I remember during my time in the Oklahoma Army National Guard after active duty time....

I was just a fly on the wall guarding a high level meeting of the state and federal big wigs discussing this very subject.

I seem to remember the general (if unofficial) consensus at the beginning of the meeting was "Don't tell anyone the nukes are on the way". I guess the thinking was there was no way to get everyone out, so let them end "peacefully".

By the end of the meeting, I think it had become official policy. Once the powers that were saw the magnitude of the issue, they knew it would take days not hours.

Now for a hurricane or something you can see coming in advance, I think the evacuations this year on the Gulf Coast are a good example. (Admittedly smaller though.)