Allied forces had been planning to storm the beaches of Normandy for years. There were many factors that had to line up in order for success to even be possible.
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A low tide was required for landing crafts to spot and maneuver around mined obstacles. Aside from the tide, weather conditions had to be suitable enough for those small crafts to safely land during the low tide.
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In early June, a strong storm nearby made that nearly impossible, but last minute adjustments to the forecast led to a window of opportunity that changed history forever.
Allied meteorologists decided to postpone the original invasion date from the June 5 to June 6 to allow the storm to exit to the east.
German meteorologists predicted strong storms to persist through mid-June, making an invasion not only unlikely, but impossible. This mistake cost the Germans their position at Normandy and was the beginning of the end for the Axis powers.
Allied forces saw conditions improve on the June 6 just enough to storm the beaches and catch the Germans off-guard.
While conditions were not ideal by any means, they were favorable enough to carry out the mission successfully.
When asked how the invasion was so successful, Gen. Dwight D. Eisenhower responded, "Because we had better meteorologists than the Germans."