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.

http://www.lrb.co.uk/v16/n10/lawrence-hogben/diary