Southwick House is a large Regency building with a stucco façade and a colonnaded front. At the beginning of June 1944, five miles to the south, Portsmouth naval base and the anchorages beyond were crowded with craft of every size and type — grey warships, transport vessels and hundreds of landing craft, all tethered together. D-Day was scheduled for Monday, 5 June, and loading had already begun.

In peacetime, Southwick could have been the setting for an Agatha Christie house party, but the Royal Navy had taken it over in 1940. Its formerly handsome grounds and the wood behind were now blighted by rows of Nissen huts, tents and cinder paths. Southwick served as the headquarters of Admiral Sir Bertram Ramsay, the naval commander-in-chief for the invasion of Europe, and also as the advanced command post of SHAEF, the Supreme Headquarters Allied Expeditionary Force. Anti-aircraft batteries on the Portsdown ridge were positioned to defend it as well as the dockyards below from the Luftwaffe.

Southern England had been enjoying a heatwave compounded by drought. Temperatures of up to 100 degrees Fahrenheit had been recorded on 29 May, yet the meteorological team attached to General Dwight D. Eisenhower’s headquarters soon became uneasy. The group was headed by Dr James Stagg, a tall, lanky Scot with a rather gaunt face and a neat moustache. Stagg, the leading civilian weather expert in the country, had just been given the rank of group captain in the RAF to lend him the necessary authority in a military milieu unused to outsiders.

Since April, Eisenhower had been testing Stagg and his team by demanding three-day forecasts delivered on a Monday which were then checked against the reality later in the week. On Thursday, 1 June, the day before the battleships were due to sail from Scapa Flow off the north-west tip of Scotland, weather stations indicated some deep depressions forming over the North Atlantic.