page_title: Operation Overlord
section_title: Weather forecasting
hierarchical_section_title: Operation Overlord / Preparations for D-Day / Weather forecasting
caption_attribution_description: IWM caption&#160;: THE BRITISH ARMY IN THE UNITED KINGDOM 1939-45. Men of 22nd Independent Parachute Company, 6th Airborne Division for the invasion. Comment: This happened during preparation for Operation Tonga, part of the airborne invasion of Normandy. The officer briefing them is Lieutenant Robert Midwood&#91;1&#93;. Others have been identified&#91;2&#93;, too.
context_page_description: Operation Overlord was the codename for the Battle of Normandy, the Allied operation that launched the successful invasion of German-occupied Western Europe during World War II. The operation was launched on 6 June 1944 with the Normandy landings. A 1,200-plane airborne assault preceded an amphibious assault involving more than 5,000 vessels. Nearly 160,000 troops crossed the English Channel on 6 June, and more than two million Allied troops were in France by the end of August.
  The decision to undertake a cross-channel invasion in 1944 was taken at the Trident Conference in Washington in May 1943. General Dwight D. Eisenhower was appointed commander of Supreme Headquarters Allied Expeditionary Force, and General Bernard Montgomery was named as commander of the 21st Army Group, which comprised all the land forces involved in the invasion. The coast of Normandy of northwestern France was chosen as the site of the invasion, with the Americans assigned to land at sectors codenamed Utah and Omaha, the British at Sword and Gold, and the Canadians at Juno.
context_section_description: The invasion planners specified a set of conditions regarding the timing of the invasion, deeming only a few days in each month suitable. A full moon was desirable, as it would provide illumination for aircraft pilots and have the highest tides. The Allies wanted to schedule the landings for shortly before dawn, midway between low and high tide, with the tide coming in. This would improve the visibility of obstacles the enemy had placed on the beach while minimising the amount of time the men had to spend exposed in the open. Specific criteria were also set for wind speed, visibility, and cloud cover. 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 from finding their targets.
  By the evening of 4 June, the Allied meteorological team, headed by Group Captain James Stagg of the Royal Air Force, predicted that the weather would improve sufficiently so that the invasion could go ahead on 6 June. He met Eisenhower and other senior commanders at their headquarters at Southwick House in Hampshire to discuss the situation. General Montgomery and Major General Walter Bedell Smith, Eisenhower's chief of staff, were eager to launch the invasion. Admiral Bertram Ramsay was prepared to commit his ships, while Air Chief Marshal Trafford Leigh-Mallory expressed concern that the conditions would be unfavourable for Allied aircraft. After much discussion, Eisenhower decided that the invasion should go ahead. Allied control of the Atlantic meant that German meteorologists did not have access to as much information as the Allies on incoming weather patterns. As the Luftwaffe meteorological centre in Paris predicted two weeks of stormy weather, many Wehrmacht commanders left their posts to attend war games in Rennes, and men in many units were given leave. Marshal Erwin Rommel returned to Germany for his wife's birthday and to meet Hitler to try to get more Panzers.
  Had Eisenhower postponed the invasion, the next available period with the right combination of tides (but without the desirable full moon) was two weeks later, from 18 to 20 June. As it happened, during this period the invaders would have encountered a major storm lasting four days, between 19 and 22 June, that would have made the initial landings impossible.
caption: Men of the British 22nd Independent Parachute Company, 6th Airborne Division being briefed for the invasion, 4–5 June 1944
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