mmckenna - I'm totally with you on this with a few notes
We already have tools to do this. It is AM broadcast radio. FM broadcast radio. Broadcast TV stations, all using the EAS system. Then we have NOAA alerts, which can handle EAS and other localized alerts.
People should have a battery powered AM/FM radio and ideally, a radio with NOAA capability. Not expensive. Most people have them in their car, but forget how to use them.
Which is one reason I keep a CCrane EP-PRO, which has modern internals, but retro-operations. Nothing to figure out when in a panic. Or as a handout to my non-radio savvy neighbors in a panic who isn't going to drill-down some menus. D-powered or AA with holders. simple.
Propogation - if it is bad enough that the locals are down, at least one can wait until nightime. Example: If there was a major emergency in L.A. to where even the locals or nearby stations are down or weak, I can still hear San Franciso, Oregon, or other out of state flamethrowers with no problem. If there was an agreement to temporarily broadcast local L.A. information from those out of state flamethrowers, that might help alleviate lack of local reception and get the word out. The locals might be willing to put up with detailed emergency announcements.
Kind of a reciprocal agreement thing - if San Francisco got into a heap of trouble, have KFI in Los Angeles broadcast the necessary detailed emergency information to that community, simple because there is a great chance they can hear us with a common AM radio.
Back when the solar cycle was good, this could have even been applied to shortwave. Let's say the west-coast got hammered badly. Radio Australia on 11780 was a flamethrower and easily found on the cheapest of shortwave portables. I always thought that if some sort of agreement could have been reached between the US and Australia to temporarily broadcast detailed information to our community, how helpful that might be. And of course if the US still had shortwave transmitters, our Australian friends dealing with wildfires might have welcome emergency info coming from this side of the pond. This is just an example, and of course wouldn't work today with the poor sunspot cycle.
But yes, I'm totally in agreement with using AM radio as a fallback - which could be enhanced with inter-state agreements to broadast detailed community information to each other in times of need.