Want to hear something truly terrifying? An EMP strike could be launched against us with no warning. It's entirely likely that a Chinese or former Soviet communications or weather satellite is really an orbiting nuclear warhead. Think Goldeneye, the weapon in the 007 movie of the same name. If it's already in orbit, we'd get no missile launch warning.
In reality, though, I don't think Putin or any other Russian would be that stupid. It just goes back to the M.A.D. doctrine of the Cold War. They can't possibly launch a nuclear or EMP strike against every asset the US has, and one or two of our ballistic missile submarines would be more than enough to render Russia or any other country on Earth to smoking, glowing rubble. And they are enjoying their free market economy a little too much to throw it all away now. China may be a bit more unpredictable, but their growing economy needs the US as a trading partner and consumer market. So, as scary as an EMP strike is, it's about as likely as full on nuclear strike, which is about as likely as the dead reanimating and eating our brains.
Best bet is to plan for the most likely scenarios: natural disasters. Hurricanes in the Southeast, earthquakes out west, massive blizzards up north and in the mountains, wild fires seemingly just about anywhere.