The Most Important Weather Forecast in the History of the World

‘Everything on the Normandy beachhead will hang on your weather,’ said the D-Day planners, assuming that we meteorologists had total control of the elements. ‘Just name us five fine, calm days and we’ll go.’ A hundred years of weather records suggested there was no hope of their getting this. They limited their demand to ‘a quiet day with not more than moderate winds and seas and not too much cloud for the airmen, to be followed by three more quiet days.’ On top of this, the military insisted on a late full moon for the parachutists, plus a tide that would be high three hours after pioneer forces had landed at dawn to clear paths through the beach mines and other obstacles. Tides and moon being fully predictable, they would determine possible dates. July would be too late, and May too early. That left just four possible days: 5, 6, 19 or 20 June. We worked out the odds on the weather on any one of these four dates conforming to requirements as being 13 to one against. So meteorologically, D-Day was bound to be a gamble against the odds.

The planners had originally wanted to know how far ahead a correct weather forecast could be made and who should make it. ‘We can make it, one, perhaps two days ahead,’ said the Meteorological Office. ‘We can make it up to five days ahead,’ said the American Air Force. ‘But do they know anything about the sea?’ asked the Royal Navy, who made no promises. To satisfy pride, and to get the best possible forecast, the solution was to form a committee made up of the three groups. However, fear that loud-mouthed Americans might shout down mild British civil servants, or even battle-hardened naval officers, brought about an administrative compromise. The D-Day forecast became a telephonic affair: the six forecasters would stay in three separate centres, joined to one another, and to Eisenhower’s headquarters, by GPO telephones (which worked). A non-forecaster ‘presenter’ would moderate our debates and then explain our agreed forecast to Ike and his staff. Today’s TV weather presenter has a single source of information, but poor James Stagg had three, sometimes hotly divergent views to harmonise. Knowing little about forecasting, he was astounded to discover that, despite all the scientific expertise simultaneously available on his phone, an agreed forecast never emerged easily, even after hours of discussion.

Nevertheless, the three pairs of top forecasters turned out to be a solid mixture, with enough confidence, diffidence, pragmatism, initiative, foresight, hindsight, optimism and pessimism to produce a valid forecast which clearly read ‘go’ or ‘no go’. We all analysed the same hundreds of observations, including decoded German ones, teleprinted to us and plotted on large maps stretching from the Urals to Newfoundland and Greenland. We drew pressure maps for the days ahead, and based our predictions mainly on these – with hunch and experience as our dangerous but necessary allies.

None of us proved to have a monopoly on rightness or wrongness. Our two West Coast Americans from the California Institute of Technology were used to forecasting for Hollywood. Ben Holzman was full of liberal optimism and Irv Krick, a salesman to his fingertips, had total faith in his own five-day forecasts. C.K.M. Douglas, the senior Met Office forecaster, was a good counterbalance, full of wise ‘buts’ and pessimistic ‘ifs’, plus a complete lack of confidence in any forecast that predicted the weather more than about a day in advance. With him, from Norway, where modern synoptic meteorology was born, was Sverre Petterssen. He had emigrated to the Massachusetts Institute of Technology, disliked the West Coasters and argued interminably against them. The Royal Navy provided two sailors who had taken part in earlier Allied landings in North Africa: Geoffrey Wolfe, an urbane Cambridge man from Hove, on the English Channel, who not surprisingly turned out to be quite the most consistent forecaster of the six, and the youngest, myself, a New Zealand Rhodes Scholar and the first ever Naval Instructor to have won a DSC in battle.

Eisenhower and his commanders had about a month to make up their minds about the quality and timing of their weather advice. During the pleasant weather of May 1944, our five-day forecasts proved to be terrible, but our presenter was able to give Ike 18 substantially correct two-day forecasts of suitable invasion weather. So the vital decision on the choice of a D-Day would have to be based on one-and-a-half to two-day forecasts.

Unfortunately, at the beginning of June, forecasting became more difficult when a steady Azores anti-cyclone started to misbehave, and a series of depressions threatened to run across the North Atlantic, with associated fronts menacing a hitherto sheltered Channel. Up to Friday, 2 June, it was still peaceful, but we suspected the calm was near its end. Then began the forecasters’ and the Supreme Commander’s nightmare: a calm period ending – to be replaced when, and by what? We speculated on Friday that the big change might arrive around Tuesday, 6 June. However, by the Saturday morning, our three forecast centres all agreed that the change was upon us, while disagreeing on what it would turn out to be, or when. On Saturday evening the moment for our crunch forecast arrived. The Americans were still relying on the Azores high pressure: they were for ‘go’ on Monday the 5th; the cautious Met Office were for ‘no go’; so were the pragmatic Royal Navy. Ike considered a postponement. Then, during the night, our observations showed that a stormy front was sweeping towards the Channel and, unwillingly, Caltech yielded. At a hairy 4 a.m. meeting on Sunday morning, faced by a unanimous prediction of strong winds, low cloud and rough seas for the Monday, Ike postponed the operation for 24 hours, only two hours before the main body was due to sail. So Monday was out, and the troops in their landing-craft tossed uncomfortably at moorings. The decision had been an emotional drain on all the participants, military and meteorological, and the whole nerve-racking process had now to continue for yet another day.

