If i needed to do it i would bring everyone to a town in Kentucky called Pineville, by way of a caravan of buses following a D.O.T. snow plow rigged diesel.

Pineville Kentucky: A small ...ish town with a (12'?) flood wall covering 2/3 of the town and a mountain on the other 3rd. The mountain has a boulder that the army core of engineers chained to the mountain so it wouldn't cause trouble.It has its own hospital and fire department (both essentials in time to come)
Heres a pic of the down town area
upload.wikimedia.org/wiki...ine_Mt.png


Or (a safer but more difficult idea) Fort Jefferson in the Dry Tortugas just of the Florida Keys.

heres a wiki on it
en.wikipedia.org/wiki/Fort_Jefferson