Shipping in the upper Great Lakes mostly stops in late fall because of ice.
VOA: special.2009.11.11
Latein the fall, during a rare lull in his work on the Mars program, Grotzinger and I took a drive to Death Valley due east from his house in lush San Marino, across the front range of the San Gabriels, past Apple Valley and Barstow, and down into the great basin of the Mojave.
Even if Germany believed it would be in Moscow by latefall, controlling the countryside in W. Russia would have required soldiers to remain in the field and conduct operations.