The telephone debates went on furiously if inconclusively. We all agreed that the key depression whose front had caused the postponement would move north-east and threaten no more. But what would it leave behind? And how quick and how real would the clearance be after the storm? Tricky meteorological questions! On the Sunday evening, in the fine weather which preceded a still-to-happen Monday storm, we had to stop arguing and forecast for Tuesday the 6th. The Americans were again for ‘go’. The Met Office initially hesitated; Sverre said: ‘This is not the invasion weather we envisaged.’ The Royal Navy, this time, opted for ‘go’. So the ‘go’s had it, and it was time to draft a psychologically suitable forecast of wind, wave and cloud, to be passed on to Channel-crossers and their commanders. After hectic interrogation of the presenter, with all the high-ups participating, a provisional decision to ‘go’ on Tuesday was taken by Eisenhower at the 9 p.m. Sunday meeting, although he had no promise of good weather for after D-Day. Our forecast firmed up favourably overnight and at 4 a.m. on Monday, Ike rapidly gave the word. So, irrevocably, Tuesday the 6th it was. Asking one another whether we might be shot at dawn on Wednesday, we forecasters waited to see the actual Monday weather. Our storm, quite a severe one, arrived more or less on time, fully justifying the postponement. Then it cleared from the West, with wind, sea and cloud slowly lessening. This enabled the bombers to bomb overnight, the moon to guide the parachutists, and the 3000 ships, with their seasick thankful for improving seas, to make for the assembly-point in the Channel, and go on to the assault on the beaches.

The gamble that the weather would be both suitable and forecastable had come off. It was the adoption of the untried, apparently cockeyed telephone committee method of forecasting that got the predictions right, aided by the discovery in the course of May that out five-day forecasts were virtually useless. Suppose, for example, that the Americans had been given, as they originally sought, sole responsibility. Their five-day forecast, confirmed by a short-range one, could have fixed D-Day on the stormy Monday the 5th, with disaster to follow. If, similarly, sole responsibility had lain with the Met Office it might not only have said ‘no go’ to the 5th, but also, lacking contra-opinion, to the 6th, thus opening the way to the next possible date, the 19th. The 19th, as it happens, would have been a meteorological disaster. On the 17th, forecasting for it, we unanimously predicted calm, quiet weather – it would have been a ‘go’ forecast. But in the event a persistent, unforecast gale on the 19th destroyed one artificial Mulberry harbour and just spared the Arromanches one – a large army being already, thankfully, ashore. Although my naval colleague and I happened to forecast correctly for the two critical days, our forecasts were as much the fruit of our discussions with the others as of any singular ability. The team forecasts saved us, and the invasion.

Unlike the Allies, German generals had faith in long-range forecasts, to which they had become attuned, perhaps by Hitler’s weather-aided successes in Poland. On Sunday, 4 June, Major Lettau, Rommel’s chief meteorologist, said that there would be no invasion in the next fortnight. ‘The enemy has not made use of three periods of fine weather in May for his invasion and further periods of fine weather in the coming weeks cannot be reckoned on.’ Rommel went home on leave; Admiral Krancke took a trip to Bordeaux; General von Salmuth went hunting in the Ardennes; General Feuchtinger and his chief of staff went on manoeuvres in the Paris night clubs; while on D-Day itself three more serious-minded German generals were conducting a mock invasion exercise at Rennes.

Would today’s weather forecasters do any better now than we did in 1944? They certainly should, with their satellites, computers and weather models. Fifty years ago our only satellites and computers were in our heads and we could find no analogue in our past records for what the Americans afterwards described as ‘a meteorological situation unique in the annals of June weather’.

I am sure that in weather like that of May 1944 today’s experts would improve on us, and make reasonable five-day forecasts. But when one type of weather is changing to something radically different, as it did the following month, I am not so sure. I have been a knowledgeable consumer of Met Office and Météo France one or two-day forecasts for many years and my observation is that the timing and nature of a total change in the weather continue to pose questions which often are not satisfactorily answered. For shorter periods, the accuracy of today’s forecasts is most impressive. In 1944 we only just got the most important weather forecast in history right. But we steered the invading army away from a potential disaster at sea and helped to make ultimate victory feasible.