I hope this isn't too minor or obscure a WI...
In the OTL, the primary target for Enola Gay and Little Boy on 8/6/45 was Hiroshima with Kokura and Nagasaki being secondaries. For Bockscar and Fat Man on 8/9/45 the primary was Kokura with Nagasaki as the secondary target.
On 8/9/45, Kokura was clouded over.
What if Hiroshima had been obscured on 8/6 instead, and the Little Boy had been dropped on Kokura instead?