Underground (reinforced concrete) is the only chance to survive, albeit slim. After you lucked out, you'd still need prepared bags of water, long-term rations, radiation monitor, heavy-duty tools to dig out, mask + hazard suit, batteries, Faraday cage for any electronics to survive etc. Sirens are unlikely to help, the government would have to interrupt any broadcasts and tell you about nukes coming.
It really depends on how far you are from the nearest blasts, and how big they were, and which way the winds are blowing. Even in a massive nuclear exchange, there will still be places in the United States hundreds of miles from the nearest blast. Those people could be fine if they take adequate protection from fallout, and if they can survive supply chain collapse and who knows how long of nuclear winter and poor agricultural output.
