If they are coming, it will take about 30mins. Far too quick for a headline to make the rounds.
What you might notice (depending on if your country thinks it useful) will be siren signals and radio/TV broadcasts. But I guess it is even too short a time for those, and you cannot do more than "move into the cellar" anyways.
What you might notice (depending on if your country thinks it useful) will be siren signals and radio/TV broadcasts. But I guess it is even too short a time for those, and you cannot do more than "move into the cellar" anyways.