That shrill siren you may hear downtown will just be part of a test. According to NotifyNYC, the city's public messaging program, "Starting at 9:00 AM this morning and continuing on and off for several hours, Hudson County, NJ will be testing their emergency alert siren system. Loud sirens associated with this test may be heard in Lower Manhattan." (Hudson County includes Jersey City, Bayonne, Seacaucus and Hoboken.) NotifyNYC is only in four pilot communities in the city—Lower Manhattan, SW Staten Island, NE Bronx, and the Rockaways —but even if you don't live there, you can still sign up.