D-Day Weather

Written by: Bob Metcalfe

I've always been intrigued with the things I've heard about how important of a factor the weather was in the D-Day invasion of Normandy in 1944. In fact, there's a book written about Sverre Pettersen, one of the key meteorologists in making the D-Day weather forecast. I've yet to read it but it's on my to-do list.

It's amazing the factors they thought about "back then". Tides had a very important part in the timing of the invasion, as well as darkness and even moonlight. Apparently the window of opportunity they picked was the right one, because bad weather the few days after the invasion tore things apart pretty well. I did a search on the internet about the weather on D-Day, and this is what I found:

RAIN FELL FROM overcast skies and gale force winds drove large waves on to the beaches of Normandy as dawn broke on Monday June 5, 1944. To the Germans watching their defences, there was nothing to show that this was the moment the Allied Armies had planned to invade Europe. In fact, the operation had been put on hold because the bad weather had been forecast 24 hours before. Had it gone ahead in these conditions, the invasion would have been a catastrophic disaster.

Nevertheless, the invasion had to occur on either the 5th, 6th or 7th of June to take advantage of the right conditions of 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 to be delayed for two weeks.

The decision to postpone the invasion for 24 hours had been taken by Eisenhower and the Supreme Command at 0430 on Sunday June 4. It was not taken lightly, because so many ships were already converging on Normandy that the risk of detection was grave.

Nor had the forecast which prompted the postponement been easily arrived at. Eisenhower's weather advice was provide by Group Captain Stagg, a forecaster seconded from the British Meteorological Office who was coordinating the advice of three forecasting teams: one from the Meteorological Office, one from the Admiralty and one from the United States Army Air Forces.

The advice of these groups was often diametrically opposed. The American team used an analog method, comparing the current map with maps from the past, and were often over-optimistic. The Meteorological Office, aided by the brilliant Norwegian theoretician Sverre Petterssen, had a more dynamic approach, using wind and temperature observations from high altitude provide by the air force, and were closer to the mark.

The decision to invade on Tuesday June 6, taken late on Sunday night and finally confirmed early Monday morning, was based on a forecast of a short period of improved weather caused by a strengthening ridge following the front that brought Monday's rain and strong winds. In the event, Monday's bad weather had already given the Allies a crucial advantage: it had put the Germans off guard.

The Germans were uncertain when and where the invasion would come, but had been led to believe the most likely place was Calais and the most likely time was July. Hitler, however, had long understood that the key to anticipating the timing of the invasion would be good weather forecasting.

But by the summer of 1944, German weather forecasters in France were hampered by a lack of weather observations over the Atlantic, because their submarine fleet was now much depleted and the Luftwaffe had largely yielded the skies to the RAF. Consequently, their forecasters could not detect the subtle changes that would lead to a temporary improvement starting on Monday evening.

Rommel, the general commanding the defence of the invasion beaches, had identified the period of June 5, 6 and 7 as high risk because of the state of the moon and tide. However, he also believed the Allies would not attempt an invasion without a guarantee of six days' fine weather. Reassured by the Luftwaffe weather forecaster's prediction that the bad weather starting on Monday the 5th would last at least three days, Rommel left France for Berlin. There he hoped to persuade Hitler to relinquish his personal control of the Panzer reserves in Holland and France to either himself or Von Rundstedt, who had overall command in the west. (As it transpired, Hitler held most of the reserves in the north, near Calais, for almost two months after the Normandy invasion, because he was persuaded Normandy was only a diversion).

Consequently, Rommel was in Germany when the invasion began, and only made it back to the front at the end of the first day.

The German Navy also dropped their guard when the bad weather commenced, and did not patrol the Channel. Only five weeks before, some of their torpedo boats had crossed the Channel and attacked a night-time dress rehearsal for the landings. In ten minutes they sank two landing craft, crippled a third, and killed over 600 sailors and soldiers.

But on the Monday night when the invasion fleet of over 6000 ships crossed the Channel, the torpedo boats did not venture out until 4am - after the fleet had been detected from the French shore. By this time the fleet had been anchored about 15km off the beaches along a front of 100km for more than an hour.

The weather on June 6 was tolerable but not ideal. Strong winds scattered the paratroops, some of whom overshot the Cherbourg Peninsula and landed in the sea and were drowned. However, the Germans were also obliged to scatter their defences.

Large waves swamped 27 out of 32 amphibious tanks, and all the artillery was lost on the run into Omaha beach, where the Allies suffered their greatest losses of the day and briefly considered withdrawing. At the end of the first day, Allied casualties were 12,000 killed, wounded and missing, as against an estimated 75,000 if surprise had not been achieved.

The weather that northern summer was among the worst on record. Several days after the landing, a storm wrecked one of the artificial harbours that had been built and caused four times the losses in ships and equipment that occurred during the landing.

Two weeks later, in the second time slot suitable for the invasion, another major storm occurred prompting Eisenhower to send Stagg a letter saying, "I thank the Gods of war we went when we did."

Correctly forecasting the weather for D-Day was crucial to the success of the invasion, which, if it had failed, could not have been repeated for another year. For the rest of his life, in moments of stress, Group Captain Stagg would remember some words spoken to him in the tension-filled days leading up to the postponement by General Morgan, Eisenhower's Chief of Staff: "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 aright."