The Overlord planners for the invasion of Europe in 1944 specified suitable weather (wind, cloud, tidal and moon conditions) for the assault landing; with only a few days in each month suitable. In May and June 1944 frequent pre-assault meetings were held at Southwick House in Hampshire near Portsmouth by Eisenhower with Group Captain James Stagg of the RAF, the Chief Meteorological Officer, SHAEF, his deputy Colonel Donald Yates of the USAAF, and his three two-man teams of meteorologists. Stagg was a "dour but canny Scot.. " He had been given the rank of group captain in the RAF "to lend him the necessary authority in a military milieu unused to outsiders". The senior commanders were General Bernard Montgomery, Admiral Sir Bertram Ramsay and Air Marshal Sir Trafford Leigh-Mallory, plus Eisenhower's deputy, Air Marshall Arthur Tedder, his chief of staff Walter Bedell Smith and his deputy chief of Staff Major General Harold R. Bull.
Stagg reported the team consensus, although this has been glossed over in popular memory. Admiral Sir George Creasy remarked on 4 June: "Here comes six feet two inches of Stagg and six foot one inch of gloom..." (1.88m &amp; 1.85m). British general Frederick Morgan (head of COSSAC) had half-jokingly said to him "Remember, if you don't read the runes (or signs) right, we'll string you up from the nearest lamppost".
Forecasting teams.
The three weather forecasting teams at Southwick House were from the UK Met Office at Dunstable, the Eighth Air Force of the USAAF at High Wycombe, and the Royal Navy at Portsmouth; they usually met twice daily; at 9:30 am (0930 hours) and 4:00 pm (1600 hours), with Eisenhower and his senior commanders.
The Royal Navy team was Commander Geoffrey Wolfe and New Zealand Lieutenant Commander Lawrence Hogben. Hogben recalled that "We six never agreed on anything except that Stagg was not a good meteorologist and that he was a bit of a glory hound". At heart a geophysist, Stagg was appointed as SHAEF weatherman for his administrative skills to co-ordinate Army and RAF forecasting; he had only two years experience of forecasting in the Iraqi desert.
The civilian Met Office team was Charles K. M. Douglas and Norwegian Sverre Petterssen of the Norwegian Air Force. Douglas had told USAAF airmen Spaatz and Doolittle in 1942 that "a forecast for more than a day or two ahead in this country can be nothing more than speculation".
The USAAF team was Irving P. Krick and Ben Holtzman. Hogben thought their forecasts for daylight bombing runs over Germany made by looking at forty years of weather maps were "pure bunk" and the Norwegian Petterssen thought that Krick was "talking nonsense" in claiming reliable forecasts for up to five days. Pre-war they had forecasted for Hollywood film studios (e.g. for the "burning of Atlanta" scene in Gone with the Wind). Krick (a physics graduate and college professor) was regarded by his rivals as "brash" and "a salesman to his fingertips". Donald Yates was a former student of Krick. The American team was based at the USAAF facility "Widewing" used for SHAEF HQ at Camp Griffiss in Bushy Park near Teddington.
Tidal predictions.
A full moon was desirable, as it would provide illumination for aircraft pilots and have the highest tides. Having the landings shortly before dawn, midway between low and high tide, with the tide coming in 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 set for wind speed, visibility, and cloud cover. The previous night had to be reasonably light (moonlight) for darkened convoys; although airborne troops needed a full or nearly full moon but late-rising so with darkness until they were dropped.
The invasion zone tidal range was about 23 ft (7m) and the landings should be on a rising tide to allow landing craft to run aground, unload and then withdraw without becoming stranded. Landings had to be at about the same time on all beaches to avoid later arrivals meeting an alerted enemy, and Omaha had an exposed tidal flat of 300 yards (275m) to cross. It was decided that the assault should be one hour after low tide and within one hour of first light.
Arthur Doodson head of the Liverpool Tidal Institute advised the Royal Navy on European coastal areas, and was asked in October 1943 by Commander Ian Farquharson to make tidal predictions for Normandy, which was disguised as "Position Z". He created two tide-prediction machines. The required light and tidal conditions occurred for only six days a month, and when strong moonlight for airborne operations was added there were only three days per month.
Choice of date for Overlord.
Originally 1 May 1944 was proposed at the Trident Conference attended by Roosevelt and Churchill in May 1943, but when the invasion force was expanded from landing three divisions to landing five divisions (as proposed by Eisenhower and Montgomery), it was delayed to acquire the extra landing craft needed. This allowed more time for air attacks on the defences. Suitable dates were 21–23 May, 5–7 June, 19–21 June and 3–5 July 1944 according to Admiral Alan Kirk.
At the Tehran Conference in November–December 1943, Thursday 1 June 1944 was tentatively chosen. For long-term planning it was called Y-day, so D-day must be Y plus 4, 5 or 6. As the assault machinery must be set in motion two days before the event, D-day must be decided by Y plus 2. For early June there were only three days (5, 6 and 7 June). For H-Hour (the hour of landing) one hour of daylight was needed beforehand. Stagg had said they might have to wait 150 years for the perfect weather which would please everyone, and also that a full moon and early low tide could be guaranteed on the 4th, 5th, 6th and 7 June.
