I haven't seen anyone mention the partnership between Nixle and Google yet:
http://www.nixle.com/google/
https://www.google.org/publicalerts/

The alerts come in via the Google Now application on my android devices
I've been receiving local emergency alerts for a couple years now,
mainly for fires in the Southern California area. I also get warnings
from National Weather Service.

Last year while on vacation out of the country I even received an alert
that told me to evacuate my home immediately because a local Police
Department drew their fire evacuation area wrong.

There is some great info on google's support page on how to enable
alerting for end users, how the system works and where they get alert
data from:
https://support.google.com/publicalerts/

There is also a form for public agencies to sign up to send their
emergency alert data to google for distribution.

-Keith