As stormy weather made a German invasion seem less likely, British intelligence from Bletchley Park, referred to as “our special source,” suggested that Hitler had probably postponed Operation Sea Lion. At the same time, the Luftwaffe’s bombing campaign was clearly changing shape. A newly captured German airman described a coming full-moon attack of exceptional scale, aimed at breaking civilian morale with mass bombing and incendiaries, while fresh intelligence from the special source warned of a gigantic operation code-named Moonlight Sonata.

Air Ministry officials tried to interpret the fragmentary warnings. They concluded that the raid might come in three parts, might use every available navigational beam, and might involve the whole German long-range bomber force under Hermann Göring’s direction. Because the threat seemed serious, the RAF prepared a defensive and retaliatory plan called Operation Cold Water: No. 80 Wing would try to disrupt German beams and bomb key transmitters, while Bomber Command considered a heavy counterstrike against a German city for moral and strategic effect.

Further updates on Wednesday confirmed that Moonlight Sonata did indeed have three named components, though British officials still could not tell whether these meant phases in one night or attacks spread over three nights. William Sholto Douglas doubted the Germans could count on three successive clear nights. On Thursday, November 14, the Air Ministry judged the threat large enough to brief Churchill directly in a highly secret memorandum placed in his yellow box, though officials still believed the main raid would likely fall on Friday, November 15, and probably on London.

That same day, amid wartime tension, John Colville attended Neville Chamberlain’s funeral at Westminster Abbey, where secrecy and bomb damage underscored the danger still hanging over London. Later, Churchill set out for Ditchley, his country refuge during full moons, but read the memorandum en route. Alarmed by the warning that the entire German long-range bomber force might strike and that London was a likely target, Churchill immediately turned the cars around and returned to Downing Street rather than spend the night in the country.

Back in London, Churchill sent his female staff away to safety, ordered John Colville and John Peck to sleep in the protected Down Street shelter, and went to the Cabinet War Rooms to await the attack. As the evening developed, new electronic and wireless intelligence corrected the earlier estimate: Moonlight Sonata would begin that very night, November 14. At 6:17 p.m., KGr 100 crossed the coast carrying incendiaries to mark the target, while a few aircraft flew over London only to continue onward; their passage proved to be a feint meant to make the RAF believe the main blow was aimed at the capital.