4 days ago
The most important weather forecast in history — D-Day
Ideal conditions — low tide, calm weather, full moonlight
There were a few must-haves when planning a history-altering invasion crossing the English Channel — calm weather, a low tide for a smooth landing, and a full moon so folks could see, especially the paratroopers.
Over 13,000 paratroopers were set to jump into the darkness just before dawn, so forecasting a night with clear skies and a full moon was imperative.
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Some 4,000 troop-landing ships needed a tide that was low enough to expose mines and other shipwrecking barriers placed by the Axis powers. But the Allies needed tides high enough to shorten the distance troops had to cover under open fire.
All of these considerations had Captain James Stagg, lead of the team of six Allied meteorologists circling the first week of June on the calendar, with the best time of attack to take place right before dawn. Specifically, June 5 was the day after a plan for May was tossed out the window due to a shortage in machinery. It was the best day for a full moon paired with the right tide height during a predawn approach.
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Ideal D-Day conditions included moonlight, lower tides, and light winds
UK MET OFFICE
Maureen Sweeney spots trouble, 500 miles away from Normandy
Maureen Sweeney, working at Blacksod Lighthouse on the west coast of Ireland, was taking weather observations on June 3 and 4, 1944, and observed falling barometric pressure, increasing clouds, and building winds.
The weather observations from this lighthouse were very important because the weather pattern usually tracks from Ireland through the English Channel and determines whether or not good or bad weather was on the way.
The weather observations reached Stagg and his team, raising concern about deteriorating conditions and the severity of impacts to the June 5 invasion. Sweeney recalled receiving follow-up phone calls from her submitted hourly observations, which were fed directly to Stagg and then reported to Dwight Eisenhower, the supreme commander of the Allies and overseeing Operation Overlord. She was pressed for accuracy but maintained and confirmed, that a storm was coming.
Blacksod Lighthouse in the 1940s.
Sweeney Family
On June 4, a strong low-pressure system pushed to the north of the United Kingdom, draping a strong cold front directly across the English Channel and bringing rounds of heavy rain and near hurricane-force winds of up to 60 miles per hour across the seas.
With bad weather across the board, the plans for a June 5 attack were aborted after hemming and hawing between meteorologists, generals, and other military leaders planning the attack.
All eyes looked at the 6th — especially since word got around that the Germans had intercepted and decoded plans for the invasion, according to the US Department of Defense. Time was of the essence.
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Allied forces had more surface observations than the Germans, identifying inclement and clear weather more accurately.
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June 6: A window to 'go' opens
Stagg and his team regrouped on June 5 while a storm packing 2 to 3 inches of rain and wind gusts between 40 and 65 miles per hour swept across Southern England and the English Channel. Things looked bleak, but good news was soon delivered.
Leveraging the hundreds to thousands of weather observations gathered from observation towers (including Blacksod), aircraft, ships, bases, and even the Nazis by breaking their code, Stagg and his team were able to pinpoint a break in the weather — or at least enough of a break for conditions to be more favorable.
The report from Blacksod read 'Heavy rain cleared, visibility on land and sea very clear.'
Changes in temperatures, pressure, and wind direction showed the cold front that delayed a June 5 attack, pulling away and June 6 showed signs of a calming period before another storm was to arrive.
It wasn't perfect — winds were forecast to be around 20 miles per hour with more clouds than General Eisenhower would have liked — but it was calm enough to proceed on to one of the greatest military victories.
A copy of the original surface forecast map ahead of D-Day from the Allied meteorologists.
UK MET OFFICE
What if they waited until the next full moon cycle?
The next window for a full moon and for the tide to be just right wasn't until the end of June, which would have given Germany more time to plan since they knew an attack was imminent.
Weather conditions proved to be worse on the morning of June 7 and beyond.
The next full moon was June 18, where during that week, the strongest storm in four decades came across the English Channel.
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When Eisenhower was asked by JFK what had given him the advantage on D-Day, Eisenhower replied, 'We had better meteorologists than the Germans.'
Ken Mahan can be reached at
