no “ifs”, “maybes” or “possibles” attached to the wording.  With definitive forecast information required and thousands of lives on the line, it is an underestimate to say that the task was daunting for chief meteorologist, Group Captain James Martin Stagg, of the British Royal Air Force.  Stagg ultimately persuaded General Eisenhower to change the date of the Allied invasion of Europe during World War II due to weather concerns from the 5th of June to the 6th of June in 1944. There were actually three different teams of weather forecasters involved with the Normandy invasion including the British Royal Navy, British Meteorological Office, and the US Strategic and Tactical Air Force, but Stagg was given the role as the chief meteorologist and the only meteorologist allowed direct contact with Eisenhower.

The opportunity for launching an invasion was limited to only a few days in each month to take advantage of the moon and tide.  Darkness was needed when the airborne troops went in, but moonlight once they were on the ground.  Spring low tide was necessary to ensure extreme low sea level so that the landing craft could spot and avoid the thousands of mined obstacles that had been deployed on the beaches.  If this narrow time slot was missed, the invasion would have been delayed for two weeks.  Eisenhower had tentatively selected June 5th as the date for the assault which was one of the few days in early June that met these criteria.

Saturday, June 3rd
By Saturday, June 3rd, the forecasts began to be highly unfavorable for a June 5th invasion. High pressure areas were over Greenland and the Azores, with low pressure centers moving east-northeast across the Atlantic.  It seemed probable that the high winds and sea would rule out the 5th as D-day.  Nevertheless, “Ike” pushed ahead for now with his plans for a June 5th invasion.

Sunday, June 4th
There were two official weather briefings given by weather forecasters on June 4th.  At the early weather briefing on that day, the weather prospect seemed completely hopeless for a June 5th invasion.  All weather experts predicted seas heavy enough to swamp landing craft and a low ceiling, which would prevent the air forces from carrying out their part of the assault. Under these circumstances, the air commanders were unwilling to take off, and Admiral Ramsey, after being advised that the winds would reach 25 to 30 miles per hour, feared that the channel would be too rough for small craft.  Only “Monty” (General Montgomery of England) wished to still carry out the schedule. […]

Monday, June 5th 
The 5th of June was indeed a miserable day and soldiers were cooped up in small beaching craft under lashing rain, and a day of intense anxiety for the top commanders watching from shore.  The surface weather chart for 4 June 1944 (not shown) featured an intense low pressure system centered to the west of England and a cold front extending southeast from the low pressure center to Ireland.  The foul weather that set in on June 4th threw all German commanders off their guard since, lacking weather observation stations west of the European continent; they were unable to predict the favorable weather that would follow the frontal system.  The German weather station in Greenland had been evacuated at the beginning of June, and no weather reporting U-boats were in a position to detect the small area of high pressure.  Hitler had long understood that the key to anticipating the timing of the invasion would be good weather forecasting.  General Rommel, who was in charge with the defense of the invasion beaches, was certain that there would be no invasion between June 5th and 8th because the tides were “not right” and the German weather forecasters predicted no letup in the stormy weather until mid-June. Rommel also thought that the Allies would not attempt an invasion without a guarantee of about six days of fine weather.  He was actually at home in Germany on the morning of D-Day when news of the landing caught up with him and only made it to the front at the end of the first day.

D-Day, Tuesday, June 6th 
On June 6th, the weather was more tolerable, but certainly not ideal.  A gusty wind blowing from the west at 15 to 20 knots produced a moderately choppy sea with waves of from 5 to 6 feet in height.  This was a heavy sea for the small craft, which had some difficulty in making way.  Even the assault area was rough for the shallow-draft vessels, although there the wind did not exceed 15 knots, and the waves averaged 3 feet.  Visibility was 8 miles with a cloud ceiling at 10,000 to 12,000 feet.  Scattered clouds from 3000 to 7000 feet covered almost half the sky over the channel, becoming denser farther inland.  Maritime polar air had moved over the channel behind the cold front as the low of 4 June that was west of England moved eastward; the deep low that was off Labrador on 4 June moved north-northeast to just off the southeast coast of Greenland (surface map for 6 June 1944 below).  This was the key to the clearing weather: if the Labrador low had tracked eastward, foul weather would have prevailed.

The midlevel overcast was most serious for air operations.  Heavy bombers assigned to hit the coastal fortifications at Omaha Beach had to bomb by instruments through the overcast.  With concurrence of General Eisenhower, the Eighth Air Force ordered a delay of several seconds in its release of bombs, in order to insure that they were not dropped among the assault craft. The result was that the 13,000 bombs dropped by 329 B-24 bombers did not hit the enemy beach and coast defenses at all, but were scattered as far as 3 miles inland. The weather also contributed to navigational difficulties. Mist mixed with the smoke and dust raised by the naval bombardment obscured landmarks on the coast; additionally, a lateral current of from 2 to 3 knots tended to carry craft east of their touchdown points by 1500 to 2000 yards and this caused some confusion.  Their difficulties were compounded by the heavier enemy opposition, which isolated boat sections only a few hundred yards apart and at first made reassembly and reorganization or improvised missions almost impossible.  Unloading at Utah beach proceeded in an orderly fashion, the chief distractions being an intermittent shelling of the beaches and air raids in the early morning hours. 

Post D-Day
By D-Day plus 12 days, the flow of men and supplies over the beaches was running smoothly: 314,514 troops, 41,000 vehicles, and 116,000 tons of supplies had been landed on the American beaches, with almost identical figures for the British beaches.  Had General Eisenhower postponed the invasion, the only option would have been to go two weeks later, and this would have encountered the “worst channel storm in 40 years” as Churchill later described it, which lasted four days between 19 and 22 June.  In fact, Eisenhower sent a letter to Captain Stagg saying in reference to the major storm that occurred in the potential second time slot for the invasion, “I thank the Gods of war we went when we did.” For the rest of his life, in moments of stress, Group Captain Stagg would remember some words spoken to him by General Morgan, Eisenhower’s Chief of Staff, in the tension-filled days leading up to the postponement: “Good luck Stagg: may all your depressions be nice little ones, but remember, we’ll string you up from the nearest lamp post if you don’t read the omens right.”

Years later, during their ride to the Capitol for his inauguration, President-elect John F. Kennedy asked President Eisenhower why the Normandy invasion had been so successful. Ike’s answer: “Because we had better meteorologists than the Germans!”

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