Difference between WEA levels?

Many new phones support the government's Wireless Emergency Alert (WEA) system. I just bought a Samsung S3 and in less than an hour, while I was driving home from the Verizon store I was startled by a loud, piercing alert tone sounding like the old Emergency Broadcast System alarm. I pulled over to see what it was and it was a flash flood warning sent by some anonymous government agency. (what Verizon calls an "Authorized Sender" but they won't say who) These things are going to cause accidents, IMO.

Anyway, there are 4 levels of Alerts - Amber Alerts, Severe Alerts, Extreme Alerts, and Presidential Alerts.

I disabled the Amber alerts - honestly, what are we supposed to do with one of those? Drive around looking for little kids who match the description?? That sounds like a good way to get arrested! We're not allowed to disable Presidential Alerts (think about that for a second). But that leaves the other two.

Does anyone know what the difference is between Severe Alert and Extreme Alert? Thanks in advance.