The weather in Europe during WW II was colder and wetter than usual, so what would happen if the the winter of 1939-1940 was not so cold that it led to the demoralized French army, a later mud season in Operation Babarsosa, a later and less cold winter 1941? There was also in 1943 the rains in the Italian Campaign slowed down the Allies advance. And Overlord benefited from the gales to surprise the German army but then the overcast skies kept the Allies planes from giving air support to the ground forces.