If you hear sirens around noon today, this is the reason. Issued by Cobb County government:
Cobb County has more than 70 outdoor warning sirens to alert residents during a weather-related emergency, including ten sirens that can also broadcast voice messages.
The intent of warning sirens is to alert people who are outside that an imminent danger is approaching; they are not designed to be heard within a home or other building.
Cobb County conducts outdoor warning siren system tests at noon on the first Wednesday of each month, sounding the sirens for 3 to 5 minutes. In the event that there is inclement weather on the first Wednesday of the month, the test will be postponed until the following day, the first Thursday of the month.
More here.