On 17 May 1944 Eisenhower chose 5 June which allowed the possibility of postponing for one or two days, although the convoys might have to be refuelled for 7 June. For 5 June, units from the north would have to depart on 3 June to arrive in time. The first to depart were the blockships to be sunk off the Normandy coast that sailed south from their Scottish ports on 31 May 1944. With bad weather on 5 June the operation was postponed for a day to 6 June, and then a slight break in the weather was predicted. As many ships had to depart one or two days beforehand some were held at sea. The bad weather had led German commanders to rule out an invasion then, so the Allied landings took them by surprise. 
The next suitable dates with the right combination of tides (but without a full moon, desirable for the airborne troops) would have been 18 to 20 June. On 17 June the team forecasts were all for good weather, but on 18 June the worst storm for forty years arrived, which would have made the initial landings impossible. Eisenhower later said to Stagg: "I thank the gods of war we went when we did".
Monday 29 May to Wednesday 31 May.
Weather conditions were excellent on the 20th, and Stagg gave an optimistic long-range forecast for the first week of June. Admiral Ramsay noted "another boiling hot day" on Tuesday but "slightly cooler" on Wednesday.
Thursday 1 June.
The first coded messages (about 160; repeated on the 2nd and 3rd ) over the BBC's Radio Londres told the French Resistance that the invasion was expected in two weeks, e.g. the quote from Paul Verlaine's 1866 poem Chanson d'automne (sung by Charles Trenet): "Les sanglots longs des violons d'automne" ("The long sobs of autumn violins"). It was intercepted by the Germans at 1:20 pm. "Il fait chaud à Suez" ("It's hot in Suez") initiated the widespread launching of disruptive guerrilla operations. 
Admiral Ramsay noted "A dull morning, Overcast with slight rain. Cooler. No doubt the general lull is but a precursor of the storm to follow." 
Friday 2 June.
May had been consistently fine, but less favourable weather was predicted for D-Day, with indications that the relatively quiet weather might end about 6 June. After discussion with his commanders Eisenhower decided to keep existing orders. So Bombarding Force D sailed from the Clyde, and HMZ Nelson left Scapa for Milford Haven. Two midget submarines X-20 &amp; X-23 left; they were to mark Sword and Juno beaches for Forces S and J (Operation Gambit). Admiral Ramsay noted "1000 hours. Commander's meeting at which main topic was the change for the worse in weather, affecting air (operations) generally and the carriage of airborne troops in particular." The Blacksod lighthouse in neutral Ireland had reported a rapidly falling barometer and a Force Six wind.
Stagg conferred and argued (without agreement) with the other meteorological centres over secure landlines. To Stagg "Had it not been fraught with such potential tragedy, the whole business was ridiculous. In less than half an hour I was expected to present to General Eisenhower an "agreed" forecast for the next five days which covered the time of launching of the greatest military operation ever mounted; no two of the expert participants in the discussion could agree on the likely weather even for the next 24 hours." At the 21:30 evening meeting Stagg was asked by Eisenhower "Well, Stagg, what have you got for us this time." He followed his instincts, overlooking the optimistic American team, saying "The whole situation from the British Isles to Newfoundland has been transformed in recent days and is now potentially full of menace". Some officers glanced out the window at the beautiful sunset in slight bewilderment (on Double British Summer Time, it was still light; France was an hour behind). Asked by Eisenhower about the likely situation on 6 and 7 June; he replied after a pause "If I answered that, Sir, I would be guessing, not behaving as your meteorological advisor".
Then Stagg and Yates left and withdrew to their tented sleeping quarters; the sky was almost clear and everything around was quiet. Later that night Eisenhower was told by his aide Harry Butcher about the message accidentally put out by Associated Press (picked up by CBS and Radio Moscow) that "Eisenhower's forces are landing in France"; the agency cancelled it 23 minutes later.
Saturday 3 June.
In the morning Stagg was "all but physically nauseated" by the weather charts and the different analysis by the teams. He said to the commanders at 21:30 hours "Gentleman, the fears my colleagues and I had yesterday about the weather for the next three or four days have been confirmed" and gave a detailed forecast of rough seas, winds up to force six and low cloud. All seemed temporarily stunned; Eisenhower was motionless and had to recommend a provisional postponement.
By the afternoon the weather had deteriorated, to the dismay of the Southwick House meeting at 16:15. The two British teams predicted a sudden and serious decline with storms that would rule out air support and could make landing treacherous, though the American team thought better weather was in the offing for 5 June. Petterssen presented his own weather map showing a violent storm brewing. And Ramsay noted that "The report was bad"; the low cloud predicted would prohibit the use of airborne troops and the majority of air action (including air spotting for naval gunfire). The sea conditions were "unpromising but not prohibitive".
Stagg did not get a consensus, so told Eisenhower on 3 June that the weather two days hence was unlikely to meet the Overlord planners' requirements. At the meeting Tedder was against going and Ramsay was neutral. But Montgomery was insistent on going "regardless of casualties or air support", he was opposed by Ramsay and the airmen Tedder and Leigh-Mallory. So Eisenhower postponed a go-no go decision until dawn on Sunday 4 June. Messages e.g. "Bowsprit" or "Regatta postponed 24 hours" were to go out, some were not received until 4 June. Convoy U2A from Salcome and Dartmouth left on 4 June for Utah Beach but did not receive the broadcast recall notices, and was headed for France alone (panic in Southwick House ops room!). A search by two destroyers was unsuccessful, then a Walrus reconnaissance biplane located the convoy after an all-day search and dropped two coded messages in canisters; the second one was acknowledged