Important Note: The counties and cities in Minnesota own and operate all sirens in their jurisdictions. They also set the policies and procedures of how and when to activate them. Neither the National Weather Service or the State of Minnesota own, operate or control any sirens.
There are many different policies regarding siren activation that are used by the various cities and counties. Some will activate sirens across the entire county for tornado warnings only. Others will activate sirens countywide for tornado warnings and all severe thunderstorm warnings. Some will activate sirens across the entire county for tornado warnings and severe thunderstorms that have winds of at least 70 or 75 mph. Others will activate sirens only for portions of counties. Local officials may also sound the sirens anytime they believe severe weather is a threat, even if there is no warning from the National Weather Service.
Sirens normally sound for about three minutes, and then go silent. It is very rare to keep the sirens sounding for the entire warning, since that would cause the backup battery to run out, which would be critical in the event that power goes out. Furthermore, the siren motor will fail much more quickly if the siren sounds continuously. Some jurisdictions may repeat siren activation every few minutes.
NOTE: There is no such thing as an "all-clear" siren.
Please check with your local public safety officials for details on when warning sirens are sounded in your community.