The window of opportunity for launching an invasion was limited to only a few days in each month as a full moon was required, for light for the aircraft pilots and for the
spring tide. Eisenhower had tentatively selected 5 June as the date for the assault. However, on 4 June, conditions were clearly unsuitable for a landing; high winds and heavy seas made it impossible to launch landing craft, and low clouds would prevent aircraft finding their targets. The Germans meanwhile took comfort from the existing poor conditions and believed an invasion would not be possible for several days. Some troops stood down, and many senior officers were absent. Rommel, for example, took leave to attend his wife's birthday.
Since April, the
Captain class frigate HMS
Grindall had been transmitting weather reports from the mid-Atlantic every three hours. From these reports, Group Captain
James Stagg RAF, Eisenhower's chief meteorologist, predicted a slight improvement in the weather for 6 June.
Grindall's reports indicated a ridge of high pressure behind a deep depression. Stagg forecast that the ridge would move eastward to reach the south-west approaches to the Channel late on 5 June, bringing a short-term improvement in the weather.
At a vital meeting late on 5 June, Eisenhower and his senior commanders discussed the situation. General Montgomery and Major General
Walter Bedell Smith, Eisenhower's chief of staff, were eager to launch the invasion. Admiral
Bertram Ramsay also was prepared to commit his ships, while Air Chief Marshal
Trafford Leigh-Mallory was concerned that the conditions would be unfavourable for Allied aircraft to operate. After much discussion, Eisenhower decided to launch the invasion that night.
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Had Eisenhower postponed the invasion, the only option was to go two weeks later but this would have encountered the 'worst channel storm in 40 years' as Churchill later described it, which lasted four days between 19 and 22 June.