The announcement (followed by a siren) starts when there is 90 seconds left. The Omega Warhead has a countdown of 4 hours this can be seen by the chat interface. The difference between the Omega Warhead and the Alpha Warheads is that instead of destroying the surface, the Omega Warhead will destroy the facility altogether. The Omega Warhead is a 5 megaton hydrogen bomb placed within the facility which is used as a last resort containment method.
