No towns that i know of around here use All Clear. Its very useless
and confusing, in my opinion.
Situation: Sirens go off for tornado warning, then stop. 3 minutes
later, they go off again, when another tornado is comming.
There is half the people who think another one is comming when they
go off again.
The other half of people think its all clear and venture outside to see
what happened (when they get slammed in the face by a tornado

)
I just don't think All Clear is a good idea, unles Attack designates danger
and Alert is only use for All Clear.