[RyanW]

"A butterfly can flap its wings in Peking and in Central Park you get nuclear winter instead of sunshine." - Ian Malcolm (paraphrased)

One thing I've always thought would be a scarily easy alternate timeline:

The cloud cover over the US is ever so slightly different on September 25, 1983. Just different enough that the Oko satellite array doesn't mistakenly report a nuclear launch by the United States against the Soviet Union. The Soviet Union proceeds blissfully unaware of the flaw in their detection system.

Instead, the weather conditions needed occur on November 9, 1983. Right during the middle of Able Archer 83, which the Soviets already feared was a cover for a real attack.