NEVER in history has so much turned on the weather forecast as it did on June 3, 4 and 5 in 1944. But in 1944 forecasters trying to see 48 hours ahead were at or beyond the limits of what was possible.

In May the previous year Washington had chosen May 1944 as the time for the invasion of Europe. Difficulties in assembling landing craft forced a postponement until June, but June 5, 1944, was fixed as the “unalterable” date by General Eisenhower, the Supreme Allied Commander, on May 17.

To begin with, conditions looked remarkably good as May ended in a glorious heatwave, but June roared in with atrocious weather as freak gales battered the country and tore up the English Channel on southwesterly winds. Worse still, a long line of depressions were ganging up in the Atlantic ready for more attacks. Nothing like it had been seen before in June.

The bad weather threatened disastrous consequences for the biggest military operation ever mounted.

It was the job of senior meteorologists to advise the Supreme Commander, but each arm of the huge operation had its own demanding requirements of the weather.

The RAF’s bomber and fighter aircraft each needed different cloud conditions. Gliders to be used in the initial airborne landings at night required moonlight with no fog or mist to identify and reach their targets. The Army needed firm dry ground, so there had to be no heavy rain before the order came to move.

The Navy wanted winds no stronger than 12 mph, good visibility and no prolonged high winds in the Western Approaches for the days immediately preceding the operation. Otherwise troops could be helplessly seasick and landing craft and “swimming” tanks capsize before they could reach the shore. The invaders then wanted quiet conditions to persist for as long as possible after the initial assault.

It would all have been much easier if the date of the attack could have been decided at very short notice, but so large an invasion force could not be kept waiting indefinitely.

It was decided that the senior meteorologists of the Meteorological Office, the Naval Meteorological Service and the Weather Service of the United States military, would work independently on predicting the likely weather patterns. Often their forecasts were diametrically opposed.

The Americans claimed to make accurate forecasts for up to five days ahead, based on looking back at past records and seeing if the weather fitted a past pattern. But there was little proof that this worked, and the Channel storms of June 1944 were unprecedented in 40 years of weather observations. Yet the American forecasters still confidently predicted good weather for June 5.

The British relied on physics and maths to calculate the forecast, but in those days without computers it was a struggle to get a 24-hour prediction. The rift between the American and British forecasters grew as Supreme Headquarters pressed for forecasts days ahead, with D-Day nearing and the storms howling.

The final weather forecasting decision rested on Dr James Stagg of the British Meteorological Office, seconded as a group captain in the RAF, who had been appointed as the co-ordinating forecaster. It was he who had to decide how to brief the Supreme Commander and his staff.

Climatology had identified May and June as the most likely months for the right conditions, and the tide, state of the Moon and time of sunrise combined favourably on Monday, June 5, and the following two days. The tide, but not the moonlight, would be right again two weeks later, but all eyes focused on June, with the right moon phases for the tides on June 5.

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