There are multiple notification systems available, I find that the twitter account tends to have a lot more info about hyperlocal and situational events vs the text-based system. I've just never straight up heard the outdoor one go off except for tests.

Folks in SF that want to sign up for the texts can do so here: http://sfdem.org/public-alerts

The national and state emergency alert systems have a cellphone program. You should get cellphone alerts in an emergency (I get them for flooding, AMBER alerts, etc.).

How would they know cell phone numbers? Where I live they call landlines for emergency alerts but there's no other way to specifically target residents.

If it was a big deal, beyond the opt-in SF text alerts, there's a thing called Wireless Emergency Alerts. It's geographically targeted. It's basically like Amber Alerts going to your phone except for imminent emergencies.

There has been some controversy about the timing and use of WEAs for evacuations for the big wildfires in California recently, but I imagine it's hard to balance alarm fatigue with the need to warn people.

The state can broadcast emergency messages, it doesn't need to send them individually to some database of cell phone numbers.

Alert About Missile Bound for Hawaii Was Sent in Error, Officials Say https://news.ycombinator.com/item?id=16140761

How do amber alerts work? When I lived in SF I got SF-related alerts. I don't know enough about how they work - but it sounds like tech that could be re-used for major emergency alerts.

As I should have realized, they're sent to all wireless phones in the area. On an iPhone at least both Amber and Emergency alerts are on by default but you can turn either of them off.