If there is going to be a nuclear attack you can expect to be warned in one or both of two ways. A warning can be either strategic or tactical.
A strategic warning happens when enemy actions are observed that lead us to believe that they are preparing to use nuclear weapons. This information could be broadcast over the news or through printed media over period of days. This does not necessarily mean that Americans would have much time to evacuate in the event of an emergency. We would only have a day or so of warning. However there is enough time in the weeks during which an attack is being speculated about to put together a quick shelter.
A tactical warning is very official. News of a nuclear attack on the United States would be received by officials just a few minutes after missiles or other nuclear weapons had been launched against the country. Information from satellites would be feeding information into our satellites almost immediately. The information would first be evaluated by the military and then decision would be made as to how it would all be handled. Only then would news of the attack be transmitted via radio and television to citizens all over the country.
This can be a very last minute warning as once missiles are launched it takes between fifteen and forty minutes for the missiles to hit their targets in the United States. This does not give most Americans the ability to even get to the nearest shelter. There may not be a lot you can do if you built your shelter at home and you are four miles away at your work and the missiles were launched fifteen minutes ago.
Aside from television and radio announcements the official warning system called NAW also gives warnings by siren. The warning signal is a single wavering and very loud wailing sound that lasts three to five minutes. In some areas of the country it may sound like a number of short blasts on a horn or a whistle. After a brief pause these signals are repeated. If you hear them go directly to the closest available shelter. Do not phone, stop to get supplies or try to get hold of loved ones. An attack is immediate and imminent.
It is important that you familiarize yourself with the announcements and sounds of official nuclear warnings. Unfortunately many people have heard too many tests of these systems over the years and like a fire alarm pulled too many times in an apartment building they do not take the sound seriously.