And this is all considering areas that are actually livable, just over-populated. Entire population of Vegas would die in a week or so. There's just nowhere to go, and nothing there to keep them alive once the available food/water runs out. Good chunk of the southwest, actually. Gonna be a big, dead dustbowl for anything resembling a city. Smaller areas maybe ok for a while, if they had their own water source and weren't near a population center. Pretty much all of California is gone, nothing to sustain them, and population is WAY too big. anyone that made it out to the north would run into the people escaping Portland and Seattle. A few might make it into the mountains, but most are going to die. On a unthinkable scale.
Here's another fun one: where are all our nuclear power plants located? How long before we've got an issue there if you just flipped the off switch for all their power, battery backups, etc? Mostly along the east coast (already screwed because of population, this won't help their problems) a stretch around Chicago (already killing the potential of this series), and southern CA (screwed anyway).
Anyone that's interested in the topic really should read Dies the Fire, by S.M. Stirling (recommended several times in this thread). A pretty realistic and believable take on this whole thing. More to the series if you like that one, but start there. Only difference between that book and the premise of this show is they tinker with the laws of physics a bit, taking away guns and really any high-energy reactions. for example, steam power just can't generate enough power to be useful.