A Fire in the Sky (1978)

"The countdown to annihilation has begun... And nothing on earth can stop it!"

Astronomers discover a comet that they believe will crash into Phoenix, Arizona. They attempt to warn officials, but without 100% certainty, the governor of Arizona is reticent to cause a panic. Even after a television news reporter discovers the truth and threatens to go live with it, the response is understated enough to doom some residents of Phoenix to certain death.