In order for the city to perform necessary upgrades to the Outdoor Public Warning System the regularly blaring sirens we're accustomed to hearing every Tuesday at noon will be going silent for the next two years.
The last, comforting "This is a test" announcement and five minutes of blaring will be happening next Tuesday, December 10. After that, the system's 109 separate sirens won't be undergoing those weekly tests for two years while the city tests out new, more secure alert equipment. Once testing is complete, the city's technology department will then oversee upgrades to all 109 stations, as the Chronicle reports.
The changes come after a security flaw was revealed in April 2018 that would have allowed hackers to seize control of the sirens and potentially create a public panic — or at the very least a major nuisance. The city of Dallas saw this happen in April 2017, when hackers gained access to its public-address and siren system, and sent the alarms ringing for two hours late one night. San Francisco's sirens randomly went off late one weekend night in November 2014, though that appeared to have been a simple malfunctioning.
In SF, in addition to the physical siren posts, we have the Emergency Alert System that is sent over television and radio broadcast stations, as well as a network of alerts that are sent out via SMS and Twitter. (You can enroll in the AlertSF text system by texting your zip code to 888-777.) In the event of an earthquake, we will hopefully all get a few seconds warning now that the ShakeAlert app is up and running — but that has yet to have a real-world test in this region. All of those systems, in addition to workers from the Department of Emergency Management roaming the streets with public-address systems, will temporarily take the place of the siren posts.
The sirens date back to 1942, at the beginning of World War II, when 50 of them around the city had siren and tone capability. In addition to broadcasting speech, the latest sirens can play seven different tones, but we only usually hear the "wail" tone being tested each Tuesday. The various speaker posts have been specifically calibrated throughout the city so that they are audible from every corner and hill.
So, when you hear the old-timey air raid sound go off next week, remember to savor it, because it won't come again until 2021.